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

[Andreas x Larsson]

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>
---
  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