[vlc-devel] [PATCH] video_output: explicit vlc_testcancel

[Quentin Chateau]

Le mer. 26 févr. 2020 à 20:47, Rémi Denis-Courmont <remi at remlab.net> a
écrit :

> Le keskiviikkona 26. helmikuuta 2020, 16.34.09 EET Quentin Chateau a écrit
> :
> > Thanks for the hindsight, I dug further:
> >
> > vlc_cond_timedwait is called, and so is pthread_cond_timedwait. The
> > issue happens when the deadline parameter is in the past: when this
> > happens, pthread_cond_timedwait does not act as a cancellation point
> > anymore...
>
> pthread_cond_timedwait() and pthread_cond_wait() are *always* cancellation
> points. The deadline does not matter.
>

That's what I though as well. Though it seemed that it did not act as a
cancellation point. I am not able to reproduce on a different machine right
now. I'll have to double check tomorrow on the machine that I had the bug
on.


> vlc_cond_timedwait() and vlc_cond_wait() are likewise supposed to always
> be
> cancellation points. By my count, there are 21 outstanding vlc_cancel()
> call
> sites still depending on that behaviour.
>
> Right now, there is a bug for which I just sent a patch
>

> > In my case deadline happens to be in the past because
> > ThreadDisplayPicture keeps failing (due to one of my video filters
> > failing on the last frame)
> >
> > I could not find this behavior in the pthread documentation, but it
> > seems to be the case anyway. Do you think that we need to:
> >
> >  1. Fix vlc_cond_timedwait so it will check for cancellation if the
> >     deadline is in the past
>
> I don't think that that's a feasible option.
>
> For a start, I couldn't find that bug in those circumstances. I did find a
> bug
> with the same symptions in different circumstances though, see patch.
>
> And then, even if the bug were there where I could not see it, there are
> no
> ways to determine that the deadline is in the past, as that changes
> asynchronously.
>
> >  2. Acknowledge the fact that vlc_cond_timedwait is not always a
> >     cancellation point and fix the issue higher in the stack, either in
> >     vout_control_Pop, or similarly to the patch I sent, directly in
> Thread
>
> You can't do that either, or at least not yet. Not when we still have 21
> or so
> code paths depending on it.
>
> --
> Rémi Denis-Courmont
> http://www.remlab.net/
>
>
>
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