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diff --git a/README.CV b/README.CV new file mode 100644 index 0000000..522fa60 --- /dev/null +++ b/README.CV @@ -0,0 +1,3036 @@ +README.CV -- Condition Variables
+--------------------------------
+
+The original implementation of condition variables in
+pthreads-win32 was based on a discussion paper:
+
+"Strategies for Implementing POSIX Condition Variables
+on Win32": http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
+
+The changes suggested below were made on Feb 6 2001. This
+file is included in the package for the benefit of anyone
+interested in understanding the pthreads-win32 implementation
+of condition variables and the (sometimes subtle) issues that
+it attempts to resolve.
+
+Thanks go to the individuals whose names appear throughout
+the following text.
+
+Ross Johnson
+
+--------------------------------------------------------------------
+
+fyi.. (more detailed problem description/demos + possible fix/patch)
+
+regards,
+alexander.
+
+
+Alexander Terekhov
+31.01.2001 17:43
+
+To: ace-bugs@cs.wustl.edu
+cc:
+From: Alexander Terekhov/Germany/IBM@IBMDE
+Subject: Implementation of POSIX CVs: spur.wakeups/lost
+ signals/deadlocks/unfairness
+
+
+
+ ACE VERSION:
+
+ 5.1.12 (pthread-win32 snapshot 2000-12-29)
+
+ HOST MACHINE and OPERATING SYSTEM:
+
+ IBM IntelliStation Z Pro, 2 x XEON 1GHz, Win2K
+
+ TARGET MACHINE and OPERATING SYSTEM, if different from HOST:
+ COMPILER NAME AND VERSION (AND PATCHLEVEL):
+
+ Microsoft Visual C++ 6.0
+
+ AREA/CLASS/EXAMPLE AFFECTED:
+
+ Implementation of POSIX condition variables - OS.cpp/.h
+
+ DOES THE PROBLEM AFFECT:
+
+ EXECUTION? YES!
+
+ SYNOPSIS:
+
+ a) spurious wakeups (minor problem)
+ b) lost signals
+ c) broadcast deadlock
+ d) unfairness (minor problem)
+
+ DESCRIPTION:
+
+ Please see attached copy of discussion thread
+ from comp.programming.threads for more details on
+ some reported problems. (i've also posted a "fyi"
+ message to ace-users a week or two ago but
+ unfortunately did not get any response so far).
+
+ It seems that current implementation suffers from
+ two essential problems:
+
+ 1) cond.waiters_count does not accurately reflect
+ number of waiters blocked on semaphore - w/o
+ proper synchronisation that could result (in the
+ time window when counter is not accurate)
+ in spurious wakeups organised by subsequent
+ _signals and _broadcasts.
+
+ 2) Always having (with no e.g. copy_and_clear/..)
+ the same queue in use (semaphore+counter)
+ neither signal nor broadcast provide 'atomic'
+ behaviour with respect to other threads/subsequent
+ calls to signal/broadcast/wait.
+
+ Each problem and combination of both could produce
+ various nasty things:
+
+ a) spurious wakeups (minor problem)
+
+ it is possible that waiter(s) which was already
+ unblocked even so is still counted as blocked
+ waiter. signal and broadcast will release
+ semaphore which will produce a spurious wakeup
+ for a 'real' waiter coming later.
+
+ b) lost signals
+
+ signalling thread ends up consuming its own
+ signal. please see demo/discussion below.
+
+ c) broadcast deadlock
+
+ last_waiter processing code does not correctly
+ handle the case with multiple threads
+ waiting for the end of broadcast.
+ please see demo/discussion below.
+
+ d) unfairness (minor problem)
+
+ without SignalObjectAndWait some waiter(s)
+ may end up consuming broadcasted signals
+ multiple times (spurious wakeups) because waiter
+ thread(s) can be preempted before they call
+ semaphore wait (but after count++ and mtx.unlock).
+
+ REPEAT BY:
+
+ See below... run problem demos programs (tennis.cpp and
+ tennisb.cpp) number of times concurrently (on multiprocessor)
+ and in multiple sessions or just add a couple of "Sleep"s
+ as described in the attached copy of discussion thread
+ from comp.programming.threads
+
+ SAMPLE FIX/WORKAROUND:
+
+ See attached patch to pthread-win32.. well, I can not
+ claim that it is completely bug free but at least my
+ test and tests provided by pthreads-win32 seem to work.
+ Perhaps that will help.
+
+ regards,
+ alexander.
+
+
+>> Forum: comp.programming.threads
+>> Thread: pthread_cond_* implementation questions
+.
+.
+.
+David Schwartz <davids@webmaster.com> wrote:
+
+> terekhov@my-deja.com wrote:
+>
+>> BTW, could you please also share your view on other perceived
+>> "problems" such as nested broadcast deadlock, spurious wakeups
+>> and (the latest one) lost signals??
+>
+>I'm not sure what you mean. The standard allows an implementation
+>to do almost whatever it likes. In fact, you could implement
+>pthread_cond_wait by releasing the mutex, sleeping a random
+>amount of time, and then reacquiring the mutex. Of course,
+>this would be a pretty poor implementation, but any code that
+>didn't work under that implementation wouldn't be strictly
+>compliant.
+
+The implementation you suggested is indeed correct
+one (yes, now I see it :). However it requires from
+signal/broadcast nothing more than to "{ return 0; }"
+That is not the case for pthread-win32 and ACE
+implementations. I do think that these implementations
+(basically the same implementation) have some serious
+problems with wait/signal/broadcast calls. I am looking
+for help to clarify whether these problems are real
+or not. I think that I can demonstrate what I mean
+using one or two small sample programs.
+.
+.
+.
+==========
+tennis.cpp
+==========
+
+#include "ace/Synch.h"
+#include "ace/Thread.h"
+
+enum GAME_STATE {
+
+ START_GAME,
+ PLAYER_A, // Player A playes the ball
+ PLAYER_B, // Player B playes the ball
+ GAME_OVER,
+ ONE_PLAYER_GONE,
+ BOTH_PLAYERS_GONE
+
+};
+
+enum GAME_STATE eGameState;
+ACE_Mutex* pmtxGameStateLock;
+ACE_Condition< ACE_Mutex >* pcndGameStateChange;
+
+void*
+ playerA(
+ void* pParm
+ )
+{
+
+ // For access to game state variable
+ pmtxGameStateLock->acquire();
+
+ // Play loop
+ while ( eGameState < GAME_OVER ) {
+
+ // Play the ball
+ cout << endl << "PLAYER-A" << endl;
+
+ // Now its PLAYER-B's turn
+ eGameState = PLAYER_B;
+
+ // Signal to PLAYER-B that now it is his turn
+ pcndGameStateChange->signal();
+
+ // Wait until PLAYER-B finishes playing the ball
+ do {
+
+ pcndGameStateChange->wait();
+
+ if ( PLAYER_B == eGameState )
+ cout << endl << "----PLAYER-A: SPURIOUS WAKEUP!!!" << endl;
+
+ } while ( PLAYER_B == eGameState );
+
+ }
+
+ // PLAYER-A gone
+ eGameState = (GAME_STATE)(eGameState+1);
+ cout << endl << "PLAYER-A GONE" << endl;
+
+ // No more access to state variable needed
+ pmtxGameStateLock->release();
+
+ // Signal PLAYER-A gone event
+ pcndGameStateChange->broadcast();
+
+ return 0;
+
+}
+
+void*
+ playerB(
+ void* pParm
+ )
+{
+
+ // For access to game state variable
+ pmtxGameStateLock->acquire();
+
+ // Play loop
+ while ( eGameState < GAME_OVER ) {
+
+ // Play the ball
+ cout << endl << "PLAYER-B" << endl;
+
+ // Now its PLAYER-A's turn
+ eGameState = PLAYER_A;
+
+ // Signal to PLAYER-A that now it is his turn
+ pcndGameStateChange->signal();
+
+ // Wait until PLAYER-A finishes playing the ball
+ do {
+
+ pcndGameStateChange->wait();
+
+ if ( PLAYER_A == eGameState )
+ cout << endl << "----PLAYER-B: SPURIOUS WAKEUP!!!" << endl;
+
+ } while ( PLAYER_A == eGameState );
+
+ }
+
+ // PLAYER-B gone
+ eGameState = (GAME_STATE)(eGameState+1);
+ cout << endl << "PLAYER-B GONE" << endl;
+
+ // No more access to state variable needed
+ pmtxGameStateLock->release();
+
+ // Signal PLAYER-B gone event
+ pcndGameStateChange->broadcast();
+
+ return 0;
+
+}
+
+
+int
+main (int, ACE_TCHAR *[])
+{
+
+ pmtxGameStateLock = new ACE_Mutex();
+ pcndGameStateChange = new ACE_Condition< ACE_Mutex >( *pmtxGameStateLock
+);
+
+ // Set initial state
+ eGameState = START_GAME;
+
+ // Create players
+ ACE_Thread::spawn( playerA );
+ ACE_Thread::spawn( playerB );
+
+ // Give them 5 sec. to play
+ Sleep( 5000 );//sleep( 5 );
+
+ // Set game over state
+ pmtxGameStateLock->acquire();
+ eGameState = GAME_OVER;
+
+ // Let them know
+ pcndGameStateChange->broadcast();
+
+ // Wait for players to stop
+ do {
+
+ pcndGameStateChange->wait();
+
+ } while ( eGameState < BOTH_PLAYERS_GONE );
+
+ // Cleanup
+ cout << endl << "GAME OVER" << endl;
+ pmtxGameStateLock->release();
+ delete pcndGameStateChange;
+ delete pmtxGameStateLock;
+
+ return 0;
+
+}
+
+===========
+tennisb.cpp
+===========
+#include "ace/Synch.h"
+#include "ace/Thread.h"
+
+enum GAME_STATE {
+
+ START_GAME,
+ PLAYER_A, // Player A playes the ball
+ PLAYER_B, // Player B playes the ball
+ GAME_OVER,
+ ONE_PLAYER_GONE,
+ BOTH_PLAYERS_GONE
+
+};
+
+enum GAME_STATE eGameState;
+ACE_Mutex* pmtxGameStateLock;
+ACE_Condition< ACE_Mutex >* pcndGameStateChange;
+
+void*
+ playerA(
+ void* pParm
+ )
+{
+
+ // For access to game state variable
+ pmtxGameStateLock->acquire();
+
+ // Play loop
+ while ( eGameState < GAME_OVER ) {
+
+ // Play the ball
+ cout << endl << "PLAYER-A" << endl;
+
+ // Now its PLAYER-B's turn
+ eGameState = PLAYER_B;
+
+ // Signal to PLAYER-B that now it is his turn
+ pcndGameStateChange->broadcast();
+
+ // Wait until PLAYER-B finishes playing the ball
+ do {
+
+ pcndGameStateChange->wait();
+
+ if ( PLAYER_B == eGameState )
+ cout << endl << "----PLAYER-A: SPURIOUS WAKEUP!!!" << endl;
+
+ } while ( PLAYER_B == eGameState );
+
+ }
+
+ // PLAYER-A gone
+ eGameState = (GAME_STATE)(eGameState+1);
+ cout << endl << "PLAYER-A GONE" << endl;
+
+ // No more access to state variable needed
+ pmtxGameStateLock->release();
+
+ // Signal PLAYER-A gone event
+ pcndGameStateChange->broadcast();
+
+ return 0;
+
+}
+
+void*
+ playerB(
+ void* pParm
+ )
+{
+
+ // For access to game state variable
+ pmtxGameStateLock->acquire();
+
+ // Play loop
+ while ( eGameState < GAME_OVER ) {
+
+ // Play the ball
+ cout << endl << "PLAYER-B" << endl;
+
+ // Now its PLAYER-A's turn
+ eGameState = PLAYER_A;
+
+ // Signal to PLAYER-A that now it is his turn
+ pcndGameStateChange->broadcast();
+
+ // Wait until PLAYER-A finishes playing the ball
+ do {
+
+ pcndGameStateChange->wait();
+
+ if ( PLAYER_A == eGameState )
+ cout << endl << "----PLAYER-B: SPURIOUS WAKEUP!!!" << endl;
+
+ } while ( PLAYER_A == eGameState );
+
+ }
+
+ // PLAYER-B gone
+ eGameState = (GAME_STATE)(eGameState+1);
+ cout << endl << "PLAYER-B GONE" << endl;
+
+ // No more access to state variable needed
+ pmtxGameStateLock->release();
+
+ // Signal PLAYER-B gone event
+ pcndGameStateChange->broadcast();
+
+ return 0;
+
+}
+
+
+int
+main (int, ACE_TCHAR *[])
+{
+
+ pmtxGameStateLock = new ACE_Mutex();
+ pcndGameStateChange = new ACE_Condition< ACE_Mutex >( *pmtxGameStateLock
+);
+
+ // Set initial state
+ eGameState = START_GAME;
+
+ // Create players
+ ACE_Thread::spawn( playerA );
+ ACE_Thread::spawn( playerB );
+
+ // Give them 5 sec. to play
+ Sleep( 5000 );//sleep( 5 );
+
+ // Make some noise
+ pmtxGameStateLock->acquire();
+ cout << endl << "---Noise ON..." << endl;
+ pmtxGameStateLock->release();
+ for ( int i = 0; i < 100000; i++ )
+ pcndGameStateChange->broadcast();
+ cout << endl << "---Noise OFF" << endl;
+
+ // Set game over state
+ pmtxGameStateLock->acquire();
+ eGameState = GAME_OVER;
+ cout << endl << "---Stopping the game..." << endl;
+
+ // Let them know
+ pcndGameStateChange->broadcast();
+
+ // Wait for players to stop
+ do {
+
+ pcndGameStateChange->wait();
+
+ } while ( eGameState < BOTH_PLAYERS_GONE );
+
+ // Cleanup
+ cout << endl << "GAME OVER" << endl;
+ pmtxGameStateLock->release();
+ delete pcndGameStateChange;
+ delete pmtxGameStateLock;
+
+ return 0;
+
+}
+.
