[alsa-devel] [RFC PATCH] ALSA: pcm: fix blocking while loop in snd_pcm_update_hw_ptr0()

[Takashi Iwai]

On Wed, 23 Oct 2019 14:01:14 +0200,
Srinivas Kandagatla wrote:
> 
> From: Preetam Singh Ranawat <apranawat at codeaurora.org>
> 
> When period interrupts are disabled, while loop in snd_pcm_update_hw_ptr0()
> results in the machine locking up if runtime->hw_ptr_buffer_jiffies is 0.
> 
> Validate runtime->hw_ptr_buffer_jiffies value before while loop to avoid
> delta check.

Although the fix itself is fine, I still wonder how this is
triggered.  Could you check in what kind of PCM state is it?

Basically snd_pcm_update_hw_ptr*() is called only in the running PCM
state, and for that, we should have set hw_ptr_buffer_jiffies at
snd_pcm_post_start().  It's

	runtime->hw_ptr_buffer_jiffies = (runtime->buffer_size * HZ) / 
							    runtime->rate;

So maybe the given buffer size is so small and hw_ptr_buffer_jiffies
is really 0?


thanks,

Takashi

> 
> Signed-off-by: Preetam Singh Ranawat <apranawat at codeaurora.org>
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla at linaro.org>
> ---
> 
> This issue was noticed in Android setup from fastcapture path.
> 
>  sound/core/pcm_lib.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index d80041ea4e01..f407f6895f8a 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -341,7 +341,8 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
>  		 * the elapsed time to detect xruns.
>  		 */
>  		jdelta = curr_jiffies - runtime->hw_ptr_jiffies;
> -		if (jdelta < runtime->hw_ptr_buffer_jiffies / 2)
> +		if ((jdelta < runtime->hw_ptr_buffer_jiffies / 2) ||
> +		    !runtime->hw_ptr_buffer_jiffies)
>  			goto no_delta_check;
>  		hdelta = jdelta - delta * HZ / runtime->rate;
>  		xrun_threshold = runtime->hw_ptr_buffer_jiffies / 2 + 1;
> -- 
> 2.21.0
>