[PATCH] ALSA: pcm: retrieve true appl_ptr where stream was unlocked

[PATCH] ALSA: pcm: retrieve true appl_ptr where stream was unlocked

[Oskar Schirmer]

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.

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

Re: [PATCH] ALSA: pcm: retrieve true appl_ptr where stream was unlocked

[Takashi Iwai]

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
>