linux-kernel - Re: [PATCH 1/1] ALSA: SOC: DMA: increment buffer pointer atomically

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Date:   Wed, 30 Nov 2016 09:36:16 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Jiada Wang <jiada_wang@...tor.com>, perex@...ex.cz, tiwai@...e.com
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        apape@...adit-jv.com
Subject: Re: [PATCH 1/1] ALSA: SOC: DMA: increment buffer pointer atomically

On 11/30/2016 09:30 AM, Lars-Peter Clausen wrote:
> On 11/30/2016 09:22 AM, Jiada Wang wrote:
>> 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 increments buffer pointer atomically to avoid this issue.
> 
> Makes sense.
> 
>>
>> Signed-off-by: Andreas Pape <apape@...adit-jv.com>
>> Signed-off-by: Jiada Wang <jiada_wang@...tor.com>
>> ---
>>  sound/core/pcm_dmaengine.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/sound/core/pcm_dmaengine.c b/sound/core/pcm_dmaengine.c
>> index 8eb58c7..6f6da11 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;
>> +	prtd->pos = new_pos;
> 
> But to really make it atomic I think this needs READ_ONCE/WRITE_ONCE.

And the access to prtd->pos in snd_dmaengine_pcm_pointer_no_residue() should
also use READ_ONCE(). It is very unlikely that the code gets mis-compiled to
generate more than one access, but having READ_ONCE() acts as a annotation
that makes it explicit that this is data that can be updated concurrently
without further synchronization.