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commit 64f73cc1f7f57ca6643b027eae63041fec408ea8
Author: Ralph Giles <giles@mozilla.com>
Date: Fri Nov 6 16:42:49 2015 -0800
Bug 1218124 - Use InterlockCompare in win32 vpx_once(). r=gerald
diff --git a/media/libvpx/vpx_ports/vpx_once.h b/media/libvpx/vpx_ports/vpx_once.h
index f1df394..da04db4 100644
--- a/media/libvpx/vpx_ports/vpx_once.h
+++ b/media/libvpx/vpx_ports/vpx_once.h
@@ -1,5 +1,5 @@
/*
- * Copyright (c) 2011 The WebM project authors. All Rights Reserved.
+ * Copyright (c) 2015 The WebM project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
@@ -13,63 +13,83 @@
#include "vpx_config.h"
+/* Implement a function wrapper to guarantee initialization
+ * thread-safety for library singletons.
+ *
+ * NOTE: These functions use static locks, and can only be
+ * used with one common argument per compilation unit. So
+ *
+ * file1.c:
+ * vpx_once(foo);
+ * ...
+ * vpx_once(foo);
+ *
+ * file2.c:
+ * vpx_once(bar);
+ *
+ * will ensure foo() and bar() are each called only once, but in
+ *
+ * file1.c:
+ * vpx_once(foo);
+ * vpx_once(bar):
+ *
+ * bar() will never be called because the lock is used up
+ * by the call to foo().
+ */
+
#if CONFIG_MULTITHREAD && defined(_WIN32)
#include <windows.h>
#include <stdlib.h>
+/* Declare a per-compilation-unit state variable to track the progress
+ * of calling func() only once. This must be at global scope because
+ * local initializers are not thread-safe in MSVC prior to Visual
+ * Studio 2015.
+ *
+ * As a static, once_state will be zero-initialized as program start.
+ */
+static LONG once_state;
static void once(void (*func)(void))
{
- static CRITICAL_SECTION *lock;
- static LONG waiters;
- static int done;
- void *lock_ptr = &lock;
-
- /* If the initialization is complete, return early. This isn't just an
- * optimization, it prevents races on the destruction of the global
- * lock.
+ /* Try to advance once_state from its initial value of 0 to 1.
+ * Only one thread can succeed in doing so.
*/
- if(done)
+ if (InterlockedCompareExchange(&once_state, 1, 0) == 0) {
+ /* We're the winning thread, having set once_state to 1.
+ * Call our function. */
+ func();
+ /* Now advance once_state to 2, unblocking any other threads. */
+ InterlockedIncrement(&once_state);
return;
-
- InterlockedIncrement(&waiters);
-
- /* Get a lock. We create one and try to make it the one-true-lock,
- * throwing it away if we lost the race.
- */
-
- {
- /* Scope to protect access to new_lock */
- CRITICAL_SECTION *new_lock = malloc(sizeof(CRITICAL_SECTION));
- InitializeCriticalSection(new_lock);
- if (InterlockedCompareExchangePointer(lock_ptr, new_lock, NULL) != NULL)
- {
- DeleteCriticalSection(new_lock);
- free(new_lock);
- }
}
- /* At this point, we have a lock that can be synchronized on. We don't
- * care which thread actually performed the allocation.
+ /* We weren't the winning thread, but we want to block on
+ * the state variable so we don't return before func()
+ * has finished executing elsewhere.
+ *
+ * Try to advance once_state from 2 to 2, which is only possible
+ * after the winning thead advances it from 1 to 2.
*/
-
- EnterCriticalSection(lock);
-
- if (!done)
- {
- func();
- done = 1;
+ while (InterlockedCompareExchange(&once_state, 2, 2) != 2) {
+ /* State isn't yet 2. Try again.
+ *
+ * We are used for singleton initialization functions,
+ * which should complete quickly. Contention will likewise
+ * be rare, so it's worthwhile to use a simple but cpu-
+ * intensive busy-wait instead of successive backoff,
+ * waiting on a kernel object, or another heavier-weight scheme.
+ *
+ * We can at least yield our timeslice.
+ */
+ Sleep(0);
}
- LeaveCriticalSection(lock);
-
- /* Last one out should free resources. The destructed objects are
- * protected by checking if(done) above.
+ /* We've seen once_state advance to 2, so we know func()
+ * has been called. And we've left once_state as we found it,
+ * so other threads will have the same experience.
+ *
+ * It's safe to return now.
*/
- if(!InterlockedDecrement(&waiters))
- {
- DeleteCriticalSection(lock);
- free(lock);
- lock = NULL;
- }
+ return;
}
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