+.
+.
+David Schwartz <davids@webmaster.com> wrote:
+>> > It's compliant
+>>
+>> That is really good.
+>
+>> Tomorrow (I have to go urgently now) I will try to
+>> demonstrate the lost-signal "problem" of current
+>> pthread-win32 and ACE-(variant w/o SingleObjectAndWait)
+>> implementations: players start suddenly drop their balls :-)
+>> (with no change in source code).
+>
+>Signals aren't lost, they're going to the main thread,
+>which isn't coded correctly to handle them. Try this:
+>
+> // Wait for players to stop
+> do {
+>
+> pthread_cond_wait( &cndGameStateChange,&mtxGameStateLock );
+>printf("Main thread stole a signal\n");
+>
+> } while ( eGameState < BOTH_PLAYERS_GONE );
+>
+>I bet everytime you thing a signal is lost, you'll see that printf.
+>The signal isn't lost, it was stolen by another thread.
+
+well, you can probably loose your bet.. it was indeed stolen
+by "another" thread but not the one you seem to think of.
+
+I think that what actually happens is the following:
+
+H:\SA\UXX\pt\PTHREADS\TESTS>tennis3.exe
+
+PLAYER-A
+
+PLAYER-B
+
+----PLAYER-B: SPURIOUS WAKEUP!!!
+
+PLAYER-A GONE
+
+PLAYER-B GONE
+
+GAME OVER
+
+H:\SA\UXX\pt\PTHREADS\TESTS>
+
+here you can see that PLAYER-B after playing his first
+ball (which came via signal from PLAYER-A) just dropped
+it down. What happened is that his signal to player A
+was consumed as spurious wakeup by himself (player B).
+
+The implementation has a problem:
+
+================
+waiting threads:
+================
+
+{ /** Critical Section
+
+ inc cond.waiters_count
+
+}
+
+ /*
+ /* Atomic only if using Win32 SignalObjectAndWait
+ /*
+ cond.mtx.release
+
+ /*** ^^-- A THREAD WHICH DID SIGNAL MAY ACQUIRE THE MUTEX,
+ /*** GO INTO WAIT ON THE SAME CONDITION AND OVERTAKE
+ /*** ORIGINAL WAITER(S) CONSUMING ITS OWN SIGNAL!
+
+ cond.sem.wait
+
+Player-A after playing game's initial ball went into
+wait (called _wait) but was pre-empted before reaching
+wait semaphore. He was counted as waiter but was not
+actually waiting/blocked yet.
+
+===============
+signal threads:
+===============
+
+{ /** Critical Section
+
+ waiters_count = cond.waiters_count
+
+}
+
+ if ( waiters_count != 0 )
+
+ sem.post 1
+
+ endif
+
+Player-B after he received signal/ball from Player A
+called _signal. The _signal did see that there was
+one waiter blocked on the condition (Player-A) and
+released the semaphore.. (but it did not unblock
+Player-A because he was not actually blocked).
+Player-B thread continued its execution, called _wait,
+was counted as second waiter BUT was allowed to slip
+through opened semaphore gate (which was opened for
+Player-B) and received his own signal. Player B remained
+blocked followed by Player A. Deadlock happened which
+lasted until main thread came in and said game over.
+
+It seems to me that the implementation fails to
+correctly implement the following statement
+from specification:
+
+http://www.opengroup.org/
+onlinepubs/007908799/xsh/pthread_cond_wait.html
+
+"These functions atomically release mutex and cause
+the calling thread to block on the condition variable
+cond; atomically here means "atomically with respect
+to access by another thread to the mutex and then the
+condition variable". That is, if another thread is
+able to acquire the mutex after the about-to-block
+thread has released it, then a subsequent call to
+pthread_cond_signal() or pthread_cond_broadcast()
+in that thread behaves as if it were issued after
+the about-to-block thread has blocked."
+
+Question: Am I right?
+
+(I produced the program output above by simply
+adding ?Sleep( 1 )?:
+
+================
+waiting threads:
+================
+
+{ /** Critical Section
+
+ inc cond.waiters_count
+
+}
+
+ /*
+ /* Atomic only if using Win32 SignalObjectAndWait
+ /*
+ cond.mtx.release
+
+Sleep( 1 ); // Win32
+
+ /*** ^^-- A THREAD WHICH DID SIGNAL MAY ACQUIRE THE MUTEX,
+ /*** GO INTO WAIT ON THE SAME CONDITION AND OVERTAKE
+ /*** ORIGINAL WAITER(S) CONSUMING ITS OWN SIGNAL!
+
+ cond.sem.wait
+
+to the source code of pthread-win32 implementation:
+
+http://sources.redhat.com/cgi-bin/cvsweb.cgi/pthreads/
+condvar.c?rev=1.36&content-type=text/
+x-cvsweb-markup&cvsroot=pthreads-win32
+
+
+ /*
+ * We keep the lock held just long enough to increment the count of
+ * waiters by one (above).
+ * Note that we can't keep it held across the
+ * call to sem_wait since that will deadlock other calls
+ * to pthread_cond_signal
+ */
+ cleanup_args.mutexPtr = mutex;
+ cleanup_args.cv = cv;
+ cleanup_args.resultPtr = &result;
+
+ pthread_cleanup_push (ptw32_cond_wait_cleanup, (void *)
+&cleanup_args);
+
+ if ((result = pthread_mutex_unlock (mutex)) == 0)
+ {((result
+Sleep( 1 ); // @AT
+
+ /*
+ * Wait to be awakened by
+ * pthread_cond_signal, or
+ * pthread_cond_broadcast, or
+ * a timeout
+ *
+ * Note:
+ * ptw32_sem_timedwait is a cancelation point,
+ * hence providing the
+ * mechanism for making pthread_cond_wait a cancelation
+ * point. We use the cleanup mechanism to ensure we
+ * re-lock the mutex and decrement the waiters count
+ * if we are canceled.
+ */
+ if (ptw32_sem_timedwait (&(cv->sema), abstime) == -1) {
+ result = errno;
+ }
+ }
+
+ pthread_cleanup_pop (1); /* Always cleanup */
+
+
+BTW, on my system (2 CPUs) I can manage to get
+signals lost even without any source code modification
+if I run the tennis program many times in different
+shell sessions.
+.
+.
+.
+David Schwartz <davids@webmaster.com> wrote:
+>terekhov@my-deja.com wrote:
+>
+>> well, it might be that the program is in fact buggy.
+>> but you did not show me any bug.
+>
+>You're right. I was close but not dead on. I was correct, however,
+>that the code is buggy because it uses 'pthread_cond_signal' even
+>though not any thread waiting on the condition variable can do the
+>job. I was wrong in which thread could be waiting on the cv but
+>unable to do the job.
+
+Okay, lets change 'pthread_cond_signal' to 'pthread_cond_broadcast'
+but also add some noise from main() right before declaring the game
+to be over (I need it in order to demonstrate another problem of
+pthread-win32/ACE implementations - broadcast deadlock)...
+.
+.
+.
+It is my understanding of POSIX conditions,
+that on correct implementation added noise
+in form of unnecessary broadcasts from main,
+should not break the tennis program. The
+only 'side effect' of added noise on correct
+implementation would be 'spurious wakeups' of
+players (in fact they are not spurious,
+players just see them as spurious) unblocked,
+not by another player but by main before
+another player had a chance to acquire the
+mutex and change the game state variable:
+.
+.
+.
+
+PLAYER-B
+
+PLAYER-A
+
+---Noise ON...
+
+PLAYER-B
+
+PLAYER-A
+
+.
+.
+.
+
+PLAYER-B
+
+PLAYER-A
+
+----PLAYER-A: SPURIOUS WAKEUP!!!
+
+PLAYER-B
+
+PLAYER-A
+
+---Noise OFF
+
+PLAYER-B
+
+---Stopping the game...
+
+PLAYER-A GONE
+
+PLAYER-B GONE
+
+GAME OVER
+
+H:\SA\UXX\pt\PTHREADS\TESTS>
+
+On pthread-win32/ACE implementations the
+program could stall:
+
+.
+.
+.
+
+PLAYER-A
+
+PLAYER-B
+
+PLAYER-A
+
+PLAYER-B
+
+PLAYER-A
+
+PLAYER-B
+
+PLAYER-A
+
+PLAYER-B
+
+---Noise ON...
+
+PLAYER-A
+
+---Noise OFF
+^C
+H:\SA\UXX\pt\PTHREADS\TESTS>
+
+
+The implementation has problems:
+
+================
+waiting threads:
+================
+
+{ /** Critical Section
+
+ inc cond.waiters_count
+
+}
+
+ /*
+ /* Atomic only if using Win32 SignalObjectAndWait
+ /*
+ cond.mtx.release
+ cond.sem.wait
+
+ /*** ^^-- WAITER CAN BE PREEMPTED AFTER BEING UNBLOCKED...
+
+{ /** Critical Section
+
+ dec cond.waiters_count
+
+ /*** ^^- ...AND BEFORE DECREMENTING THE COUNT (1)
+
+ last_waiter = ( cond.was_broadcast &&
+ cond.waiters_count == 0 )
+
+ if ( last_waiter )
+
+ cond.was_broadcast = FALSE
+
+ endif
+
+}
+
+ if ( last_waiter )
+
+ /*
+ /* Atomic only if using Win32 SignalObjectAndWait
+ /*
+ cond.auto_reset_event_or_sem.post /* Event for Win32
+ cond.mtx.acquire
+
+ /*** ^^-- ...AND BEFORE CALL TO mtx.acquire (2)
+
+ /*** ^^-- NESTED BROADCASTS RESULT IN A DEADLOCK
+
+
+ else
+
+ cond.mtx.acquire
+
+ /*** ^^-- ...AND BEFORE CALL TO mtx.acquire (3)
+
+ endif
+
+
+==================
+broadcast threads:
+==================
+
+{ /** Critical Section
+
+ waiters_count = cond.waiters_count
+
+ if ( waiters_count != 0 )
+
+ cond.was_broadcast = TRUE
+
+ endif
+
+}
+
+if ( waiters_count != 0 )
+
+ cond.sem.post waiters_count
+
+ /*** ^^^^^--- SPURIOUS WAKEUPS DUE TO (1)
+
+ cond.auto_reset_event_or_sem.wait /* Event for Win32
+
+ /*** ^^^^^--- DEADLOCK FOR FURTHER BROADCASTS IF THEY
+ HAPPEN TO GO INTO WAIT WHILE PREVIOUS
+ BROADCAST IS STILL IN PROGRESS/WAITING
+
+endif
+
+a) cond.waiters_count does not accurately reflect
+number of waiters blocked on semaphore - that could
+result (in the time window when counter is not accurate)
+in spurios wakeups organised by subsequent _signals
+and _broadcasts. From standard compliance point of view
+that is OK but that could be a real problem from
+performance/efficiency point of view.
