linux-kernel - Re: [PATCH 3/3] sound: dummy, avoid races with timer

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Date:	Mon, 19 Jan 2015 10:49:01 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	linux-kernel@...r.kernel.org, Jaroslav Kysela <perex@...ex.cz>,
	alsa-devel@...a-project.org
Subject: Re: [PATCH 3/3] sound: dummy, avoid races with timer

At Mon, 19 Jan 2015 10:42:56 +0100,
Jiri Slaby wrote:
> 
> There is a race between timer and process contexts. Process context
> does not disable irqs, so when a timer ticks inside process' critical
> section, the system can deadlock. Fix this by a traditional _irqsave
> variant of spin_lock.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Jaroslav Kysela <perex@...ex.cz>
> Cc: Takashi Iwai <tiwai@...e.de>
> Cc: alsa-devel@...a-project.org
> ---
>  sound/drivers/dummy.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
> index 6d6bf4093583..18a7cfc99078 100644
> --- a/sound/drivers/dummy.c
> +++ b/sound/drivers/dummy.c
> @@ -271,10 +271,13 @@ static void dummy_systimer_update(struct dummy_systimer_pcm *dpcm)
>  static int dummy_systimer_start(struct snd_pcm_substream *substream)
>  {
>  	struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
> -	spin_lock(&dpcm->lock);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&dpcm->lock, flags);
>  	dpcm->base_time = jiffies;
>  	dummy_systimer_rearm(dpcm);
> -	spin_unlock(&dpcm->lock);
> +	spin_unlock_irqrestore(&dpcm->lock, flags);
> +

This looks correct, but...


>  	return 0;
>  }
>  
> @@ -322,12 +325,14 @@ static snd_pcm_uframes_t
>  dummy_systimer_pointer(struct snd_pcm_substream *substream)
>  {
>  	struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
> +	unsigned long flags;
>  	snd_pcm_uframes_t pos;
>  
> -	spin_lock(&dpcm->lock);
> +	spin_lock_irqsave(&dpcm->lock, flags);
>  	dummy_systimer_update(dpcm);
>  	pos = dpcm->frac_pos / HZ;
> -	spin_unlock(&dpcm->lock);
> +	spin_unlock_irqrestore(&dpcm->lock, flags);
> +

This chunk is superfluous.  The pointer callback is guaranteed to be
called in the irq-disabled context.


thanks,

Takashi
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