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@xxxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxxx>
>> Signed-off-by: Jiada Wang <jiada_wang@xxxxxxxxxx>
>> ---
>> 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.
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