+
+b) If subsequent broadcast happen to go into wait on
+cond.auto_reset_event_or_sem before previous
+broadcast was unblocked from cond.auto_reset_event_or_sem
+by its last waiter, one of two blocked threads will
+remain blocked because last_waiter processing code
+fails to unblock both threads.
+
+In the situation with tennisb.c the Player-B was put
+in a deadlock by noise (broadcast) coming from main
+thread. And since Player-B holds the game state
+mutex when it calls broadcast, the whole program
+stalled: Player-A was deadlocked on mutex and
+main thread after finishing with producing the noise
+was deadlocked on mutex too (needed to declare the
+game over)
+
+(I produced the program output above by simply
+adding ?Sleep( 1 )?:
+
+==================
+broadcast threads:
+==================
+
+{ /** Critical Section
+
+ waiters_count = cond.waiters_count
+
+ if ( waiters_count != 0 )
+
+ cond.was_broadcast = TRUE
+
+ endif
+
+}
+
+if ( waiters_count != 0 )
+
+Sleep( 1 ); //Win32
+
+ cond.sem.post waiters_count
+
+ /*** ^^^^^--- SPURIOUS WAKEUPS DUE TO (1)
+
+ cond.auto_reset_event_or_sem.wait /* Event for Win32
+
+ /*** ^^^^^--- DEADLOCK FOR FURTHER BROADCASTS IF THEY
+ HAPPEN TO GO INTO WAIT WHILE PREVIOUS
+ BROADCAST IS STILL IN PROGRESS/WAITING
+
+endif
+
+to the source code of pthread-win32 implementation:
+
+http://sources.redhat.com/cgi-bin/cvsweb.cgi/pthreads/
+condvar.c?rev=1.36&content-type=text/
+x-cvsweb-markup&cvsroot=pthreads-win32
+
+ if (wereWaiters)
+ {(wereWaiters)sroot=pthreads-win32eb.cgi/pthreads/Yem...m
+ /*
+ * Wake up all waiters
+ */
+
+Sleep( 1 ); //@AT
+
+#ifdef NEED_SEM
+
+ result = (ptw32_increase_semaphore( &cv->sema, cv->waiters )
+ ? 0
+ : EINVAL);
+
+#else /* NEED_SEM */
+
+ result = (ReleaseSemaphore( cv->sema, cv->waiters, NULL )
+ ? 0
+ : EINVAL);
+
+#endif /* NEED_SEM */
+
+ }
+
+ (void) pthread_mutex_unlock(&(cv->waitersLock));
+
+ if (wereWaiters && result == 0)
+ {(wereWaiters
+ /*
+ * Wait for all the awakened threads to acquire their part of
+ * the counting semaphore
+ */
+
+ if (WaitForSingleObject (cv->waitersDone, INFINITE)
+ == WAIT_OBJECT_0)
+ {
+ result = 0;
+ }
+ else
+ {
+ result = EINVAL;
+ }
+
+ }
+
+ return (result);
+
+}
+
+BTW, on my system (2 CPUs) I can manage to get
+the program stalled even without any source code
+modification if I run the tennisb program many
+times in different shell sessions.
+
+===================
+pthread-win32 patch
+===================
+struct pthread_cond_t_ {
+ long nWaitersBlocked; /* Number of threads blocked
+*/
+ long nWaitersUnblocked; /* Number of threads unblocked
+*/
+ long nWaitersToUnblock; /* Number of threads to unblock
+*/
+ sem_t semBlockQueue; /* Queue up threads waiting for the
+*/
+ /* condition to become signalled
+*/
+ sem_t semBlockLock; /* Semaphore that guards access to
+*/
+ /* | waiters blocked count/block queue
+*/
+ /* +-> Mandatory Sync.LEVEL-1
+*/
+ pthread_mutex_t mtxUnblockLock; /* Mutex that guards access to
+*/
+ /* | waiters (to)unblock(ed) counts
+*/
+ /* +-> Optional* Sync.LEVEL-2
+*/
+}; /* Opt*) for _timedwait and
+cancellation*/
+
+int
+pthread_cond_init (pthread_cond_t * cond, const pthread_condattr_t * attr)
+ int result = EAGAIN;
+ pthread_cond_t cv = NULL;
+
+ if (cond == NULL)
+ {(cond
+ return EINVAL;
+ }
+
+ if ((attr != NULL && *attr != NULL) &&
+ ((*attr)->pshared == PTHREAD_PROCESS_SHARED))
+ {
+ /*
+ * Creating condition variable that can be shared between
+ * processes.
+ */
+ result = ENOSYS;
+
+ goto FAIL0;
+ }
+
+ cv = (pthread_cond_t) calloc (1, sizeof (*cv));
+
+ if (cv == NULL)
+ {(cv
+ result = ENOMEM;
+ goto FAIL0;
+ }
+
+ cv->nWaitersBlocked = 0;
+ cv->nWaitersUnblocked = 0;
+ cv->nWaitersToUnblock = 0;
+
+ if (sem_init (&(cv->semBlockLock), 0, 1) != 0)
+ {(sem_init
+ goto FAIL0;
+ }
+
+ if (sem_init (&(cv->semBlockQueue), 0, 0) != 0)
+ {(sem_init
+ goto FAIL1;
+ }
+
+ if (pthread_mutex_init (&(cv->mtxUnblockLock), 0) != 0)
+ {(pthread_mutex_init
+ goto FAIL2;
+ }
+
+
+ result = 0;
+
+ goto DONE;
+
+ /*
+ * -------------
+ * Failed...
+ * -------------
+ */
+FAIL2:
+ (void) sem_destroy (&(cv->semBlockQueue));
+
+FAIL1:
+ (void) sem_destroy (&(cv->semBlockLock));
+
+FAIL0:
+DONE:
+ *cond = cv;
+
+ return (result);
+
+} /* pthread_cond_init */
+
+int
+pthread_cond_destroy (pthread_cond_t * cond)
+{
+ int result = 0;
+ pthread_cond_t cv;
+
+ /*
+ * Assuming any race condition here is harmless.
+ */
+ if (cond == NULL
+ || *cond == NULL)
+ {
+ return EINVAL;
+ }
+
+ if (*cond != (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
+ {(*cond
+ cv = *cond;
+
+ /*
+ * Synchronize access to waiters blocked count (LEVEL-1)
+ */
+ if (sem_wait(&(cv->semBlockLock)) != 0)
+ {(sem_wait(&(cv->semBlockLock))
+ return errno;
+ }
+
+ /*
+ * Synchronize access to waiters (to)unblock(ed) counts (LEVEL-2)
+ */
+ if ((result = pthread_mutex_lock(&(cv->mtxUnblockLock))) != 0)
+ {((result
+ (void) sem_post(&(cv->semBlockLock));
+ return result;
+ }
+
+ /*
+ * Check whether cv is still busy (still has waiters blocked)
+ */
+ if (cv->nWaitersBlocked - cv->nWaitersUnblocked > 0)
+ {(cv->nWaitersBlocked
+ (void) sem_post(&(cv->semBlockLock));
+ (void) pthread_mutex_unlock(&(cv->mtxUnblockLock));
+ return EBUSY;
+ }
+
+ /*
+ * Now it is safe to destroy
+ */
+ (void) sem_destroy (&(cv->semBlockLock));
+ (void) sem_destroy (&(cv->semBlockQueue));
+ (void) pthread_mutex_unlock (&(cv->mtxUnblockLock));
+ (void) pthread_mutex_destroy (&(cv->mtxUnblockLock));
+
+ free(cv);
+ *cond = NULL;
+ }
+ else
+ {
+ /*
+ * See notes in ptw32_cond_check_need_init() above also.
+ */
+ EnterCriticalSection(&ptw32_cond_test_init_lock);
+
+ /*
+ * Check again.
+ */
+ if (*cond == (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
+ {(*cond
+ /*
+ * This is all we need to do to destroy a statically
+ * initialised cond that has not yet been used (initialised).
+ * If we get to here, another thread
+ * waiting to initialise this cond will get an EINVAL.
+ */
+ *cond = NULL;
+ }
+ else
+ {
+ /*
+ * The cv has been initialised while we were waiting
+ * so assume it's in use.
+ */
+ result = EBUSY;
+ }
+
+ LeaveCriticalSection(&ptw32_cond_test_init_lock);
+ }
+
+ return (result);
+}
+
+/*
+ * Arguments for cond_wait_cleanup, since we can only pass a
+ * single void * to it.
+ */
+typedef struct {
+ pthread_mutex_t * mutexPtr;
+ pthread_cond_t cv;
+ int * resultPtr;
+} ptw32_cond_wait_cleanup_args_t;
+
+static void
+ptw32_cond_wait_cleanup(void * args)
+{
+ ptw32_cond_wait_cleanup_args_t * cleanup_args =
+(ptw32_cond_wait_cleanup_args_t *) args;
+ pthread_cond_t cv = cleanup_args->cv;
+ int * resultPtr = cleanup_args->resultPtr;
+ int eLastSignal; /* enum: 1=yes 0=no -1=cancelled/timedout w/o signal(s)
+*/
+ int result;
+
+ /*
+ * Whether we got here as a result of signal/broadcast or because of
+ * timeout on wait or thread cancellation we indicate that we are no
+ * longer waiting. The waiter is responsible for adjusting waiters
+ * (to)unblock(ed) counts (protected by unblock lock).
+ * Unblock lock/Sync.LEVEL-2 supports _timedwait and cancellation.
+ */
+ if ((result = pthread_mutex_lock(&(cv->mtxUnblockLock))) != 0)
+ {((result
+ *resultPtr = result;
+ return;
+ }
+
+ cv->nWaitersUnblocked++;
+
+ eLastSignal = (cv->nWaitersToUnblock == 0) ?
+ -1 : (--cv->nWaitersToUnblock == 0);
+
+ /*
+ * No more LEVEL-2 access to waiters (to)unblock(ed) counts needed
+ */
+ if ((result = pthread_mutex_unlock(&(cv->mtxUnblockLock))) != 0)
+ {((result
+ *resultPtr = result;
+ return;
+ }
+
+ /*
+ * If last signal...
+ */
+ if (eLastSignal == 1)
+ {(eLastSignal
+ /*
+ * ...it means that we have end of 'atomic' signal/broadcast
+ */
+ if (sem_post(&(cv->semBlockLock)) != 0)
+ {(sem_post(&(cv->semBlockLock))
+ *resultPtr = errno;
+ return;
+ }
+ }
+ /*
+ * If not last signal and not timed out/cancelled wait w/o signal...
+ */
+ else if (eLastSignal == 0)
+ {
+ /*
+ * ...it means that next waiter can go through semaphore
+ */
+ if (sem_post(&(cv->semBlockQueue)) != 0)
+ {(sem_post(&(cv->semBlockQueue))
+ *resultPtr = errno;
+ return;
+ }
+ }
+
+ /*
+ * XSH: Upon successful return, the mutex has been locked and is owned
+ * by the calling thread
+ */
+ if ((result = pthread_mutex_lock(cleanup_args->mutexPtr)) != 0)
+ {((result
+ *resultPtr = result;
+ }
+
+} /* ptw32_cond_wait_cleanup */
+
+static int
+ptw32_cond_timedwait (pthread_cond_t * cond,
+ pthread_mutex_t * mutex,
+ const struct timespec *abstime)
+{
+ int result = 0;
+ pthread_cond_t cv;
+ ptw32_cond_wait_cleanup_args_t cleanup_args;
+
+ if (cond == NULL || *cond == NULL)
+ {(cond
+ return EINVAL;
+ }
+
+ /*
+ * We do a quick check to see if we need to do more work
+ * to initialise a static condition variable. We check
+ * again inside the guarded section of ptw32_cond_check_need_init()
+ * to avoid race conditions.
