[alsa-devel] async between dmaengine_pcm_dma_complete and snd_pcm_release

[Qiao Zhou]

Hi Mark, Liam, Jaroslav, Takashi

I met an issue in which kernel panic appears in 
dmaengine_pcm_dma_complete function on a quad-core system. The 
dmaengine_pcm_dma_complete is running core0, while snd_pcm_release has 
already been executed on core1, due to in low memory stress oom killer 
kills the audio thread to release some memory.

snd_pcm_release frees the runtime parameters, and runtime is used in 
dmaengine_pcm_dma_complete, which is a callback from tasklet in 
dmaengine. In current audio driver, we can't promise that 
dmaengine_pcm_dma_complete is not executed after snd_pcm_release on 
multi cores. Maybe we should add some protection. Do you have any 
suggestion?

I have tried to apply below workaround, which can fix the panic, but I'm 
not confident it's proper. Need your comment and better suggestion.


 From d568a88e8f66ee21d44324bdfb48d2a3106cf0d1 Mon Sep 17 00:00:00 2001
From: Qiao Zhou <zhouqiao at marvell.com>
Date: Wed, 9 Oct 2013 15:24:29 +0800
Subject: [PATCH] ASoC: soc-dmaengine: add mutex to protect runtime param

under SMP arch, the dmaengine_pcm_dma_complete callback has the
chance to run on one cpu while at the same time, the substream is
released on another cpu. thus it may access param which is already
freed. we need to add mutes to protect such access, and check PCM
availability before using it.

Signed-off-by: Qiao Zhou <zhouqiao at marvell.com>
---
  sound/soc/soc-dmaengine-pcm.c |   11 ++++++++++-
  1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/sound/soc/soc-dmaengine-pcm.c b/sound/soc/soc-dmaengine-pcm.c
index 111b7d9..5917029 100644
--- a/sound/soc/soc-dmaengine-pcm.c
+++ b/sound/soc/soc-dmaengine-pcm.c
@@ -125,13 +125,22 @@ EXPORT_SYMBOL_GPL(snd_hwparams_to_dma_slave_config);
  static void dmaengine_pcm_dma_complete(void *arg)
  {
  	struct snd_pcm_substream *substream = arg;
-	struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
+	struct dmaengine_pcm_runtime_data *prtd;
+
+	mutex_lock(&substream->pcm->open_mutex);
+	if (!substream || !substream->runtime) {
+		mutex_unlock(&substream->pcm->open_mutex);
+		return;
+	}
+
+	prtd = substream_to_prtd(substream);

  	prtd->pos += snd_pcm_lib_period_bytes(substream);
  	if (prtd->pos >= snd_pcm_lib_buffer_bytes(substream))
  		prtd->pos = 0;

  	snd_pcm_period_elapsed(substream);
+	mutex_unlock(&substream->pcm->open_mutex);
  }

  static int dmaengine_pcm_prepare_and_submit(struct snd_pcm_substream 
*substream)
-- 
1.7.0.4


-- 

Best Regards
Qiao