On 28.09.2017 19:18, Gustaw Smolarczyk wrote:
2017-09-28 18:52 GMT+02:00 Marek Olšák <[email protected]>:
A clearer comment would be: "Don't destroy the fence when it's in the
middle of util_queue_fence_signal (signalled but not unlocked yet
because util_queue_fence_is_signalled doesn't lock). Instead, wait
until util_queue_fence_signal returns and then destroy it."
Sure, I can change that.
[snip]
What about the ones that wait in util_queue_fence_wait? I think it's
possible for the following:
-----------------------
util_queue_fence_wait (begin)
...
cnd_wait(&fence->cond, &fence->mutex);
util_queue_fence_signal (begin)
mtx_lock(&fence->mutex);
fence->signalled = true;
cnd_broadcast(&fence->cond);
mtx_unlock(&fence->mutex);
util_queue_fence_signal (end)
util_queue_fence_is_signalled
util_queue_fence_destroy (begin)
mtx_lock(&fence->mutex);
mtx_unlock(&fence->mutex);
cnd_destroy(&fence->cond);
mtx_destroy(&fence->mutex);
util_queue_fence_destroy (end)
mtx_unlock(&fence->mutex); <--- use after free
util_queue_fence_wait (end)
-----------------------
Unless it is defined that the race between mtx_unlock and mtx_destroy
is automagically fixed (like mtx_destroy performs a lock operation
inside of itself; pthread_mutex_* don't do that IIUC).
If I'm understanding you correctly, there are three threads in play
here. The question is: In your example, what is the contract between the
different threads?
The thread which calls util_queue_fence_destroy is responsible for
ensuring that no other thread accesses the fence anymore.
The contract between the util_queue and its users is that after
signaling the fence of a job, the queue will not touch that job anymore.
So the user of util_queue can destroy the job (and hence its fence)
after the fence is signaled.
I'm pretty sure your example cannot possibly make sense. You have to
have some synchronization external to u_queue.c between the thread
running util_queue_fence_wait and the thread running
util_queue_fence_destroy to ensure that this doesn't happen.
Cheers,
Nicolai
Regards,
Gustaw
--
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.
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