[alsa-devel] [PATCH] alsa-lib:Make snd_atomic_write_* truly atomic

[Takashi Iwai]

On Wed, 13 Apr 2016 12:42:01 +0200,
Andreas x Larsson wrote:
> 
> Under some usecases a race condition appears inside
> the snd_atomic_write_* functions. The 'begin' and 'end'
> variables are updated with the ++ operator which is not
> atomic but needs to be. This can be achieved with the
> gcc atomic_* built-ins. Combined with __ATOMIC_SEQ_CST
> as the memory model, memory barriers are introduced so
> those can also be removed from the code.
> 
> Signed-off-by: Andreas x Larsson <andrelan at axis.com>
> Signed-off-by: Arvid Nihlgård Lindell <arvidnl at axis.com>

Isn't this dependent on gcc version?

And, if we use the gcc atomic operation, the whole macros should be
rewritten in another way -- or consider to drop the whole.  It's only
partly used in pcm_rate.c and pcm_plugin.c in anyway.


thanks,

Takashi

> ---
>   alsa-lib/include/iatomic.h | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/alsa-lib/include/iatomic.h b/alsa-lib/include/iatomic.h
> index acdd3e2..7fafe20 100644
> --- a/alsa-lib/include/iatomic.h
> +++ b/alsa-lib/include/iatomic.h
> @@ -140,14 +140,12 @@ static __inline__ void 
> snd_atomic_write_init(snd_atomic_write_t *w)
> 
>   static __inline__ void snd_atomic_write_begin(snd_atomic_write_t *w)
>   {
> -   w->begin++;
> -   wmb();
> +   __atomic_add_fetch(&w->begin, 1,__ATOMIC_SEQ_CST);
>   }
> 
>   static __inline__ void snd_atomic_write_end(snd_atomic_write_t *w)
>   {
> -   wmb();
> -   w->end++;
> +   __atomic_add_fetch(&w->end, 1, __ATOMIC_SEQ_CST);
>   }
> 
>   static __inline__ void snd_atomic_read_init(snd_atomic_read_t *r, 
> snd_atomic_write_t *w)
> -- 
> 2.1.4
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