Re: [PATCH 1/1] ALSA: SOC: DMA: increment buffer pointer atomically
On 11/30/2016 09:36 AM, Lars-Peter Clausen wrote:
> On 11/30/2016 09:30 AM, Lars-Peter Clausen wrote:
>> On 11/30/2016 09:22 AM, Jiada Wang wrote:
>>> From: Andreas Pape
>>>
>>> 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
>>> Signed-off-by: Jiada Wang
>>> ---
>>> 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.
Having given this some additional thoughts, I think a READ_ONCE() in
dmaengine_pcm_dma_complete() is not necessary. dmaengine_pcm_dma_complete()
is the only writer of prtd->pos and it is not running concurrently to
itself. So we'll always observe consistent state, even if the compiler
decides to issue multiple reads. The WRITE_ONCE() is required to make sure
that the prtd->pos state stays consistent to concurrent readers. And the
READ_ONCE() in snd_dmaengine_pcm_pointer_no_residue() is required to make
sure that consistent state is observed from concurrent writers.
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
>>
>> 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
>> Signed-off-by: Jiada Wang
>> ---
>> 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.
Re: [PATCH 1/1] ALSA: SOC: DMA: increment buffer pointer atomically
On 11/30/2016 09:22 AM, Jiada Wang wrote:
> From: Andreas Pape
>
> 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
> Signed-off-by: Jiada Wang
> ---
> 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.
