[FFmpeg-devel] [PATCH v3 2/3] libavcodec/utils.c: simplify avcodec locking with atomics

[Rostislav Pehlivanov]

On 25 November 2017 at 18:40, James Almer <jamrial at gmail.com> wrote:

> On 11/25/2017 2:01 PM, Rostislav Pehlivanov wrote:
> > Also makes it more robust than using volatiles.
> >
> > Signed-off-by: Rostislav Pehlivanov <atomnuker at gmail.com>
> > ---
> >  libavcodec/internal.h |  1 -
> >  libavcodec/utils.c    | 12 ++++++------
> >  2 files changed, 6 insertions(+), 7 deletions(-)
> >
> > diff --git a/libavcodec/internal.h b/libavcodec/internal.h
> > index d3310b6afe..1c54966f37 100644
> > --- a/libavcodec/internal.h
> > +++ b/libavcodec/internal.h
> > @@ -246,7 +246,6 @@ int ff_init_buffer_info(AVCodecContext *s, AVFrame
> *frame);
> >
> >  void ff_color_frame(AVFrame *frame, const int color[4]);
> >
> > -extern volatile int ff_avcodec_locked;
> >  int ff_lock_avcodec(AVCodecContext *log_ctx, const AVCodec *codec);
> >  int ff_unlock_avcodec(const AVCodec *codec);
> >
> > diff --git a/libavcodec/utils.c b/libavcodec/utils.c
> > index 3a0f3c11f5..96bc9ff4a4 100644
> > --- a/libavcodec/utils.c
> > +++ b/libavcodec/utils.c
> > @@ -114,7 +114,7 @@ static int (*lockmgr_cb)(void **mutex, enum AVLockOp
> op) = NULL;
> >  #endif
> >
> >
> > -volatile int ff_avcodec_locked;
> > +static atomic_bool ff_avcodec_locked;
> >  static atomic_int entangled_thread_counter = ATOMIC_VAR_INIT(0);
> >  static void *codec_mutex;
> >  static void *avformat_mutex;
> > @@ -1937,6 +1937,7 @@ int av_lockmgr_register(int (*cb)(void **mutex,
> enum AVLockOp op))
> >
> >  int ff_lock_avcodec(AVCodecContext *log_ctx, const AVCodec *codec)
> >  {
> > +    _Bool exp = 1;
> >      if (codec->caps_internal & FF_CODEC_CAP_INIT_THREADSAFE ||
> !codec->init)
> >          return 0;
> >
> > @@ -1952,22 +1953,21 @@ int ff_lock_avcodec(AVCodecContext *log_ctx,
> const AVCodec *codec)
> >                 atomic_load(&entangled_thread_counter));
> >          if (!lockmgr_cb)
> >              av_log(log_ctx, AV_LOG_ERROR, "No lock manager is set,
> please see av_lockmgr_register()\n");
> > -        ff_avcodec_locked = 1;
> > +        atomic_store(&ff_avcodec_locked, 1);
> >          ff_unlock_avcodec(codec);
> >          return AVERROR(EINVAL);
> >      }
> > -    av_assert0(!ff_avcodec_locked);
> > -    ff_avcodec_locked = 1;
> > +    av_assert0(atomic_compare_exchange_strong(&ff_avcodec_locked,
> &exp, 1));
>
> _Bool atomic_compare_exchange_strong( volatile A* obj,
>                                       C* expected, C desired );
>
> "Atomically compares the value pointed to by obj with the value pointed
> to by expected, and if those are equal, replaces the former with desired
> (performs read-modify-write operation).
> Return value: The result of the comparison: true if *obj was equal to
> *exp, false otherwise."
>
> exp should be 0. You need to assert that ff_avcodec_locked == 0, then
> set it to 1.
>

The whole experssion seems fine to me as-is and works as expected.


>
> >      return 0;
> >  }
> >
> >  int ff_unlock_avcodec(const AVCodec *codec)
> >  {
> > +    _Bool exp = 0;
> >      if (codec->caps_internal & FF_CODEC_CAP_INIT_THREADSAFE ||
> !codec->init)
> >          return 0;
> >
> > -    av_assert0(ff_avcodec_locked);
> > -    ff_avcodec_locked = 0;
> > +    av_assert0(atomic_compare_exchange_strong(&ff_avcodec_locked,
> &exp, 0));
>
> And here exp should be 1.
>
> >      atomic_fetch_add(&entangled_thread_counter, -1);
> >      if (lockmgr_cb) {
> >          if ((*lockmgr_cb)(&codec_mutex, AV_LOCK_RELEASE))
> >
>
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>

Seems like I sent an older version of the patch (had to rebase to fix older
commit), attached patch fixed both.
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