Tue Jul 28 14:04:29 1998 Ross Johnson <rpj@swan.canberra.edu.au>

	* private.c: Added detailed explanation of the new thread
	allocation scheme.
	(_pthread_new_thread): Totally rewritten to use
	new thread allocation scheme.
	(_pthread_delete_thread): Ditto.
	(_pthread_find_thread): Obsolete.

1 files changed, 120 insertions, 83 deletions

diff --git a/private.c b/private.c
index 44afd4f..db26563 100644
--- a/private.c
+++ b/private.c
@@ -9,121 +9,158 @@
 #include "pthread.h"
 #include "implement.h"
 
-/* The threads table works as follows:
-   hash into the table,
-   if the thread in this slot doesn't match then start single
-   stepping from there until we find it, or we hit an empty slot, or
-   we end up where we started from.
-
-   The scheme should have these characteristics:
-   - if the thread handle is a sequence number then the hash will
-     succeed first time every time,
-   - if the thread handle is a pseudo randomish value (eg. a pointer)
-     then the hash should succeed first time most times.
+/* Thread ID management.
+   ---------------------
+
+   We started by simply mapping the Win32 thread handle to directly to
+   pthread_t. Then, is order to process pthread_join()'s, needed to be
+   able to keep our POSIX thread ID (pthread_t) around after the Win32
+   thread has terminated and possibly reused the Win32 handle.
+
+   The pthread_t value is now actually the pointer to a thread struct:
+
+   typedef struct _pthread * pthread_t;
+
+   which amongst other things stores the Win32 thread handle:
+
+   struct _pthread {
+     HANDLE  win32handle;
+     int     ptstatus;
+     ...
+   };
+
+   So now whereever we need to use the Win32 handle it can be accessed
+   as:
+
+   pthread_t T = pthread_this();
+   HANDLE    H;
+
+   H = T->win32handle;
+
+   // or (which is NOT preferred, let the compiler optimise to this).
+
+   H = (HANDLE) *T;
+
+
+   POSIX Threads Table
+   -------------------
+
+   Having the thread ID as a pointer to the thread struct itself
+   avoids the need to search the threads table in all but the initial
+   occation where we create the thread.
+
+   Initially we used a hash function to select a free thread struct
+   from the table, possibly needing a walk through the table if the
+   hash collided with an already in-use thread.
+
+   The scheme used now is more efficient and is done as follows:
+
+   We use two tables and two counters:
+
+   struct _pthread  _pthread_virgins[PTHREAD_THREADS_MAX];
+   pthread_t        _pthread_reuse[PTHREAD_THREADS_MAX];
+
+   int       _pthread_virgin_next = 0;
+   int       _pthread_reuse_top = -1;
+
+   The counter _pthread_virgin_next is an index into _pthread_virgins[],
+   which can be thought of as a list, and _pthread_reuse_top is an
+   index into _pthread_reuse[], which can be thought of as a LIFO stack.
+
+   Once taken from _pthread_virgins[], used and freed threads are only
+   ever pushed back onto _pthread_reuse[].
+
+   The code for choosing a new (pthread_t) thread from the pool of
+   free thread structs looks like:
+
+   if (_pthread_reuse_top == -1)
+     {
+       if (_pthread_virgin_next >= PTHREAD_THREADS_MAX)
+         {
+	   return EAGAIN;
+	 }
+       else
+         {
+	   thread = _pthread_virgin[_pthread_virgin_next++];
+	 }
+     }
+   else
+     {
+       thread = _pthread_reuse[_pthread_reuse_top--];
+     }
+
+
+   The code to free a thread is:
+
+   _pthread_reuse[++_pthread_reuse_top] = thread;
+
  */
 
 int
-_pthread_new_thread_entry(_pthread_t * entry)
+_pthread_new_thread(pthread_t * thread)
 {
-  _pthread_t new;
+  pthread_t new;
 
