[FFmpeg-devel] [PATCH 1/3] avutil/get_pool: Remove redundant initial atomic operation

[Michael Niedermayer]

On Sun, Mar 17, 2013 at 06:54:35PM +0100, Hendrik Leppkes wrote:
> On Sun, Mar 17, 2013 at 6:46 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > 602->442 dezicycles
> >
> > Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
> > ---
> >  libavutil/buffer.c |    6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/libavutil/buffer.c b/libavutil/buffer.c
> > index d268a7f..3475e57 100644
> > --- a/libavutil/buffer.c
> > +++ b/libavutil/buffer.c
> > @@ -239,14 +239,14 @@ void av_buffer_pool_uninit(AVBufferPool **ppool)
> >  /* remove the whole buffer list from the pool and return it */
> >  static BufferPoolEntry *get_pool(AVBufferPool *pool)
> >  {
> > -    BufferPoolEntry *cur = NULL, *last = NULL;
> > +    BufferPoolEntry *cur = *(void * volatile *)&pool->pool, *last = NULL;
> >
> > -    do {
> > +    while (cur != last) {
> >          FFSWAP(BufferPoolEntry*, cur, last);
> >          cur = avpriv_atomic_ptr_cas((void * volatile *)&pool->pool, last, NULL);
> >          if (!cur)
> >              return NULL;
> > -    } while (cur != last);
> > +    }
> >
> >      return cur;
> >  }
> > --
> > 1.7.9.5
> >
> 
> 
> I don't think you can safely assume that the initial get is atomic
> because there is no memory barrier, and considering this function is
> called only a handful of times per frame, i would stay on the safe
> side.

It should be safe because the initial value of cur doesnt matter

consider cur is set to NULL, this can already happen as get_pool()
can always switch the whole buffer chain out making it NULL until it
gets reinserted

now consider its non NULL but wrong
while (cur != last) will be true

cur = avpriv_atomic_ptr_cas((void * volatile *)&pool->pool, last, NULL);
will compare it against the actual correct value and just do an extra
iteration

but iam not insisting on this, it just looked inefficient to me how it
was done

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The misfortune of the wise is better than the prosperity of the fool.
-- Epicurus
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