+ */
+ if (*cond == (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
+ {(*cond
+ result = ptw32_cond_check_need_init(cond);
+ }
+
+ if (result != 0 && result != EBUSY)
+ {(result
+ return result;
+ }
+
+ cv = *cond;
+
+ /*
+ * Synchronize access to waiters blocked count (LEVEL-1)
+ */
+ if (sem_wait(&(cv->semBlockLock)) != 0)
+ {(sem_wait(&(cv->semBlockLock))
+ return errno;
+ }
+
+ cv->nWaitersBlocked++;
+
+ /*
+ * Thats it. Counted means waiting, no more access needed
+ */
+ if (sem_post(&(cv->semBlockLock)) != 0)
+ {(sem_post(&(cv->semBlockLock))
+ return errno;
+ }
+
+ /*
+ * Setup this waiter cleanup handler
+ */
+ cleanup_args.mutexPtr = mutex;
+ cleanup_args.cv = cv;
+ cleanup_args.resultPtr = &result;
+
+ pthread_cleanup_push (ptw32_cond_wait_cleanup, (void *) &cleanup_args);
+
+ /*
+ * Now we can release 'mutex' and...
+ */
+ if ((result = pthread_mutex_unlock (mutex)) == 0)
+ {((result
+
+ /*
+ * ...wait to be awakened by
+ * pthread_cond_signal, or
+ * pthread_cond_broadcast, or
+ * timeout, or
+ * thread cancellation
+ *
+ * Note:
+ *
+ * ptw32_sem_timedwait is a cancellation point,
+ * hence providing the mechanism for making
+ * pthread_cond_wait a cancellation point.
+ * We use the cleanup mechanism to ensure we
+ * re-lock the mutex and adjust (to)unblock(ed) waiters
+ * counts if we are cancelled, timed out or signalled.
+ */
+ if (ptw32_sem_timedwait (&(cv->semBlockQueue), abstime) != 0)
+ {(ptw32_sem_timedwait
+ result = errno;
+ }
+ }
+
+ /*
+ * Always cleanup
+ */
+ pthread_cleanup_pop (1);
+
+
+ /*
+ * "result" can be modified by the cleanup handler.
+ */
+ return (result);
+
+} /* ptw32_cond_timedwait */
+
+
+static int
+ptw32_cond_unblock (pthread_cond_t * cond,
+ int unblockAll)
+{
+ int result;
+ pthread_cond_t cv;
+
+ if (cond == NULL || *cond == NULL)
+ {(cond
+ return EINVAL;
+ }
+
+ cv = *cond;
+
+ /*
+ * No-op if the CV is static and hasn't been initialised yet.
+ * Assuming that any race condition is harmless.
+ */
+ if (cv == (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
+ {(cv
+ return 0;
+ }
+
+ /*
+ * Synchronize access to waiters blocked count (LEVEL-1)
+ */
+ if (sem_wait(&(cv->semBlockLock)) != 0)
+ {(sem_wait(&(cv->semBlockLock))
+ return errno;
+ }
+
+ /*
+ * Synchronize access to waiters (to)unblock(ed) counts (LEVEL-2)
+ * This sync.level supports _timedwait and cancellation
+ */
+ if ((result = pthread_mutex_lock(&(cv->mtxUnblockLock))) != 0)
+ {((result
+ return result;
+ }
+
+ /*
+ * Adjust waiters blocked and unblocked counts (collect garbage)
+ */
+ if (cv->nWaitersUnblocked != 0)
+ {(cv->nWaitersUnblocked
+ cv->nWaitersBlocked -= cv->nWaitersUnblocked;
+ cv->nWaitersUnblocked = 0;
+ }
+
+ /*
+ * If (after adjustment) there are still some waiters blocked counted...
+ */
+ if ( cv->nWaitersBlocked > 0)
+ {(
+ /*
+ * We will unblock first waiter and leave semBlockLock/LEVEL-1 locked
+ * LEVEL-1 access is left disabled until last signal/unblock
+completes
+ */
+ cv->nWaitersToUnblock = (unblockAll) ? cv->nWaitersBlocked : 1;
+
+ /*
+ * No more LEVEL-2 access to waiters (to)unblock(ed) counts needed
+ * This sync.level supports _timedwait and cancellation
+ */
+ if ((result = pthread_mutex_unlock(&(cv->mtxUnblockLock))) != 0)
+ {((result
+ return result;
+ }
+
+
+ /*
+ * Now, with LEVEL-2 lock released let first waiter go through
+semaphore
+ */
+ if (sem_post(&(cv->semBlockQueue)) != 0)
+ {(sem_post(&(cv->semBlockQueue))
+ return errno;
+ }
+ }
+ /*
+ * No waiter blocked - no more LEVEL-1 access to blocked count needed...
+ */
+ else if (sem_post(&(cv->semBlockLock)) != 0)
+ {
+ return errno;
+ }
+ /*
+ * ...and no more LEVEL-2 access to waiters (to)unblock(ed) counts needed
+too
+ * This sync.level supports _timedwait and cancellation
+ */
+ else
+ {
+ result = pthread_mutex_unlock(&(cv->mtxUnblockLock));
+ }
+
+ return(result);
+
+} /* ptw32_cond_unblock */
+
+int
+pthread_cond_wait (pthread_cond_t * cond,
+ pthread_mutex_t * mutex)
+{
+ /* The NULL abstime arg means INFINITE waiting. */
+ return(ptw32_cond_timedwait(cond, mutex, NULL));
+} /* pthread_cond_wait */
+
+
+int
+pthread_cond_timedwait (pthread_cond_t * cond,
+ pthread_mutex_t * mutex,
+ const struct timespec *abstime)
+{
+ if (abstime == NULL)
+ {(abstime
+ return EINVAL;
+ }
+
+ return(ptw32_cond_timedwait(cond, mutex, abstime));
+} /* pthread_cond_timedwait */
+
+
+int
+pthread_cond_signal (pthread_cond_t * cond)
+{
+ /* The '0'(FALSE) unblockAll arg means unblock ONE waiter. */
+ return(ptw32_cond_unblock(cond, 0));
+} /* pthread_cond_signal */
+
+int
+pthread_cond_broadcast (pthread_cond_t * cond)
+{
+ /* The '1'(TRUE) unblockAll arg means unblock ALL waiters. */
+ return(ptw32_cond_unblock(cond, 1));
+} /* pthread_cond_broadcast */
+
+
+
+
+TEREKHOV@de.ibm.com on 17.01.2001 01:00:57
+
+Please respond to TEREKHOV@de.ibm.com
+
+To: pthreads-win32@sourceware.cygnus.com
+cc: schmidt@uci.edu
+Subject: win32 conditions: sem+counter+event = broadcast_deadlock +
+ spur.wakeup/unfairness/incorrectness ??
+
+
+
+
+
+
+
+Hi,
+
+Problem 1: broadcast_deadlock
+
+It seems that current implementation does not provide "atomic"
+broadcasts. That may lead to "nested" broadcasts... and it seems
+that nested case is not handled correctly -> producing a broadcast
+DEADLOCK as a result.
+
+Scenario:
+
+N (>1) waiting threads W1..N are blocked (in _wait) on condition's
+semaphore.
+
+Thread B1 calls pthread_cond_broadcast, which results in "releasing" N
+W threads via incrementing semaphore counter by N (stored in
+cv->waiters) BUT cv->waiters counter does not change!! The caller
+thread B1 remains blocked on cv->waitersDone event (auto-reset!!) BUT
+condition is not protected from starting another broadcast (when called
+on another thread) while still waiting for the "old" broadcast to
+complete on thread B1.
+
+M (>=0, <N) W threads are fast enough to go thru their _wait call and
+decrement cv->waiters counter.
+
+L (N-M) "late" waiter W threads are a) still blocked/not returned from
+their semaphore wait call or b) were preempted after sem_wait but before
+lock( &cv->waitersLock ) or c) are blocked on cv->waitersLock.
+
+cv->waiters is still > 0 (= L).
+
+Another thread B2 (or some W thread from M group) calls
+pthread_cond_broadcast and gains access to counter... neither a) nor b)
+prevent thread B2 in pthread_cond_broadcast from gaining access to
+counter and starting another broadcast ( for c) - it depends on
+cv->waitersLock scheduling rules: FIFO=OK, PRTY=PROBLEM,... )
+
+That call to pthread_cond_broadcast (on thread B2) will result in
+incrementing semaphore by cv->waiters (=L) which is INCORRECT (all
+W1..N were in fact already released by thread B1) and waiting on
+_auto-reset_ event cv->waitersDone which is DEADLY WRONG (produces a
+deadlock)...
+
+All late W1..L threads now have a chance to complete their _wait call.
+Last W_L thread sets an auto-reselt event cv->waitersDone which will
+release either B1 or B2 leaving one of B threads in a deadlock.
+
+Problem 2: spur.wakeup/unfairness/incorrectness
+
+It seems that:
+
+a) because of the same problem with counter which does not reflect the
+actual number of NOT RELEASED waiters, the signal call may increment
+a semaphore counter w/o having a waiter blocked on it. That will result
+in (best case) spurious wake ups - performance degradation due to
+unnecessary context switches and predicate re-checks and (in worth case)
+unfairness/incorrectness problem - see b)
+
+b) neither signal nor broadcast prevent other threads - "new waiters"
+(and in the case of signal, the caller thread as well) from going into
+_wait and overtaking "old" waiters (already released but still not returned
+from sem_wait on condition's semaphore). Win semaphore just [API DOC]:
+"Maintains a count between zero and some maximum value, limiting the number
+of threads that are simultaneously accessing a shared resource." Calling
+ReleaseSemaphore does not imply (at least not documented) that on return
+from ReleaseSemaphore all waiters will in fact become released (returned
+from their Wait... call) and/or that new waiters calling Wait... afterwards
+will become less importance. It is NOT documented to be an atomic release
+of
+waiters... And even if it would be there is still a problem with a thread
+being preempted after Wait on semaphore and before Wait on cv->waitersLock
+and scheduling rules for cv->waitersLock itself
+(??WaitForMultipleObjects??)
+That may result in unfairness/incorrectness problem as described
+for SetEvent impl. in "Strategies for Implementing POSIX Condition
+Variables
+on Win32": http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
+
+Unfairness -- The semantics of the POSIX pthread_cond_broadcast function is
+to wake up all threads currently blocked in wait calls on the condition
+variable. The awakened threads then compete for the external_mutex. To
+ensure
+fairness, all of these threads should be released from their
+pthread_cond_wait calls and allowed to recheck their condition expressions
+before other threads can successfully complete a wait on the condition
+variable.
+
+Unfortunately, the SetEvent implementation above does not guarantee that
+all
+threads sleeping on the condition variable when cond_broadcast is called
+will
+acquire the external_mutex and check their condition expressions. Although
+the Pthreads specification does not mandate this degree of fairness, the
+lack of fairness can cause starvation.
+
+To illustrate the unfairness problem, imagine there are 2 threads, C1 and
+C2,
+that are blocked in pthread_cond_wait on condition variable not_empty_ that
+is guarding a thread-safe message queue. Another thread, P1 then places two
+messages onto the queue and calls pthread_cond_broadcast. If C1 returns
+from
+pthread_cond_wait, dequeues and processes the message, and immediately
+waits
+again then it and only it may end up acquiring both messages. Thus, C2 will
+never get a chance to dequeue a message and run.
+
+The following illustrates the sequence of events:
+
+1. Thread C1 attempts to dequeue and waits on CV non_empty_
+2. Thread C2 attempts to dequeue and waits on CV non_empty_
+3. Thread P1 enqueues 2 messages and broadcasts to CV not_empty_
+4. Thread P1 exits
+5. Thread C1 wakes up from CV not_empty_, dequeues a message and runs
+6. Thread C1 waits again on CV not_empty_, immediately dequeues the 2nd
+ message and runs
+7. Thread C1 exits
+8. Thread C2 is the only thread left and blocks forever since
+ not_empty_ will never be signaled
+
+Depending on the algorithm being implemented, this lack of fairness may
+yield
+concurrent programs that have subtle bugs. Of course, application
+developers
+should not rely on the fairness semantics of pthread_cond_broadcast.
