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 <jsl...@suse.cz>
> Cc: Jaroslav Kysela <pe...@perex.cz>
> Cc: Takashi Iwai <ti...@suse.de>
> Cc: alsa-de...@alsa-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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