-  if (_pthread_threads_count >= PTHREAD_THREADS_MAX)
+  if (_pthread_reuse_top == -1)
     {
-      return EAGAIN;
-    }
-
-  /* Use a preloaded reuse stack of pthread_t. */
-  new = 
-
-  while (new->thread != NULL)
-    {
-      new++;
-
-      if (new == &_pthread_threads_table[PTHREAD_THREADS_MAX])
+      if (_pthread_virgin_next >= PTHREAD_THREADS_MAX)
 	{
-	  /* Wrap to the top of the table. */
-	  new == _pthread_threads_table;
+	  return EAGAIN;
+	}
+      else
+	{
+	  new = _pthread_virgin[_pthread_virgin_next++];
 	}
-    }
-
-  if (new->thread != NULL)
-    {
-      /* INTERNAL ERROR: There should be at least one slot left. */
-      return ESRCH;
     }
   else
     {
-      new->thread = thread;
-      pthread_attr_init(&(new->attr));
-      new->joinvalueptr = NULL;
-      new->cancelstate = PTHREAD_CANCEL_ENABLE;
-      new->canceltype = PTHREAD_CANCEL_DEFERRED;
-      new->cancel_pending = FALSE;
-      new->cleanupstack = NULL;
-      new->destructorstack = NULL;
-      new->forkpreparestack = NULL;
-      new->forkparentstack = NULL;
-      new->forkchildstack = NULL;
+      new = _pthread_reuse[_pthread_reuse_top--];
     }
 
-  _pthread_threads_count++;
-  entry = new;
+  new->win32handle = NULL;
+  new->ptstatus = _PTHREAD_NEW;
+  pthread_attr_init(&(new->attr));
+  new->joinvalueptr = NULL;
+  new->cancelstate = PTHREAD_CANCEL_ENABLE;
+  new->canceltype = PTHREAD_CANCEL_DEFERRED;
+  new->cancel_pending = FALSE;
+  new->cleanupstack = NULL;
+  new->destructorstack = NULL;
+  new->forkpreparestack = NULL;
+  new->forkparentstack = NULL;
+  new->forkchildstack = NULL;
+
+  *thread = new;
 
   return 0;
 }
 
 _pthread_threads_thread *
-_pthread_find_thread_entry(pthread_t thread)
+_pthread_find_thread(pthread_t thread)
 {
-  _pthread_threads_thread_t * entry;
-  _pthread_threads_thread_t * start;
-
-  start = entry = &_pthread_threads_table[_PTHREAD_HASH_INDEX(thread)];
-
-  while (entry->thread != thread)
-    {
-      entry++;
-
-      if (entry == &_pthread_threads_table[PTHREAD_THREADS_MAX])
-	{
-	  /* Wrap to top of table. */
-	  entry = _pthread_threads_table;
-	}
-    }
-
-  if (entry->thread == NULL || entry == start)
-    {
-      /* Failed to find the thread. */
-      return NULL;
-    }
-
-  return entry;
+  /* Should no longer be needed */
 }
 
-void
-_pthread_delete_thread_entry(_pthread_threads_thread_t * entry)
+int
+_pthread_delete_thread(_pthread_t * thread)
 {
   /* We don't check that the thread has been properly cleaned up, so
      it had better be done already. */
 
   /* Remove the thread entry if necessary. */
 
-  if (entry->thread != NULL)
+  if (thread != NULL
+      && thread->ptstatus == _PTHREAD_EXITED)
     {
       pthread_attr_destroy(&(entry->attr));
-      entry->thread = NULL;
+      thread->win32handle = NULL;
+      thread_ptstatus = _PTHREAD_REUSE;
 
-      if (_pthread_threads_count > 0)
-	{
-	  _pthread_threads_count--;
-	}
-      else
-	{
-	  /* FIXME: INTERNAL ERROR: This should not happen. */
-	}
+      _pthread_reuse[++_pthread_reuse_top] = thread;
     }
   else
     {
-      /* FIXME: INTERNAL ERROR: This should not happen. */
+      return EINVAL
     }
+  return 0;
 }