[FFmpeg-devel] [PATCH 3/3] pthread : Protect progress & progress_used access with progress_mutex.

Aaron Colwell acolwell at chromium.org
Thu Mar 22 20:20:28 CET 2012

On Thu, Mar 22, 2012 at 11:09 AM, Reimar Döffinger <Reimar.Doeffinger at gmx.de
> wrote:

> On Thu, Mar 22, 2012 at 09:58:17AM -0700, Aaron Colwell wrote:
> > Questions:
> > Why is the existing code considered safe?
> > How are thread protected from accidentally observing a MAX_INT progress
> > right before progress gets set to -1? It seems like this could cause
> > ff_thread_await_progress() calls to return early.
> Assuming the progress_used part is working correctly:
> It will only be reused when progress_used is 0.
> progress_used is only 0 when there is no frame using
> it any more.
> progress can only by accessed through a frame.
> Thus it can only be used after the frame allocation is done,
> i.e. ff_thread_get_buffer has returned, but by that time
> it already has been reset to -1.
> Now as to progress_used working correctly...
> It is in a per-thread context, and most of its accesses
> are from that single thread (in particular, the check
> for setting progress to INT_MAX and the code one setting
> progress_used to 1 are always in the same thread).
> The exception is the free_progress. However it and allocate_progress
> are protected with the buffer lock against each other.
> So that leaves the INT_MAX related code as an issue.
> However due to being in the same thread, it cannot
> miss progress_used being set to 1.
> The worst that can happen is that is misses progress_used
> being set to 0, in which case it will set a value to INT_MAX
> that we know is no longer reachable except via allocate_progress.
> No, this is not obvious, possibly can be improved and at
> least can be documented. I do not think it is wrong though,
> not even with some of the more strict models.

Thank you very much. This information is very helpful. It should be enough
for me to annotate the code so our tool can understand what is going on.

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