diff options
Diffstat (limited to 'absl/synchronization')
| -rw-r--r-- | absl/synchronization/mutex.cc | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc index 821047fd..52e2455d 100644 --- a/absl/synchronization/mutex.cc +++ b/absl/synchronization/mutex.cc @@ -2575,6 +2575,23 @@ bool CondVar::WaitCommon(Mutex *mutex, KernelTimeout t) { while (waitp.thread->state.load(std::memory_order_acquire) == PerThreadSynch::kQueued) { if (!Mutex::DecrementSynchSem(mutex, waitp.thread, t)) { + // DecrementSynchSem returned due to timeout. + // Now we will either (1) remove ourselves from the wait list in Remove + // below, in which case Remove will set thread.state = kAvailable and + // we will not call DecrementSynchSem again; or (2) Signal/SignalAll + // has removed us concurrently and is calling Wakeup, which will set + // thread.state = kAvailable and post to the semaphore. + // It's important to reset the timeout for the case (2) because otherwise + // we can live-lock in this loop since DecrementSynchSem will always + // return immediately due to timeout, but Signal/SignalAll is not + // necessary set thread.state = kAvailable yet (and is not scheduled + // due to thread priorities or other scheduler artifacts). + // Note this could also be resolved if Signal/SignalAll would set + // thread.state = kAvailable while holding the wait list spin lock. + // But this can't be easily done for SignalAll since it grabs the whole + // wait list with a single compare-exchange and does not really grab + // the spin lock. + t = KernelTimeout::Never(); this->Remove(waitp.thread); rc = true; } |