linux-kernel - Re: [PATCH] ASoC: soc-pcm: use GFP_ATOMIC in dpcm_create_debugfs

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Date:   Mon, 17 Jan 2022 20:56:31 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        alsa-devel@...a-project.org,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Takashi Iwai <tiwai@...e.com>, Mark Brown <broonie@...nel.org>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>
Subject: Re: [PATCH] ASoC: soc-pcm: use GFP_ATOMIC in dpcm_create_debugfs_state()

On Mon, 17 Jan 2022 18:11:42 +0100,
Pierre-Louis Bossart wrote:
> 
> 
> 
> On 1/17/22 2:49 AM, Takashi Iwai wrote:
> > On Sun, 16 Jan 2022 12:18:17 +0100,
> > Christophe JAILLET wrote:
> >>
> >> The commit below states that dpcm_be_connect() may be called from atomic
> >> context. It changes a GFP_KERNEL into a GFP_ATOMIC to deal with it.
> >>
> >> Another memory allocation is done in dpcm_create_debugfs_state() which is
> >> called by dpcm_be_connect(). Also use GFP_ATOMIC there to be consistent
> >> and be compliant with atomic context.
> >>
> >> Fixes: d8a9c6e1f676 ("ASoC: soc-pcm: use GFP_ATOMIC for dpcm structure")
> >> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> >> ---
> >> Not clear to me how dpcm_be_connect() can be called from an atomic context,
> >> though. But better safe than sorry.
> > 
> > I don't think this no longer valid for the very latest code.
> > The commit b7898396f4bb dropped the spurious dpcm_lock spinlock, so
> > the code path you touched must be always sleepable.
> > 
> > Similarly, the commit d8a9c6e1f676 can be reverted now.
> 
> Can we really revert d8a9c6e1f676?
> 
> We did propagate the non-atomic FE property to the BE, but if both FE
> and BE are both atomic that constraint would be required, no?

At the kzalloc() call in dpcm_be_connect, there is no spin lock
involved.  It's merely protected by card->pcm_mutex, instead.  The
spinlock is applied at the later call with
snd_soc_pcm_stream_lock_irq() only for the list manipulations.  (See
it's *_irq(), not *_irqsave(); that means the context being sleepable
at that point.)  So, we can use GFP_KERNEL safely there.

GFP_ATOMIC was needed in the past where dpcm_be_connect() itself is
called in dpcm_lock spinlock.  It was removed recently.


Takashi

> 
> 
> > 
> > thanks,
> > 
> > Takashi
> > 
> >> ---
> >>  sound/soc/soc-pcm.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/sound/soc/soc-pcm.c b/sound/soc/soc-pcm.c
> >> index 7abfc48b26ca..1a536a2b9dc3 100644
> >> --- a/sound/soc/soc-pcm.c
> >> +++ b/sound/soc/soc-pcm.c
> >> @@ -212,7 +212,7 @@ static void dpcm_create_debugfs_state(struct snd_soc_dpcm *dpcm, int stream)
> >>  {
> >>  	char *name;
> >>  
> >> -	name = kasprintf(GFP_KERNEL, "%s:%s", dpcm->be->dai_link->name,
> >> +	name = kasprintf(GFP_ATOMIC, "%s:%s", dpcm->be->dai_link->name,
> >>  			 stream ? "capture" : "playback");
> >>  	if (name) {
> >>  		dpcm->debugfs_state = debugfs_create_dir(
> >> -- 
> >> 2.32.0
> >>
>