+However,
+there are many cases where fair implementations of condition variables can
+simplify application code.
+
+Incorrectness -- A variation on the unfairness problem described above
+occurs
+when a third consumer thread, C3, is allowed to slip through even though it
+was not waiting on condition variable not_empty_ when a broadcast occurred.
+
+To illustrate this, we will use the same scenario as above: 2 threads, C1
+and
+C2, are blocked dequeuing messages from the message queue. Another thread,
+P1
+then places two messages onto the queue and calls pthread_cond_broadcast.
+C1
+returns from pthread_cond_wait, dequeues and processes the message. At this
+time, C3 acquires the external_mutex, calls pthread_cond_wait and waits on
+the events in WaitForMultipleObjects. Since C2 has not had a chance to run
+yet, the BROADCAST event is still signaled. C3 then returns from
+WaitForMultipleObjects, and dequeues and processes the message in the
+queue.
+Thus, C2 will never get a chance to dequeue a message and run.
+
+The following illustrates the sequence of events:
+
+1. Thread C1 attempts to dequeue and waits on CV non_empty_
+2. Thread C2 attempts to dequeue and waits on CV non_empty_
+3. Thread P1 enqueues 2 messages and broadcasts to CV not_empty_
+4. Thread P1 exits
+5. Thread C1 wakes up from CV not_empty_, dequeues a message and runs
+6. Thread C1 exits
+7. Thread C3 waits on CV not_empty_, immediately dequeues the 2nd
+ message and runs
+8. Thread C3 exits
+9. Thread C2 is the only thread left and blocks forever since
+ not_empty_ will never be signaled
+
+In the above case, a thread that was not waiting on the condition variable
+when a broadcast occurred was allowed to proceed. This leads to incorrect
+semantics for a condition variable.
+
+
+COMMENTS???
+
+regards,
+alexander.
+
+-----------------------------------------------------------------------------
+
+Subject: RE: FYI/comp.programming.threads/Re: pthread_cond_*
+ implementation questions
+Date: Wed, 21 Feb 2001 11:54:47 +0100
+From: TEREKHOV@de.ibm.com
+To: lthomas@arbitrade.com
+CC: rpj@ise.canberra.edu.au, Thomas Pfaff <tpfaff@gmx.net>,
+ Nanbor Wang <nanbor@cs.wustl.edu>
+
+Hi Louis,
+
+generation number 8..
+
+had some time to revisit timeouts/spurious wakeup problem..
+found some bugs (in 7.b/c/d) and something to improve
+(7a - using IPC semaphores but it should speedup Win32
+version as well).
+
+regards,
+alexander.
+
+---------- Algorithm 8a / IMPL_SEM,UNBLOCK_STRATEGY == UNBLOCK_ALL ------
+given:
+semBlockLock - bin.semaphore
+semBlockQueue - semaphore
+mtxExternal - mutex or CS
+mtxUnblockLock - mutex or CS
+nWaitersGone - int
+nWaitersBlocked - int
+nWaitersToUnblock - int
+
+wait( timeout ) {
+
+ [auto: register int result ] // error checking omitted
+ [auto: register int nSignalsWasLeft ]
+ [auto: register int nWaitersWasGone ]
+
+ sem_wait( semBlockLock );
+ nWaitersBlocked++;
+ sem_post( semBlockLock );
+
+ unlock( mtxExternal );
+ bTimedOut = sem_wait( semBlockQueue,timeout );
+
+ lock( mtxUnblockLock );
+ if ( 0 != (nSignalsWasLeft = nWaitersToUnblock) ) {
+ if ( bTimeout ) { // timeout (or canceled)
+ if ( 0 != nWaitersBlocked ) {
+ nWaitersBlocked--;
+ }
+ else {
+ nWaitersGone++; // count spurious wakeups
+ }
+ }
+ if ( 0 == --nWaitersToUnblock ) {
+ if ( 0 != nWaitersBlocked ) {
+ sem_post( semBlockLock ); // open the gate
+ nSignalsWasLeft = 0; // do not open the gate below
+again
+ }
+ else if ( 0 != (nWaitersWasGone = nWaitersGone) ) {
+ nWaitersGone = 0;
+ }
+ }
+ }
+ else if ( INT_MAX/2 == ++nWaitersGone ) { // timeout/canceled or spurious
+semaphore :-)
+ sem_wait( semBlockLock );
+ nWaitersBlocked -= nWaitersGone; // something is going on here -
+test of timeouts? :-)
+ sem_post( semBlockLock );
+ nWaitersGone = 0;
+ }
+ unlock( mtxUnblockLock );
+
+ if ( 1 == nSignalsWasLeft ) {
+ if ( 0 != nWaitersWasGone ) {
+ // sem_adjust( -nWaitersWasGone );
+ while ( nWaitersWasGone-- ) {
+ sem_wait( semBlockLock ); // better now than spurious
+later
+ }
+ }
+ sem_post( semBlockLock ); // open the gate
+ }
+
+ lock( mtxExternal );
+
+ return ( bTimedOut ) ? ETIMEOUT : 0;
+}
+
+signal(bAll) {
+
+ [auto: register int result ]
+ [auto: register int nSignalsToIssue]
+
+ lock( mtxUnblockLock );
+
+ if ( 0 != nWaitersToUnblock ) { // the gate is closed!!!
+ if ( 0 == nWaitersBlocked ) { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+ if (bAll) {
+ nWaitersToUnblock += nSignalsToIssue=nWaitersBlocked;
+ nWaitersBlocked = 0;
+ }
+ else {
+ nSignalsToIssue = 1;
+ nWaitersToUnblock++;
+ nWaitersBlocked--;
+ }
+ }
+ else if ( nWaitersBlocked > nWaitersGone ) { // HARMLESS RACE CONDITION!
+ sem_wait( semBlockLock ); // close the gate
+ if ( 0 != nWaitersGone ) {
+ nWaitersBlocked -= nWaitersGone;
+ nWaitersGone = 0;
+ }
+ if (bAll) {
+ nSignalsToIssue = nWaitersToUnblock = nWaitersBlocked;
+ nWaitersBlocked = 0;
+ }
+ else {
+ nSignalsToIssue = nWaitersToUnblock = 1;
+ nWaitersBlocked--;
+ }
+ }
+ else { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+
+ unlock( mtxUnblockLock );
+ sem_post( semBlockQueue,nSignalsToIssue );
+ return result;
+}
+
+---------- Algorithm 8b / IMPL_SEM,UNBLOCK_STRATEGY == UNBLOCK_ONEBYONE
+------
+given:
+semBlockLock - bin.semaphore
+semBlockQueue - bin.semaphore
+mtxExternal - mutex or CS
+mtxUnblockLock - mutex or CS
+nWaitersGone - int
+nWaitersBlocked - int
+nWaitersToUnblock - int
+
+wait( timeout ) {
+
+ [auto: register int result ] // error checking omitted
+ [auto: register int nWaitersWasGone ]
+ [auto: register int nSignalsWasLeft ]
+
+ sem_wait( semBlockLock );
+ nWaitersBlocked++;
+ sem_post( semBlockLock );
+
+ unlock( mtxExternal );
+ bTimedOut = sem_wait( semBlockQueue,timeout );
+
+ lock( mtxUnblockLock );
+ if ( 0 != (nSignalsWasLeft = nWaitersToUnblock) ) {
+ if ( bTimeout ) { // timeout (or canceled)
+ if ( 0 != nWaitersBlocked ) {
+ nWaitersBlocked--;
+ nSignalsWasLeft = 0; // do not unblock next waiter
+below (already unblocked)
+ }
+ else {
+ nWaitersGone = 1; // spurious wakeup pending!!
+ }
+ }
+ if ( 0 == --nWaitersToUnblock &&
+ if ( 0 != nWaitersBlocked ) {
+ sem_post( semBlockLock ); // open the gate
+ nSignalsWasLeft = 0; // do not open the gate below
+again
+ }
+ else if ( 0 != (nWaitersWasGone = nWaitersGone) ) {
+ nWaitersGone = 0;
+ }
+ }
+ }
+ else if ( INT_MAX/2 == ++nWaitersGone ) { // timeout/canceled or spurious
+semaphore :-)
+ sem_wait( semBlockLock );
+ nWaitersBlocked -= nWaitersGone; // something is going on here -
+test of timeouts? :-)
+ sem_post( semBlockLock );
+ nWaitersGone = 0;
+ }
+ unlock( mtxUnblockLock );
+
+ if ( 1 == nSignalsWasLeft ) {
+ if ( 0 != nWaitersWasGone ) {
+ // sem_adjust( -1 );
+ sem_wait( semBlockQueue ); // better now than spurious
+later
+ }
+ sem_post( semBlockLock ); // open the gate
+ }
+ else if ( 0 != nSignalsWasLeft ) {
+ sem_post( semBlockQueue ); // unblock next waiter
+ }
+
+ lock( mtxExternal );
+
+ return ( bTimedOut ) ? ETIMEOUT : 0;
+}
+
+signal(bAll) {
+
+ [auto: register int result ]
+
+ lock( mtxUnblockLock );
+
+ if ( 0 != nWaitersToUnblock ) { // the gate is closed!!!
+ if ( 0 == nWaitersBlocked ) { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+ if (bAll) {
+ nWaitersToUnblock += nWaitersBlocked;
+ nWaitersBlocked = 0;
+ }
+ else {
+ nWaitersToUnblock++;
+ nWaitersBlocked--;
+ }
+ unlock( mtxUnblockLock );
+ }
+ else if ( nWaitersBlocked > nWaitersGone ) { // HARMLESS RACE CONDITION!
+ sem_wait( semBlockLock ); // close the gate
+ if ( 0 != nWaitersGone ) {
+ nWaitersBlocked -= nWaitersGone;
+ nWaitersGone = 0;
+ }
+ if (bAll) {
+ nWaitersToUnblock = nWaitersBlocked;
+ nWaitersBlocked = 0;
+ }
+ else {
+ nWaitersToUnblock = 1;
+ nWaitersBlocked--;
+ }
+ unlock( mtxUnblockLock );
+ sem_post( semBlockQueue );
+ }
+ else { // NO-OP
+ unlock( mtxUnblockLock );
+ }
+
+ return result;
+}
+
+---------- Algorithm 8c / IMPL_EVENT,UNBLOCK_STRATEGY == UNBLOCK_ONEBYONE
+---------
+given:
+hevBlockLock - auto-reset event
+hevBlockQueue - auto-reset event
+mtxExternal - mutex or CS
+mtxUnblockLock - mutex or CS
+nWaitersGone - int
+nWaitersBlocked - int
+nWaitersToUnblock - int
+
+wait( timeout ) {
+
+ [auto: register int result ] // error checking omitted
+ [auto: register int nSignalsWasLeft ]
+ [auto: register int nWaitersWasGone ]
+
+ wait( hevBlockLock,INFINITE );
+ nWaitersBlocked++;
+ set_event( hevBlockLock );
+
+ unlock( mtxExternal );
+ bTimedOut = wait( hevBlockQueue,timeout );
+
+ lock( mtxUnblockLock );
+ if ( 0 != (SignalsWasLeft = nWaitersToUnblock) ) {
+ if ( bTimeout ) { // timeout (or canceled)
+ if ( 0 != nWaitersBlocked ) {
+ nWaitersBlocked--;
+ nSignalsWasLeft = 0; // do not unblock next waiter
+below (already unblocked)
+ }
+ else {
+ nWaitersGone = 1; // spurious wakeup pending!!
+ }
+ }
+ if ( 0 == --nWaitersToUnblock )
+ if ( 0 != nWaitersBlocked ) {
+ set_event( hevBlockLock ); // open the gate
+ nSignalsWasLeft = 0; // do not open the gate below
+again
+ }
+ else if ( 0 != (nWaitersWasGone = nWaitersGone) ) {
+ nWaitersGone = 0;
+ }
+ }
+ }
+ else if ( INT_MAX/2 == ++nWaitersGone ) { // timeout/canceled or spurious
+event :-)
+ wait( hevBlockLock,INFINITE );
+ nWaitersBlocked -= nWaitersGone; // something is going on here -
+test of timeouts? :-)
+ set_event( hevBlockLock );
+ nWaitersGone = 0;
+ }
+ unlock( mtxUnblockLock );
+
+ if ( 1 == nSignalsWasLeft ) {
+ if ( 0 != nWaitersWasGone ) {
+ reset_event( hevBlockQueue ); // better now than spurious
+later
+ }
+ set_event( hevBlockLock ); // open the gate
+ }
+ else if ( 0 != nSignalsWasLeft ) {
+ set_event( hevBlockQueue ); // unblock next waiter
+ }
+
+ lock( mtxExternal );
+
+ return ( bTimedOut ) ? ETIMEOUT : 0;
+}
+
+signal(bAll) {
+
+ [auto: register int result ]
+
+ lock( mtxUnblockLock );
+
+ if ( 0 != nWaitersToUnblock ) { // the gate is closed!!!
