linux-kernel - Re: [PATCH] ALSA: pcm: retrieve true appl

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Date:	Tue, 12 Nov 2013 17:29:51 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	"Oskar Schirmer" <oskar@...ra.com>
Cc:	"Jaroslav Kysela" <perex@...ex.cz>, linux-kernel@...r.kernel.org,
	alsa-devel@...a-project.org,
	"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: [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@...ra.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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