[PATCH 1/1] ALSA: SOC: DMA: increment buffer pointer atomically

[PATCH 1/1] ALSA: SOC: DMA: increment buffer pointer atomically

[Jiada Wang]

From: Andreas Pape <apape@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.

Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Jiada Wang <jiada_wang@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;
 
 	snd_pcm_period_elapsed(substream);
 }
-- 
2.9.3

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

[Lars-Peter Clausen]

On 11/30/2016 09:22 AM, Jiada Wang wrote:
> From: Andreas Pape <apape@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 <apape@de.adit-jv.com>
> Signed-off-by: Jiada Wang <jiada_wang@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.

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

[Lars-Peter Clausen]

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@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 <apape@de.adit-jv.com>
>> Signed-off-by: Jiada Wang <jiada_wang@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.

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

[Lars-Peter Clausen]

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 <apape@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 <apape@de.adit-jv.com>
>>> Signed-off-by: Jiada Wang <jiada_wang@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.

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.