+ if ( 0 == nWaitersBlocked ) { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+ if (bAll) {
+ nWaitersToUnblock += nWaitersBlocked;
+ nWaitersBlocked = 0;
+ }
+ else {
+ nWaitersToUnblock++;
+ nWaitersBlocked--;
+ }
+ unlock( mtxUnblockLock );
+ }
+ else if ( nWaitersBlocked > nWaitersGone ) { // HARMLESS RACE CONDITION!
+ wait( hevBlockLock,INFINITE ); // close the gate
+ if ( 0 != nWaitersGone ) {
+ nWaitersBlocked -= nWaitersGone;
+ nWaitersGone = 0;
+ }
+ if (bAll) {
+ nWaitersToUnblock = nWaitersBlocked;
+ nWaitersBlocked = 0;
+ }
+ else {
+ nWaitersToUnblock = 1;
+ nWaitersBlocked--;
+ }
+ unlock( mtxUnblockLock );
+ set_event( hevBlockQueue );
+ }
+ else { // NO-OP
+ unlock( mtxUnblockLock );
+ }
+
+ return result;
+}
+
+---------- Algorithm 8d / IMPL_EVENT,UNBLOCK_STRATEGY == UNBLOCK_ALL ------
+given:
+hevBlockLock - auto-reset event
+hevBlockQueueS - auto-reset event // for signals
+hevBlockQueueB - manual-reset even // for broadcasts
+mtxExternal - mutex or CS
+mtxUnblockLock - mutex or CS
+eBroadcast - int // 0: no broadcast, 1: broadcast, 2:
+broadcast after signal(s)
+nWaitersGone - int
+nWaitersBlocked - int
+nWaitersToUnblock - int
+
+wait( timeout ) {
+
+ [auto: register int result ] // error checking omitted
+ [auto: register int eWasBroadcast ]
+ [auto: register int nSignalsWasLeft ]
+ [auto: register int nWaitersWasGone ]
+
+ wait( hevBlockLock,INFINITE );
+ nWaitersBlocked++;
+ set_event( hevBlockLock );
+
+ unlock( mtxExternal );
+ bTimedOut = waitformultiple( hevBlockQueueS,hevBlockQueueB,timeout,ONE );
+
+ lock( mtxUnblockLock );
+ if ( 0 != (SignalsWasLeft = nWaitersToUnblock) ) {
+ if ( bTimeout ) { // timeout (or canceled)
+ if ( 0 != nWaitersBlocked ) {
+ nWaitersBlocked--;
+ nSignalsWasLeft = 0; // do not unblock next waiter
+below (already unblocked)
+ }
+ else if ( 1 != eBroadcast ) {
+ nWaitersGone = 1;
+ }
+ }
+ if ( 0 == --nWaitersToUnblock ) {
+ if ( 0 != nWaitersBlocked ) {
+ set_event( hevBlockLock ); // open the gate
+ nSignalsWasLeft = 0; // do not open the gate below
+again
+ }
+ else {
+ if ( 0 != (eWasBroadcast = eBroadcast) ) {
+ eBroadcast = 0;
+ }
+ if ( 0 != (nWaitersWasGone = nWaitersGone ) {
+ nWaitersGone = 0;
+ }
+ }
+ }
+ else if ( 0 != eBroadcast ) {
+ nSignalsWasLeft = 0; // do not unblock next waiter
+below (already unblocked)
+ }
+ }
+ else if ( INT_MAX/2 == ++nWaitersGone ) { // timeout/canceled or spurious
+event :-)
+ wait( hevBlockLock,INFINITE );
+ nWaitersBlocked -= nWaitersGone; // something is going on here -
+test of timeouts? :-)
+ set_event( hevBlockLock );
+ nWaitersGone = 0;
+ }
+ unlock( mtxUnblockLock );
+
+ if ( 1 == nSignalsWasLeft ) {
+ if ( 0 != eWasBroadcast ) {
+ reset_event( hevBlockQueueB );
+ }
+ if ( 0 != nWaitersWasGone ) {
+ reset_event( hevBlockQueueS ); // better now than spurious
+later
+ }
+ set_event( hevBlockLock ); // open the gate
+ }
+ else if ( 0 != nSignalsWasLeft ) {
+ set_event( hevBlockQueueS ); // unblock next waiter
+ }
+
+ lock( mtxExternal );
+
+ return ( bTimedOut ) ? ETIMEOUT : 0;
+}
+
+signal(bAll) {
+
+ [auto: register int result ]
+ [auto: register HANDLE hevBlockQueue ]
+
+ lock( mtxUnblockLock );
+
+ if ( 0 != nWaitersToUnblock ) { // the gate is closed!!!
+ if ( 0 == nWaitersBlocked ) { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+ if (bAll) {
+ nWaitersToUnblock += nWaitersBlocked;
+ nWaitersBlocked = 0;
+ eBroadcast = 2;
+ hevBlockQueue = hevBlockQueueB;
+ }
+ else {
+ nWaitersToUnblock++;
+ nWaitersBlocked--;
+ return unlock( mtxUnblockLock );
+ }
+ }
+ else if ( nWaitersBlocked > nWaitersGone ) { // HARMLESS RACE CONDITION!
+ wait( hevBlockLock,INFINITE ); // close the gate
+ if ( 0 != nWaitersGone ) {
+ nWaitersBlocked -= nWaitersGone;
+ nWaitersGone = 0;
+ }
+ if (bAll) {
+ nWaitersToUnblock = nWaitersBlocked;
+ nWaitersBlocked = 0;
+ eBroadcast = 1;
+ hevBlockQueue = hevBlockQueueB;
+ }
+ else {
+ nWaitersToUnblock = 1;
+ nWaitersBlocked--;
+ hevBlockQueue = hevBlockQueueS;
+ }
+ }
+ else { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+
+ unlock( mtxUnblockLock );
+ set_event( hevBlockQueue );
+ return result;
+}
+---------------------- Forwarded by Alexander Terekhov/Germany/IBM on
+02/21/2001 09:13 AM ---------------------------
+
+Alexander Terekhov
+02/20/2001 04:33 PM
+
+To: Louis Thomas <lthomas@arbitrade.com>
+cc:
+
+From: Alexander Terekhov/Germany/IBM@IBMDE
+Subject: RE: FYI/comp.programming.threads/Re: pthread_cond_* implementatio
+ n questions
+Importance: Normal
+
+>Sorry, gotta take a break and work on something else for a while.
+>Real work
+>calls, unfortunately. I'll get back to you in two or three days.
+
+ok. no problem. here is some more stuff for pauses you might have
+in between :)
+
+---------- Algorithm 7d / IMPL_EVENT,UNBLOCK_STRATEGY == UNBLOCK_ALL ------
+given:
+hevBlockLock - auto-reset event
+hevBlockQueueS - auto-reset event // for signals
+hevBlockQueueB - manual-reset even // for broadcasts
+mtxExternal - mutex or CS
+mtxUnblockLock - mutex or CS
+bBroadcast - int
+nWaitersGone - int
+nWaitersBlocked - int
+nWaitersToUnblock - int
+
+wait( timeout ) {
+
+ [auto: register int result ] // error checking omitted
+ [auto: register int bWasBroadcast ]
+ [auto: register int nSignalsWasLeft ]
+
+ wait( hevBlockLock,INFINITE );
+ nWaitersBlocked++;
+ set_event( hevBlockLock );
+
+ unlock( mtxExternal );
+ bTimedOut = waitformultiple( hevBlockQueueS,hevBlockQueueB,timeout,ONE );
+
+ lock( mtxUnblockLock );
+ if ( 0 != (SignalsWasLeft = nWaitersToUnblock) ) {
+ if ( bTimeout ) { // timeout (or canceled)
+ if ( 0 != nWaitersBlocked ) {
+ nWaitersBlocked--;
+ nSignalsWasLeft = 0; // do not unblock next waiter
+below (already unblocked)
+ }
+ else if ( !bBroadcast ) {
+ wait( hevBlockQueueS,INFINITE ); // better now than spurious
+later
+ }
+ }
+ if ( 0 == --nWaitersToUnblock ) {
+ if ( 0 != nWaitersBlocked ) {
+ if ( bBroadcast ) {
+ reset_event( hevBlockQueueB );
+ bBroadcast = false;
+ }
+ set_event( hevBlockLock ); // open the gate
+ nSignalsWasLeft = 0; // do not open the gate below
+again
+ }
+ else if ( false != (bWasBroadcast = bBroadcast) ) {
+ bBroadcast = false;
+ }
+ }
+ else {
+ bWasBroadcast = bBroadcast;
+ }
+ }
+ else if ( INT_MAX/2 == ++nWaitersGone ) { // timeout/canceled or spurious
+event :-)
+ wait( hevBlockLock,INFINITE );
+ nWaitersBlocked -= nWaitersGone; // something is going on here -
+test of timeouts? :-)
+ set_event( hevBlockLock );
+ nWaitersGone = 0;
+ }
+ unlock( mtxUnblockLock );
+
+ if ( 1 == nSignalsWasLeft ) {
+ if ( bWasBroadcast ) {
+ reset_event( hevBlockQueueB );
+ }
+ set_event( hevBlockLock ); // open the gate
+ }
+ else if ( 0 != nSignalsWasLeft && !bWasBroadcast ) {
+ set_event( hevBlockQueueS ); // unblock next waiter
+ }
+
+ lock( mtxExternal );
+
+ return ( bTimedOut ) ? ETIMEOUT : 0;
+}
+
+signal(bAll) {
+
+ [auto: register int result ]
+ [auto: register HANDLE hevBlockQueue ]
+
+ lock( mtxUnblockLock );
+
+ if ( 0 != nWaitersToUnblock ) { // the gate is closed!!!
+ if ( 0 == nWaitersBlocked ) { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+ if (bAll) {
+ nWaitersToUnblock += nWaitersBlocked;
+ nWaitersBlocked = 0;
+ bBroadcast = true;
+ hevBlockQueue = hevBlockQueueB;
+ }
+ else {
+ nWaitersToUnblock++;
+ nWaitersBlocked--;
+ return unlock( mtxUnblockLock );
+ }
+ }
+ else if ( nWaitersBlocked > nWaitersGone ) { // HARMLESS RACE CONDITION!
+ wait( hevBlockLock,INFINITE ); // close the gate
+ if ( 0 != nWaitersGone ) {
+ nWaitersBlocked -= nWaitersGone;
+ nWaitersGone = 0;
+ }
+ if (bAll) {
+ nWaitersToUnblock = nWaitersBlocked;
+ nWaitersBlocked = 0;
+ bBroadcast = true;
+ hevBlockQueue = hevBlockQueueB;
+ }
+ else {
+ nWaitersToUnblock = 1;
+ nWaitersBlocked--;
+ hevBlockQueue = hevBlockQueueS;
+ }
+ }
+ else { // NO-OP
+ return unlock( mtxUnblockLock );
+ }
+
+ unlock( mtxUnblockLock );
+ set_event( hevBlockQueue );
+ return result;
+}
+
+
+----------------------------------------------------------------------------
+
+Subject: RE: FYI/comp.programming.threads/Re: pthread_cond_* implementatio
+ n questions
+Date: Mon, 26 Feb 2001 22:20:12 -0600
+From: Louis Thomas <lthomas@arbitrade.com>
+To: "'TEREKHOV@de.ibm.com'" <TEREKHOV@de.ibm.com>
+CC: rpj@ise.canberra.edu.au, Thomas Pfaff <tpfaff@gmx.net>,
+ Nanbor Wang
+ <nanbor@cs.wustl.edu>
+
+Sorry all. Busy week.
