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tl;dr: This fixes deadlocks that would occur in most applications in Serenity when SMP was enabled. As an optimization to `pthread_mutex_unlock()`, if only one thread holds a mutex lock, it will avoid calling `futex()` to wake, since no threads are actually waiting in `futex()`. If a thread manages to synchronously unlock and relock the same mutex, the state will be set to indicate that a wake is not needed, regardless of whether any other threads are waiting for the lock. This should be fine, as a thread that is waiting will set the mutex to needing a wake and check if it is unlocked to unblock itself in the same atomic operation. However, when `pthread_mutex_lock()` was called with only one thread holding the mutex lock, instead of telling `futex()` to atomically sleep the thread if the state is set to indicate that a wake is needed, the first wait would check if the state was set to indicate a wake was _not_ needed. This means it is possible for the call to `futex()` to wait when any subsequent call to `pthread_mutex_unlock()` would not call `futex()` to wake, causing it to wait forever. After that, any other thread that tries to take the lock will see that the lock is taken and also `futex()` wait. Despite the fact that these other threads would set the state to needing a wake, there will be no unblocked thread holding the lock to actually wake them. By making it wait only if the state indicates to other threads that a wake is needed, heavily contended mutexes should no longer cause deadlocks. Most applications would encounter these deadlocks due to the mutex used by `malloc()`, some sooner than others. The worst offenders (other than Ladybird) were most likely VideoPlayer and SoundPlayer. |
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