On Wed, 13 Apr 2016, Takashi Iwai wrote:
> 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@xxxxxxxx>
> > Signed-off-by: Arvid Nihlgård Lindell <arvidnl@xxxxxxxx>
>
> Isn't this dependent on gcc version?
Do you mean that the problem is dependent on gcc version, or the solution?
If the problem only occurs with certain gcc versions, but the _atomic_*
functions are the proper solution in all cases, then I would think that is
the way to go.
> 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.
Do you mean that the inline functions should be replaced with
#define macros, or that they should be omitted entirely and the resulting
code be integrated into the relevant places in the .c files?
/Ricard
>
> > ---
> > 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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>
--
Ricard Wolf Wanderlöf ricardw(at)axis.com
Axis Communications AB, Lund, Sweden www.axis.com
Phone +46 46 272 2016 Fax +46 46 13 61 30
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