+
+> this insures the fairness
+> which POSIX does not (e.g. two subsequent broadcasts - the gate does
+insure
+> that first wave waiters will start the race for the mutex before waiters
+> from the second wave - Linux pthreads process/unblock both waves
+> concurrently...)
+
+I'm not sure how we are any more fair about this than Linux. We certainly
+don't guarantee that the threads released by the first broadcast will get
+the external mutex before the threads of the second wave. In fact, it is
+possible that those threads will never get the external mutex if there is
+enough contention for it.
+
+> e.g. i was thinking about implementation with a pool of
+> N semaphores/counters [...]
+
+I considered that too. The problem is as you mentioned in a). You really
+need to assign threads to semaphores once you know how you want to wake them
+up, not when they first begin waiting which is the only time you can assign
+them.
+
+> well, i am not quite sure that i've fully understood your scenario,
+
+Hmm. Well, it think it's an important example, so I'll try again. First, we
+have thread A which we KNOW is waiting on a condition. As soon as it becomes
+unblocked for any reason, we will know because it will set a flag. Since the
+flag is not set, we are 100% confident that thread A is waiting on the
+condition. We have another thread, thread B, which has acquired the mutex
+and is about to wait on the condition. Thus it is pretty clear that at any
+point, either just A is waiting, or A and B are waiting. Now thread C comes
+along. C is about to do a broadcast on the condition. A broadcast is
+guaranteed to unblock all threads currently waiting on a condition, right?
+Again, we said that either just A is waiting, or A and B are both waiting.
+So, when C does its broadcast, depending upon whether B has started waiting
+or not, thread C will unblock A or unblock A and B. Either way, C must
+unblock A, right?
+
+Now, you said anything that happens is correct so long as a) "a signal is
+not lost between unlocking the mutex and waiting on the condition" and b) "a
+thread must not steal a signal it sent", correct? Requirement b) is easy to
+satisfy: in this scenario, thread C will never wait on the condition, so it
+won't steal any signals. Requirement a) is not hard either. The only way we
+could fail to meet requirement a) in this scenario is if thread B was
+started waiting but didn't wake up because a signal was lost. This will not
+happen.
+
+Now, here is what happens. Assume thread C beats thread B. Thread C looks to
+see how many threads are waiting on the condition. Thread C sees just one
+thread, thread A, waiting. It does a broadcast waking up just one thread
+because just one thread is waiting. Next, before A can become unblocked,
+thread B begins waiting. Now there are two threads waiting, but only one
+will be unblocked. Suppose B wins. B will become unblocked. A will not
+become unblocked, because C only unblocked one thread (sema_post cond, 1).
+So at the end, B finishes and A remains blocked.
+
+We have met both of your requirements, so by your rules, this is an
+acceptable outcome. However, I think that the spec says this is an
+unacceptable outcome! We know for certain that A was waiting and that C did
+a broadcast, but A did not become unblocked! Yet, the spec says that a
+broadcast wakes up all waiting threads. This did not happen. Do you agree
+that this shows your rules are not strict enough?
+
+> and what about N2? :) this one does allow almost everything.
+
+Don't get me started about rule #2. I'll NEVER advocate an algorithm that
+uses rule 2 as an excuse to suck!
+
+> but it is done (decrement)under mutex protection - this is not a subject
+> of a race condition.
+
+You are correct. My mistake.
+
+> i would remove "_bTimedOut=false".. after all, it was a real timeout..
+
+I disagree. A thread that can't successfully retract its waiter status can't
+really have timed out. If a thread can't return without executing extra code
+to deal with the fact that someone tried to unblock it, I think it is a poor
+idea to pretend we
+didn't realize someone was trying to signal us. After all, a signal is more
+important than a time out.
+
+> when nSignaled != 0, it is possible to update nWaiters (--) and do not
+> touch nGone
+
+I realize this, but I was thinking that writing it the other ways saves
+another if statement.
+
+> adjust only if nGone != 0 and save one cache memory write - probably much
+slower than 'if'
+
+Hmm. You are probably right.
+
+> well, in a strange (e.g. timeout test) program you may (theoretically)
+> have an overflow of nWaiters/nGone counters (with waiters repeatedly
+timing
+> out and no signals at all).
+
+Also true. Not only that, but you also have the possibility that one could
+overflow the number of waiters as well! However, considering the limit you
+have chosen for nWaitersGone, I suppose it is unlikely that anyone would be
+able to get INT_MAX/2 threads waiting on a single condition. :)
+
+Analysis of 8a:
+
+It looks correct to me.
+
+What are IPC semaphores?
+
+In the line where you state, "else if ( nWaitersBlocked > nWaitersGone ) {
+// HARMLESS RACE CONDITION!" there is no race condition for nWaitersGone
+because nWaitersGone is never modified without holding mtxUnblockLock. You
+are correct that there is a harmless race on nWaitersBlocked, which can
+increase and make the condition become true just after we check it. If this
+happens, we interpret it as the wait starting after the signal.
+
+I like your optimization of this. You could improve Alg. 6 as follows:
+---------- Algorithm 6b ----------
+signal(bAll) {
+ _nSig=0
+ lock counters
+ // this is safe because nWaiting can only be decremented by a thread that
+ // owns counters and nGone can only be changed by a thread that owns
+counters.
+ if (nWaiting>nGone) {
+ if (0==nSignaled) {
+ sema_wait gate // close gate if not already closed
+ }
+ if (nGone>0) {
+ nWaiting-=nGone
+ nGone=0
+ }
+ _nSig=bAll?nWaiting:1
+ nSignaled+=_nSig
+ nWaiting-=_nSig
+ }
+ unlock counters
+ if (0!=_nSig) {
+ sema_post queue, _nSig
+ }
+}
+---------- ---------- ----------
+I guess this wouldn't apply to Alg 8a because nWaitersGone changes meanings
+depending upon whether the gate is open or closed.
+
+In the loop "while ( nWaitersWasGone-- ) {" you do a sema_wait on
+semBlockLock. Perhaps waiting on semBlockQueue would be a better idea.
+
+What have you gained by making the last thread to be signaled do the waits
+for all the timed out threads, besides added complexity? It took me a long
+time to figure out what your objective was with this, to realize you were
+using nWaitersGone to mean two different things, and to verify that you
+hadn't introduced any bug by doing this. Even now I'm not 100% sure.
+
+What has all this playing about with nWaitersGone really gained us besides a
+lot of complexity (it is much harder to verify that this solution is
+correct), execution overhead (we now have a lot more if statements to
+evaluate), and space overhead (more space for the extra code, and another
+integer in our data)? We did manage to save a lock/unlock pair in an
+uncommon case (when a time out occurs) at the above mentioned expenses in
+the common cases.
+
+As for 8b, c, and d, they look ok though I haven't studied them thoroughly.
+What would you use them for?
+
+ Later,
+ -Louis! :)
+
+-----------------------------------------------------------------------------
+
+Subject: RE: FYI/comp.programming.threads/Re: pthread_cond_* implementatio
+ n questions
+Date: Tue, 27 Feb 2001 15:51:28 +0100
+From: TEREKHOV@de.ibm.com
+To: Louis Thomas <lthomas@arbitrade.com>
+CC: rpj@ise.canberra.edu.au, Thomas Pfaff <tpfaff@gmx.net>,
+ Nanbor Wang <nanbor@cs.wustl.edu>
+
+Hi Louis,
+
+>> that first wave waiters will start the race for the mutex before waiters
+>> from the second wave - Linux pthreads process/unblock both waves
+>> concurrently...)
+>
+>I'm not sure how we are any more fair about this than Linux. We certainly
+>don't guarantee that the threads released by the first broadcast will get
+>the external mutex before the threads of the second wave. In fact, it is
+>possible that those threads will never get the external mutex if there is
+>enough contention for it.
+
+correct. but gate is nevertheless more fair than Linux because of the
+barrier it establishes between two races (1st and 2nd wave waiters) for
+the mutex which under 'normal' circumstances (e.g. all threads of equal
+priorities,..) will 'probably' result in fair behaviour with respect to
+mutex ownership.
+
+>> well, i am not quite sure that i've fully understood your scenario,
+>
+>Hmm. Well, it think it's an important example, so I'll try again. ...
+
+ok. now i seem to understand this example. well, now it seems to me
+that the only meaningful rule is just:
+
+a) "a signal is not lost between unlocking the mutex and waiting on the
+condition"
+
+and that the rule
+
+b) "a thread must not steal a signal it sent"
+
+is not needed at all because a thread which violates b) also violates a).
+
+i'll try to explain..
+
+i think that the most important thing is how POSIX defines waiter's
+visibility:
+
+"if another thread is able to acquire the mutex after the about-to-block
+thread
+has released it, then a subsequent call to pthread_cond_signal() or
+pthread_cond_broadcast() in that thread behaves as if it were issued after
+the about-to-block thread has blocked. "
+
+my understanding is the following:
+
+1) there is no guarantees whatsoever with respect to whether
+signal/broadcast
+will actually unblock any 'waiter' if it is done w/o acquiring the mutex
+first
+(note that a thread may release it before signal/broadcast - it does not
+matter).
+
+2) it is guaranteed that waiters become 'visible' - eligible for unblock as
+soon
+as signalling thread acquires the mutex (but not before!!)
+
+so..
+
+>So, when C does its broadcast, depending upon whether B has started
+waiting
+>or not, thread C will unblock A or unblock A and B. Either way, C must
+>unblock A, right?
+
+right. but only if C did acquire the mutex prior to broadcast (it may
+release it before broadcast as well).
+
+implementation will violate waiters visibility rule (signal will become
+lost)
+if C will not unblock A.
+
+>Now, here is what happens. Assume thread C beats thread B. Thread C looks
+to
+>see how many threads are waiting on the condition. Thread C sees just one
+>thread, thread A, waiting. It does a broadcast waking up just one thread
+>because just one thread is waiting. Next, before A can become unblocked,
+>thread B begins waiting. Now there are two threads waiting, but only one
+>will be unblocked. Suppose B wins. B will become unblocked. A will not
+>become unblocked, because C only unblocked one thread (sema_post cond, 1).
+>So at the end, B finishes and A remains blocked.
+
+thread C did acquire the mutex ("Thread C sees just one thread, thread A,
+waiting"). beginning from that moment it is guaranteed that subsequent
+broadcast will unblock A. Otherwise we will have a lost signal with respect
+to A. I do think that it does not matter whether the signal was physically
+(completely) lost or was just stolen by another thread (B) - in both cases
+it was simply lost with respect to A.
+
+>..Do you agree that this shows your rules are not strict enough?
+
+probably the opposite.. :-) i think that it shows that the only meaningful
+rule is
+
+a) "a signal is not lost between unlocking the mutex and waiting on the
+condition"
+
+with clarification of waiters visibility as defined by POSIX above.
+
+>> i would remove "_bTimedOut=false".. after all, it was a real timeout..
+>
+>I disagree. A thread that can't successfully retract its waiter status
+can't
+>really have timed out. If a thread can't return without executing extra
+code
+>to deal with the fact that someone tried to unblock it, I think it is a
+poor
+>idea to pretend we
+>didn't realize someone was trying to signal us. After all, a signal is
+more
+>important than a time out.
+
+a) POSIX does allow timed out thread to consume a signal (cancelled is
+not).
+b) ETIMEDOUT status just says that: "The time specified by abstime to
+pthread_cond_timedwait() has passed."
+c) it seem to me that hiding timeouts would violate "The
+pthread_cond_timedwait()
+function is the same as pthread_cond_wait() except that an error is
+returned if
+the absolute time specified by abstime passes (that is, system time equals
+or
+exceeds abstime) before the condition cond is signaled or broadcasted"
+because
+the abs. time did really pass before cond was signaled (waiter was
+released via semaphore). however, if it really matters, i could imaging
+that we
+can save an abs. time of signal/broadcast and compare it with timeout after
+unblock to find out whether it was a 'real' timeout or not. absent this
+check
+i do think that hiding timeouts would result in technical violation of
+specification.. but i think that this check is not important and we can
+simply
+trust timeout error code provided by wait since we are not trying to make
+'hard' realtime implementation.
