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+/*
+ * ptw32_MCS_lock.c
+ *
+ * Description:
+ * This translation unit implements queue-based locks.
+ *
+ * --------------------------------------------------------------------------
+ *
+ * Pthreads-win32 - POSIX Threads Library for Win32
+ * Copyright(C) 1998 John E. Bossom
+ * Copyright(C) 1999,2005 Pthreads-win32 contributors
+ *
+ * Contact Email: rpj@callisto.canberra.edu.au
+ *
+ * The current list of contributors is contained
+ * in the file CONTRIBUTORS included with the source
+ * code distribution. The list can also be seen at the
+ * following World Wide Web location:
+ * http://sources.redhat.com/pthreads-win32/contributors.html
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library in the file COPYING.LIB;
+ * if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
+ */
+
+/*
+ * About MCS locks:
+ *
+ * MCS locks are queue-based locks, where the queue nodes are local to the
+ * thread. The 'lock' is nothing more than a global pointer that points to
+ * the last node in the queue, or is NULL if the queue is empty.
+ *
+ * Originally designed for use as spin locks requiring no kernel resources
+ * for synchronisation or blocking, the implementation below has adapted
+ * the MCS spin lock for use as a general mutex that will suspend threads
+ * when there is lock contention.
+ *
+ * Because the queue nodes are thread-local, most of the memory read/write
+ * operations required to add or remove nodes from the queue do not trigger
+ * cache-coherence updates.
+ *
+ * Like 'named' mutexes, MCS locks consume system recourses transiently -
+ * they are able to acquire and free recourses automatically - but MCS
+ * locks do not require any unique 'name' to identify the lock to all
+ * threads using it.
+ *
+ * Usage of MCS locks:
+ *
+ * - you need a global ptw32_mcs_lock_t instance initialised to 0 or NULL.
+ * - you need a local thread-scope ptw32_mcs_local_node_t instance, which
+ * may serve several different locks but you need at least one node for
+ * every lock held concurrently by a thread.
+ *
+ * E.g.:
+ *
+ * ptw32_mcs_lock_t lock1 = 0;
+ * ptw32_mcs_lock_t lock2 = 0;
+ *
+ * void *mythread(void *arg)
+ * {
+ * ptw32_mcs_local_node_t node;
+ *
+ * ptw32_mcs_acquire (&lock1, &node);
+ * ptw32_mcs_release (&node);
+ *
+ * ptw32_mcs_acquire (&lock2, &node);
+ * ptw32_mcs_release (&node);
+ * {
+ * ptw32_mcs_local_node_t nodex;
+ *
+ * ptw32_mcs_acquire (&lock1, &node);
+ * ptw32_mcs_acquire (&lock2, &nodex);
+ *
+ * ptw32_mcs_release (&nodex);
+ * ptw32_mcs_release (&node);
+ * }
+ * return (void *)0;
+ * }
+ */
+
+#include "implement.h"
+#include "pthread.h"
+
+/*
+ * ptw32_mcs_flag_set -- notify another thread about an event.
+ *
+ * Set event if an event handle has been stored in the flag, and
+ * set flag to -1 otherwise. Note that -1 cannot be a valid handle value.
+ */
+INLINE void
+ptw32_mcs_flag_set (LONG * flag)
+{
+ HANDLE e = (HANDLE)PTW32_INTERLOCKED_COMPARE_EXCHANGE(
+ (PTW32_INTERLOCKED_LPLONG)flag,
+ (PTW32_INTERLOCKED_LONG)-1,
+ (PTW32_INTERLOCKED_LONG)0);
+ if ((HANDLE)0 != e)
+ {
+ /* another thread has already stored an event handle in the flag */
+ SetEvent(e);
+ }
+}
+
+/*
+ * ptw32_mcs_flag_set -- wait for notification from another.
+ *
+ * Store an event handle in the flag and wait on it if the flag has not been
+ * set, and proceed without creating an event otherwise.
+ */
+INLINE void
+ptw32_mcs_flag_wait (LONG * flag)
+{
+ if (0 == InterlockedExchangeAdd((LPLONG)flag, 0)) /* MBR fence */
+ {
+ /* the flag is not set. create event. */
+
+ HANDLE e = CreateEvent(NULL, PTW32_FALSE, PTW32_FALSE, NULL);
+
+ if (0 == PTW32_INTERLOCKED_COMPARE_EXCHANGE(
+ (PTW32_INTERLOCKED_LPLONG)flag,
+ (PTW32_INTERLOCKED_LONG)e,
+ (PTW32_INTERLOCKED_LONG)0))
+ {
+ /* stored handle in the flag. wait on it now. */
+ WaitForSingleObject(e, INFINITE);
+ }
+
+ CloseHandle(e);
+ }
+}
+
+/*
+ * ptw32_mcs_lock_acquire -- acquire an MCS lock.
+ *
+ * See:
+ * J. M. Mellor-Crummey and M. L. Scott.
+ * Algorithms for Scalable Synchronization on Shared-Memory Multiprocessors.
+ * ACM Transactions on Computer Systems, 9(1):21-65, Feb. 1991.
+ */
+INLINE void
+ptw32_mcs_lock_acquire (ptw32_mcs_lock_t * lock, ptw32_mcs_local_node_t * node)
+{
+ ptw32_mcs_local_node_t *pred;
+
+ node->lock = lock;
+ node->nextFlag = 0;
+ node->readyFlag = 0;
+ node->next = 0; /* initially, no successor */
+
+ /* queue for the lock */
+ pred = (ptw32_mcs_local_node_t *)PTW32_INTERLOCKED_EXCHANGE((LPLONG)lock,
+ (LONG)node);
+
+ if (0 != pred)
+ {
+ /* the lock was not free. link behind predecessor. */
+ pred->next = node;
+ ptw32_mcs_flag_set(&pred->nextFlag);
+ ptw32_mcs_flag_wait(&node->readyFlag);
+ }
+}
+
+/*
+ * ptw32_mcs_lock_release -- release an MCS lock.
+ *
+ * See:
+ * J. M. Mellor-Crummey and M. L. Scott.
+ * Algorithms for Scalable Synchronization on Shared-Memory Multiprocessors.
+ * ACM Transactions on Computer Systems, 9(1):21-65, Feb. 1991.
+ */
+INLINE void
+ptw32_mcs_lock_release (ptw32_mcs_local_node_t * node)
+{
+ ptw32_mcs_lock_t *lock = node->lock;
+ ptw32_mcs_local_node_t *next = (ptw32_mcs_local_node_t *)
+ InterlockedExchangeAdd((LPLONG)&node->next, 0); /* MBR fence */
+
+ if (0 == next)
+ {
+ /* no known successor */
+
+ if (node == (ptw32_mcs_local_node_t *)
+ PTW32_INTERLOCKED_COMPARE_EXCHANGE((PTW32_INTERLOCKED_LPLONG)lock,
+ (PTW32_INTERLOCKED_LONG)0,
+ (PTW32_INTERLOCKED_LONG)node))
+ {
+ /* no successor, lock is free now */
+ return;
+ }
+
+ /* wait for successor */
+ ptw32_mcs_flag_wait(&node->nextFlag);
+ next = (ptw32_mcs_local_node_t *)
+ InterlockedExchangeAdd((LPLONG)&node->next, 0); /* MBR fence */
+ }
+
+ /* pass the lock */
+ ptw32_mcs_flag_set(&next->readyFlag);
+}