On 11/30/2016 09:30 AM, Lars-Peter Clausen wrote: > On 11/30/2016 09:22 AM, Jiada Wang wrote: >> From: Andreas Pape <ap...@de.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 <ap...@de.adit-jv.com> >> Signed-off-by: Jiada Wang <jiada_w...@mentor.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.