Re: [FFmpeg-devel] [PATCH v3 2/3] lavf/dashdec: Multithreaded DASH initialization

[Lukas Fellechner <Lukas.Fellechner@gmx.net>]

> Gesendet: Montag, 05. September 2022 um 00:50 Uhr
> Von: "Andreas Rheinhardt" <andreas.rheinhardt@outlook.com>
> An: ffmpeg-devel@ffmpeg.org
> Betreff: Re: [FFmpeg-devel] [PATCH v3 2/3] lavf/dashdec: Multithreaded DASH initialization
> Lukas Fellechner:
> > Andreas Rheinhardt andreas.rheinhardt at outlook.com
> > Wed Aug 31 05:54:12 EEST 2022
> >>
> >
> >> 1. We actually have an API to process multiple tasks by different
> >> threads: Look at libavutil/slicethread.h. Why can't you reuse that?
> >> 2. In case initialization of one of the conditions/mutexes fails, you
> >> are nevertheless destroying them; you are even destroying completely
> >> uninitialized mutexes. This is undefined behaviour. Checking the result
> >> of it does not fix this.
> >>
> >> - Andreas
> >
> > 1. The slicethread implementation is pretty hard to understand.
> > I was not sure if it can be used for normal parallelization, because
> > it looked more like some kind of thread pool for continuous data
> > processing. But after taking a second look, I think I can use it here.
> > I will try and see if it works well.
> >
> > 2. I was not aware that this is undefined behavior. Will switch to
> > PTHREAD_MUTEX_INITIALIZER and PTHREAD_COND_INITIALIZER macros,
> > which return a safely initialized mutex/cond.
> >
> 
> "The behavior is undefined if the value specified by the mutex argument
> to pthread_mutex_destroy() does not refer to an initialized mutex."
> (From
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_destroy.html)
> 
> Furthermore: "In cases where default mutex attributes are appropriate,
> the macro PTHREAD_MUTEX_INITIALIZER can be used to initialize mutexes.
> The effect shall be equivalent to dynamic initialization by a call to
> pthread_mutex_init() with parameter attr specified as NULL, except that
> no error checks are performed." The last sentence sounds as if one would
> then have to check mutex locking.
> 
> Moreover, older pthread standards did not allow to use
> PTHREAD_MUTEX_INITIALIZER with non-static mutexes, so I don't know
> whether we can use that. Also our pthreads-wrapper on top of
> OS/2-threads does not provide PTHREAD_COND_INITIALIZER (which is used
> nowhere in the codebase).

I missed that detail about the initializer macro. Thank you for clearing
that up.

After looking more into threads implementation in ffmpeg, I wonder if I
really need to check any results of init/destroy or other functions.
In slicethreads.c, there is zero checking on any of the lock functions.
The pthreads-based implementation does internally check the results of all
function calls and calls abort() in case of errors ("strict_" wrappers).
The Win32 implementation uses SRW locks which cannot even return errors. 
And the OS2 implementation returns 0 on all calls as well.

So right now, I think that I should continue with normal _init() calls
(no macros) and drop all error checking, just like slicethread does.
Are you fine with that approach?
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