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

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Date:   Wed, 30 Nov 2016 09:30:36 +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 09:22 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.