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SubjectRe: [PATCH] ALSA: pcm: retrieve true appl_ptr where stream was unlocked
At 12 Nov 2013 16:14:17 +0000,
Oskar Schirmer wrote:
>
> Calculated for data transfer, the local variable appl_ptr is reused,
> changed and written back later, though the lock was not held during
> the transfer earlier. Admitted, destiny of the lock is not obvious
> at all, but this looks much like a race condition candidate, so make
> sure to have some original appl_ptr value to work on.

No, appl_ptr doesn't have to be re-read at that point. Otherwise the
value will become inconsistent.

This is the write operation to a stream and it's exclusive. The lock
is needed to protect the PCM state change (e.g. stopped from an irq
handler).


thanks,

Takashi

>
> Two occurences.
>
> Signed-off-by: Oskar Schirmer <oskar@scara.com>
> ---
> sound/core/pcm_lib.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index 6e03b46..47e836a 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -2051,6 +2051,7 @@ static snd_pcm_sframes_t snd_pcm_lib_write1(struct snd_pcm_substream *substream,
> default:
> break;
> }
> + appl_ptr = runtime->control->appl_ptr;
> appl_ptr += frames;
> if (appl_ptr >= runtime->boundary)
> appl_ptr -= runtime->boundary;
> @@ -2283,6 +2284,7 @@ static snd_pcm_sframes_t snd_pcm_lib_read1(struct snd_pcm_substream *substream,
> default:
> break;
> }
> + appl_ptr = runtime->control->appl_ptr;
> appl_ptr += frames;
> if (appl_ptr >= runtime->boundary)
> appl_ptr -= runtime->boundary;
> --
> 1.7.9.5
>


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