linux-kernel - [PATCH v2 1/1] ALSA: SOC: DMA: access buffer pointer atomically

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Message-ID: <20161201052422.18775-1-jiada_wang@mentor.com>
Date:   Thu, 1 Dec 2016 14:24:22 +0900
From:   Jiada Wang <jiada_wang@...tor.com>
To:     <lars@...afoo.de>, <perex@...ex.cz>, <tiwai@...e.com>
CC:     <alsa-devel@...a-project.org>, <linux-kernel@...r.kernel.org>,
        <apape@...adit-jv.com>, <jiada_wang@...tor.com>
Subject: [PATCH v2 1/1] ALSA: SOC: DMA: access buffer pointer atomically

From: Andreas Pape <apape@...adit-jv.com>

Setting pointer and afterwards check for wrap around leads
to the possibility of returning the inconsistent pointer position.

This patch accesses buffer pointer atomically to avoid this issue.

v2: use READ_ONCE and WRITE_ONCE to ensure read/write of buffer pointer in
    only one access
    also updates snd_dmaengine_pcm_pointer_no_residue to ensure atomic
    access

Signed-off-by: Andreas Pape <apape@...adit-jv.com>
Signed-off-by: Jiada Wang <jiada_wang@...tor.com>
---
 sound/core/pcm_dmaengine.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/sound/core/pcm_dmaengine.c b/sound/core/pcm_dmaengine.c
index 8eb58c7..b6255fd 100644
--- a/sound/core/pcm_dmaengine.c
+++ b/sound/core/pcm_dmaengine.c
@@ -139,12 +139,14 @@ EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_set_config_from_dai_data);
 
 static void dmaengine_pcm_dma_complete(void *arg)
 {
+	unsigned int new_pos;
 	struct snd_pcm_substream *substream = arg;
 	struct dmaengine_pcm_runtime_data *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;
+	new_pos = prtd->pos + snd_pcm_lib_period_bytes(substream);
+	if (new_pos >= snd_pcm_lib_buffer_bytes(substream))
+		new_pos = 0;
+	WRITE_ONCE(prtd->pos, new_pos);
 
 	snd_pcm_period_elapsed(substream);
 }
@@ -235,7 +237,7 @@ EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_trigger);
 snd_pcm_uframes_t snd_dmaengine_pcm_pointer_no_residue(struct snd_pcm_substream *substream)
 {
 	struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
-	return bytes_to_frames(substream->runtime, prtd->pos);
+	return bytes_to_frames(substream->runtime, READ_ONCE(prtd->pos));
 }
 EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_pointer_no_residue);
 
-- 
2.9.3