+
+>What are IPC semaphores?
+
+<sys/sem.h>
+int semctl(int, int, int, ...);
+int semget(key_t, int, int);
+int semop(int, struct sembuf *, size_t);
+
+they support adjustment of semaphore counter (semvalue)
+in one single call - imaging Win32 ReleaseSemaphore( hsem,-N )
+
+>In the line where you state, "else if ( nWaitersBlocked > nWaitersGone ) {
+>// HARMLESS RACE CONDITION!" there is no race condition for nWaitersGone
+>because nWaitersGone is never modified without holding mtxUnblockLock. You
+>are correct that there is a harmless race on nWaitersBlocked, which can
+>increase and make the condition become true just after we check it. If
+this
+>happens, we interpret it as the wait starting after the signal.
+
+well, the reason why i've asked on comp.programming.threads whether this
+race
+condition is harmless or not is that in order to be harmless it should not
+violate the waiters visibility rule (see above). Fortunately, we increment
+the counter under protection of external mutex.. so that any (signalling)
+thread which will acquire the mutex next, should see the updated counter
+(in signal) according to POSIX memory visibility rules and mutexes
+(memory barriers). But i am not so sure how it actually works on
+Win32/INTEL
+which does not explicitly define any memory visibility rules :(
+
+>I like your optimization of this. You could improve Alg. 6 as follows:
+>---------- Algorithm 6b ----------
+>signal(bAll) {
+> _nSig=0
+> lock counters
+> // this is safe because nWaiting can only be decremented by a thread
+that
+> // owns counters and nGone can only be changed by a thread that owns
+>counters.
+> if (nWaiting>nGone) {
+> if (0==nSignaled) {
+> sema_wait gate // close gate if not already closed
+> }
+> if (nGone>0) {
+> nWaiting-=nGone
+> nGone=0
+> }
+> _nSig=bAll?nWaiting:1
+> nSignaled+=_nSig
+> nWaiting-=_nSig
+> }
+> unlock counters
+> if (0!=_nSig) {
+> sema_post queue, _nSig
+> }
+>}
+>---------- ---------- ----------
+>I guess this wouldn't apply to Alg 8a because nWaitersGone changes
+meanings
+>depending upon whether the gate is open or closed.
+
+agree.
+
+>In the loop "while ( nWaitersWasGone-- ) {" you do a sema_wait on
+>semBlockLock. Perhaps waiting on semBlockQueue would be a better idea.
+
+you are correct. my mistake.
+
+>What have you gained by making the last thread to be signaled do the waits
+>for all the timed out threads, besides added complexity? It took me a long
+>time to figure out what your objective was with this, to realize you were
+>using nWaitersGone to mean two different things, and to verify that you
+>hadn't introduced any bug by doing this. Even now I'm not 100% sure.
+>
+>What has all this playing about with nWaitersGone really gained us besides
+a
+>lot of complexity (it is much harder to verify that this solution is
+>correct), execution overhead (we now have a lot more if statements to
+>evaluate), and space overhead (more space for the extra code, and another
+>integer in our data)? We did manage to save a lock/unlock pair in an
+>uncommon case (when a time out occurs) at the above mentioned expenses in
+>the common cases.
+
+well, please consider the following:
+
+1) with multiple waiters unblocked (but some timed out) the trick with
+counter
+seem to ensure potentially higher level of concurrency by not delaying
+most of unblocked waiters for semaphore cleanup - only the last one
+will be delayed but all others would already contend/acquire/release
+the external mutex - the critical section protected by mtxUnblockLock is
+made smaller (increment + couple of IFs is faster than system/kernel call)
+which i think is good in general. however, you are right, this is done
+at expense of 'normal' waiters..
+
+2) some semaphore APIs (e.g. POSIX IPC sems) do allow to adjust the
+semaphore counter in one call => less system/kernel calls.. imagine:
+
+if ( 1 == nSignalsWasLeft ) {
+ if ( 0 != nWaitersWasGone ) {
+ ReleaseSemaphore( semBlockQueue,-nWaitersWasGone ); // better now
+than spurious later
+ }
+ sem_post( semBlockLock ); // open the gate
+ }
+
+3) even on win32 a single thread doing multiple cleanup calls (to wait)
+will probably result in faster execution (because of processor caching)
+than multiple threads each doing a single call to wait.
+
+>As for 8b, c, and d, they look ok though I haven't studied them
+thoroughly.
+>What would you use them for?
+
+8b) for semaphores which do not allow to unblock multiple waiters
+in a single call to post/release (e.g. POSIX realtime semaphores -
+<semaphore.h>)
+
+8c/8d) for WinCE prior to 3.0 (WinCE 3.0 does have semaphores)
+
+ok. so, which one is the 'final' algorithm(s) which we should use in
+pthreads-win32??
+
+regards,
+alexander.
+
+----------------------------------------------------------------------------
+
+Louis Thomas <lthomas@arbitrade.com> on 02/27/2001 05:20:12 AM
+
+Please respond to Louis Thomas <lthomas@arbitrade.com>
+
+To: Alexander Terekhov/Germany/IBM@IBMDE
+cc: rpj@ise.canberra.edu.au, Thomas Pfaff <tpfaff@gmx.net>, Nanbor Wang
+ <nanbor@cs.wustl.edu>
+Subject: RE: FYI/comp.programming.threads/Re: pthread_cond_* implementatio
+ n questions
+
+Sorry all. Busy week.
+
+> this insures the fairness
+> which POSIX does not (e.g. two subsequent broadcasts - the gate does
+insure
+> that first wave waiters will start the race for the mutex before waiters
+> from the second wave - Linux pthreads process/unblock both waves
+> concurrently...)
+
+I'm not sure how we are any more fair about this than Linux. We certainly
+don't guarantee that the threads released by the first broadcast will get
+the external mutex before the threads of the second wave. In fact, it is
+possible that those threads will never get the external mutex if there is
+enough contention for it.
+
+> e.g. i was thinking about implementation with a pool of
+> N semaphores/counters [...]
+
+I considered that too. The problem is as you mentioned in a). You really
+need to assign threads to semaphores once you know how you want to wake
+them
+up, not when they first begin waiting which is the only time you can assign
+them.
+
+> well, i am not quite sure that i've fully understood your scenario,
+
+Hmm. Well, it think it's an important example, so I'll try again. First, we
+have thread A which we KNOW is waiting on a condition. As soon as it
+becomes
+unblocked for any reason, we will know because it will set a flag. Since
+the
+flag is not set, we are 100% confident that thread A is waiting on the
+condition. We have another thread, thread B, which has acquired the mutex
+and is about to wait on the condition. Thus it is pretty clear that at any
+point, either just A is waiting, or A and B are waiting. Now thread C comes
+along. C is about to do a broadcast on the condition. A broadcast is
+guaranteed to unblock all threads currently waiting on a condition, right?
+Again, we said that either just A is waiting, or A and B are both waiting.
+So, when C does its broadcast, depending upon whether B has started waiting
+or not, thread C will unblock A or unblock A and B. Either way, C must
+unblock A, right?
+
+Now, you said anything that happens is correct so long as a) "a signal is
+not lost between unlocking the mutex and waiting on the condition" and b)
+"a
+thread must not steal a signal it sent", correct? Requirement b) is easy to
+satisfy: in this scenario, thread C will never wait on the condition, so it
+won't steal any signals. Requirement a) is not hard either. The only way
+we
+could fail to meet requirement a) in this scenario is if thread B was
+started waiting but didn't wake up because a signal was lost. This will not
+happen.
+
+Now, here is what happens. Assume thread C beats thread B. Thread C looks
+to
+see how many threads are waiting on the condition. Thread C sees just one
+thread, thread A, waiting. It does a broadcast waking up just one thread
+because just one thread is waiting. Next, before A can become unblocked,
+thread B begins waiting. Now there are two threads waiting, but only one
+will be unblocked. Suppose B wins. B will become unblocked. A will not
+become unblocked, because C only unblocked one thread (sema_post cond, 1).
+So at the end, B finishes and A remains blocked.
+
+We have met both of your requirements, so by your rules, this is an
+acceptable outcome. However, I think that the spec says this is an
+unacceptable outcome! We know for certain that A was waiting and that C did
+a broadcast, but A did not become unblocked! Yet, the spec says that a
+broadcast wakes up all waiting threads. This did not happen. Do you agree
+that this shows your rules are not strict enough?
+
+> and what about N2? :) this one does allow almost everything.
+
+Don't get me started about rule #2. I'll NEVER advocate an algorithm that
+uses rule 2 as an excuse to suck!
+
+> but it is done (decrement)under mutex protection - this is not a subject
+> of a race condition.
+
+You are correct. My mistake.
+
+> i would remove "_bTimedOut=false".. after all, it was a real timeout..
+
+I disagree. A thread that can't successfully retract its waiter status
+can't
+really have timed out. If a thread can't return without executing extra
+code
+to deal with the fact that someone tried to unblock it, I think it is a
+poor
+idea to pretend we
+didn't realize someone was trying to signal us. After all, a signal is more
+important than a time out.
+
+> when nSignaled != 0, it is possible to update nWaiters (--) and do not
+> touch nGone
+
+I realize this, but I was thinking that writing it the other ways saves
+another if statement.
+
+> adjust only if nGone != 0 and save one cache memory write - probably much
+slower than 'if'
+
+Hmm. You are probably right.
+
+> well, in a strange (e.g. timeout test) program you may (theoretically)
+> have an overflow of nWaiters/nGone counters (with waiters repeatedly
+timing
+> out and no signals at all).
+
+Also true. Not only that, but you also have the possibility that one could
+overflow the number of waiters as well! However, considering the limit you
+have chosen for nWaitersGone, I suppose it is unlikely that anyone would be
+able to get INT_MAX/2 threads waiting on a single condition. :)
+
+Analysis of 8a:
+
+It looks correct to me.
+
+What are IPC semaphores?
+
+In the line where you state, "else if ( nWaitersBlocked > nWaitersGone ) {
+// HARMLESS RACE CONDITION!" there is no race condition for nWaitersGone
+because nWaitersGone is never modified without holding mtxUnblockLock. You
+are correct that there is a harmless race on nWaitersBlocked, which can
+increase and make the condition become true just after we check it. If this
+happens, we interpret it as the wait starting after the signal.
+
+I like your optimization of this. You could improve Alg. 6 as follows:
+---------- Algorithm 6b ----------
+signal(bAll) {
+ _nSig=0
+ lock counters
+ // this is safe because nWaiting can only be decremented by a thread that
+ // owns counters and nGone can only be changed by a thread that owns
+counters.
+ if (nWaiting>nGone) {
+ if (0==nSignaled) {
+ sema_wait gate // close gate if not already closed
+ }
+ if (nGone>0) {
+ nWaiting-=nGone
+ nGone=0
+ }
+ _nSig=bAll?nWaiting:1
+ nSignaled+=_nSig
+ nWaiting-=_nSig
+ }
+ unlock counters
+ if (0!=_nSig) {
+ sema_post queue, _nSig
+ }
+}
+---------- ---------- ----------
+I guess this wouldn't apply to Alg 8a because nWaitersGone changes meanings
+depending upon whether the gate is open or closed.
+
+In the loop "while ( nWaitersWasGone-- ) {" you do a sema_wait on
+semBlockLock. Perhaps waiting on semBlockQueue would be a better idea.
+
+What have you gained by making the last thread to be signaled do the waits
+for all the timed out threads, besides added complexity? It took me a long
+time to figure out what your objective was with this, to realize you were
+using nWaitersGone to mean two different things, and to verify that you
+hadn't introduced any bug by doing this. Even now I'm not 100% sure.
+
+What has all this playing about with nWaitersGone really gained us besides
+a
+lot of complexity (it is much harder to verify that this solution is
+correct), execution overhead (we now have a lot more if statements to
+evaluate), and space overhead (more space for the extra code, and another
+integer in our data)? We did manage to save a lock/unlock pair in an
+uncommon case (when a time out occurs) at the above mentioned expenses in
+the common cases.
+
+As for 8b, c, and d, they look ok though I haven't studied them thoroughly.
+What would you use them for?
+
+ Later,
+ -Louis! :)
+
|