Is this usage of condition variables ALWAYS subject to a lost-signal race?

Question

Suppose a condition variable is used in a situation where the signaling thread modifies the state affecting the truth value of the predicate and calls pthread_cond_signal without holding the mutex associated with the condition variable? Is it true that this type of usage is always subject to race conditions where the signal may be missed?

To me, there seems to always be an obvious race:

1. Waiter evaluates the predicate as false, but before it can begin waiting...
2. Another thread changes state in a way that makes the predicate true.
3. That other thread calls pthread_cond_signal, which does nothing because there are no waiters yet.
4. The waiter thread enters pthread_cond_wait, unaware that the predicate is now true, and waits indefinitely.

But does this same kind of race condition always exist if the situation is changed so that either (A) the mutex is held while calling pthread_cond_signal, just not while changing the state, or (B) so that the mutex is held while changing the state, just not while calling pthread_cond_signal?

I'm asking from a standpoint of wanting to know if there are any valid uses of the above not-best-practices usages, i.e. whether a correct condition-variable implementation needs to account for such usages in avoiding race conditions itself, or whether it can ignore them because they're already inherently racy.

Answer 1

The state must be modified inside a mutex, if for no other reason than the possibility of spurious wake-ups, which would lead to the reader reading the state while the writer is in the middle of writing it.

You can call pthread_cond_signal anytime after the state is changed. It doesn't have to be inside the mutex. POSIX guarantees that at least one waiter will awaken to check the new state. More to the point:

1. Calling pthread_cond_signal doesn't guarantee that a reader will acquire the mutex first. Another writer might get in before a reader gets a chance to check the new status. Condition variables don't guarantee that readers immediately follow writers (After all, what if there are no readers?)
2. Calling it after releasing the lock is actually better, since you don't risk having the just-awoken reader immediately going back to sleep trying to acquire the lock that the writer is still holding.

EDIT: @DietrichEpp makes a good point in the comments. The writer must change the state in such a way that the reader can never access an inconsistent state. It can do so either by acquiring the mutex used in the condition-variable, as I indicate above, or by ensuring that all state-changes are atomic.

Answer 2

The fundamental race here looks like this:

THREAD A        THREAD B
Mutex lock
Check state
                Change state
                Signal
cvar wait
(never awakens)

If we take a lock EITHER on the state change OR the signal, OR both, then we avoid this; it's not possible for both the state-change and the signal to occur while thread A is in its critical section and holding the lock.

If we consider the reverse case, where thread A interleaves into thread B, there's no problem:

THREAD A        THREAD B
                Change state
Mutex lock
Check state
( no need to wait )
Mutex unlock
                Signal (nobody cares)

So there's no particular need for thread B to hold a mutex over the entire operation; it just need to hold the mutex for some, possible infinitesimally small interval, between the state change and signal. Of course, if the state itself requires locking for safe manipulation, then the lock must be held over the state change as well.

Finally, note that dropping the mutex early is unlikely to be a performance improvement in most cases. Requiring the mutex to be held reduces contention over the internal locks in the condition variable, and in modern pthreads implementations, the system can 'move' the waiting thread from waiting on the cvar to waiting on the mutex without waking it up (thus avoiding it waking up only to immediately block on the mutex). As pointed out in the comments, dropping the mutex may improve performance in some cases, by reducing the number of syscalls needed. Then again it could also lead to extra contention on the condition variable's internal mutex. Hard to say. It's probably not worth worrying about in any case.

Note that the applicable standards require that pthread_cond_signal be safely callable without holding the mutex:

The pthread_cond_signal() or pthread_cond_broadcast() functions may be called by a thread whether or not it currently owns the mutex that threads calling pthread_cond_wait() or pthread_cond_timedwait() have associated with the condition variable during their waits [...]

This usually means that condition variables have an internal lock over their internal data structures, or otherwise use some very careful lock-free algorithm.

Answer 3

The answer is, there is a race, and to eliminate that race, you must do this:

/* atomic op outside of mutex, and then: */

pthread_mutex_lock(&m);
pthread_mutex_unlock(&m);

pthread_cond_signal(&c);

The protection of the data doesn't matter, because you don't hold the mutex when calling pthread_cond_signal anyway.

See, by locking and unlocking the mutex, you have created a barrier. During that brief moment when the signaler has the mutex, there is a certainty: no other thread has the mutex. This means no other thread is executing any critical regions.

This means that all threads are either about to get the mutex to discover the change you have posted, or else they have already found that change and ran off with it (releasing the mutex), or else have not found they are looking for and have atomically given up the mutex to gone to sleep (and are guaranteed to be waiting nicely on the condition).

Without the mutex lock/unlock, you have no synchronization. The signal will sometimes fire as threads which didn't see the changed atomic value are transitioning to their atomic sleep to wait for it.

So this is what the mutex does from the point of view of a thread which is signaling. You can get the atomicity of access from something else, but not the synchronization.

P.S. I have implemented this logic before. The situation was in the Linux kernel (using my own mutexes and condition variables).

In my situation, it was impossible for the signaler to hold the mutex for the atomic operation on shared data. Why? Because the signaler did the operation in user space, inside a buffer shared between the kernel and user, and then (in some situations) made a system call into the kernel to wake up a thread. User space simply made some modifications to the buffer, and then if some conditions were satisfied, it would perform an ioctl.

So in the ioctl call I did the mutex lock/unlock thing, and then hit the condition variable. This ensured that the thread would not miss the wake up related to that latest modification posted by user space.

At first I just had the condition variable signal, but it looked wrong without the involvement of the mutex, so I reasoned about the situation a little bit and realized that the mutex must simply be locked and unlocked to conform to the synchronization ritual which eliminates the lost wakeup.