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Mutex: remove mutex reentrancy crasher.

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This change finally removes the reentrancy crasher after a
period without reported reentrancies.

The change saves 0.8% in a downstream project.

Bug: webrtc:11567
Change-Id: Ia98ad873f02cf5114b3b3518eed7dd8f746f7408
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/186046
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32257}
This commit is contained in:
Markus Handell
2020-09-30 12:59:10 +02:00
committed by Commit Bot
parent d324a79a85
commit 2a23bd276b
1 changed files with 1 additions and 39 deletions
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40
rtc_base/synchronization/mutex.h
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@ -15,7 +15,6 @@
#include "absl/base/const_init.h"
#include "rtc_base/checks.h"
#include "rtc_base/platform_thread_types.h"
#include "rtc_base/system/unused.h"
#include "rtc_base/thread_annotations.h"
@ -40,54 +39,17 @@ class RTC_LOCKABLE Mutex final {
Mutex& operator=(const Mutex&) = delete;
void Lock() RTC_EXCLUSIVE_LOCK_FUNCTION() {
rtc::PlatformThreadRef current = CurrentThreadRefAssertingNotBeingHolder();
impl_.Lock();
// |holder_| changes from 0 to CurrentThreadRef().
holder_.store(current, std::memory_order_relaxed);
}
RTC_WARN_UNUSED_RESULT bool TryLock() RTC_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
rtc::PlatformThreadRef current = CurrentThreadRefAssertingNotBeingHolder();
if (impl_.TryLock()) {
// |holder_| changes from 0 to CurrentThreadRef().
holder_.store(current, std::memory_order_relaxed);
return true;
}
return false;
return impl_.TryLock();
}
void Unlock() RTC_UNLOCK_FUNCTION() {
// |holder_| changes from CurrentThreadRef() to 0. If something else than
// CurrentThreadRef() is stored in |holder_|, the Unlock results in
// undefined behavior as mutexes can't be unlocked from another thread than
// the one that locked it, or called while not being locked.
holder_.store(0, std::memory_order_relaxed);
impl_.Unlock();
}
private:
rtc::PlatformThreadRef CurrentThreadRefAssertingNotBeingHolder() {
rtc::PlatformThreadRef holder = holder_.load(std::memory_order_relaxed);
rtc::PlatformThreadRef current = rtc::CurrentThreadRef();
// TODO(bugs.webrtc.org/11567): remove this temporary check after migrating
// fully to Mutex.
RTC_CHECK_NE(holder, current);
return current;
}
MutexImpl impl_;
// TODO(bugs.webrtc.org/11567): remove |holder_| after migrating fully to
// Mutex.
// |holder_| contains the PlatformThreadRef of the thread currently holding
// the lock, or 0.
// Remarks on the used memory orders: the atomic load in
// CurrentThreadRefAssertingNotBeingHolder() observes either of two things:
// 1. our own previous write to holder_ with our thread ID.
// 2. another thread (with ID y) writing y and then 0 from an initial value of
// 0. If we're observing case 1, our own stores are obviously ordered before
// the load, and hit the CHECK. If we're observing case 2, the value observed
// w.r.t |impl_| being locked depends on the memory order. Since we only care
// that it's different from CurrentThreadRef()), we use the more performant
// option, memory_order_relaxed.
std::atomic<rtc::PlatformThreadRef> holder_ = {0};
};
// MutexLock, for serializing execution through a scope.