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Subject: Re: [RFC PATCH v3 05/13] ASoC: soc-pcm: align BE 'atomicity' with
 that of the FE
To:     Takashi Iwai <tiwai@suse.de>
Cc:     alsa-devel@alsa-project.org,
        Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>,
        open list <linux-kernel@vger.kernel.org>,
        Sameer Pujar <spujar@nvidia.com>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Takashi Iwai <tiwai@suse.com>, vkoul@kernel.org,
        broonie@kernel.org, Gyeongtaek Lee <gt82.lee@samsung.com>,
        Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
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From:   Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Message-ID: <c0803288-efb1-aaeb-218f-e1a6ba87abd7@linux.intel.com>
Date:   Fri, 15 Oct 2021 12:08:48 -0500
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>> I have not been able to figure out when you need
>> a) the pcm_mutex only
>> b) the fe stream lock only
>> c) both pcm_mutex and fe stream lock
> 
> The pcm_mutex is needed for every sleepable function that treat DPCM
> FE link, but the mutex is taken only at the upper level, i.e. the
> top-most caller like PCM ops FE itself or the DAPM calls.
> 
> That said, pcm_mutex is the top-most protection of BE links in FE.
> But, there is a code path where a mutex can't be used, and that's the
> FE and BE trigger.  For protecting against this, the FE stream lock is
> taken only at the placing both adding and deleting a BE *in addition*.
> At those places, both pcm_mutex and FE stream lock are taken.
> 
> BE stream lock is taken in addition below the above mutex and FE
> locks.

Thanks Takashi, now I get the idea. Makes sense indeed. I'll make sure
to add this explanation to the commit message/cover so that it's not lost.

I added your three patches to my tests, so far so good, code is that
https://github.com/thesofproject/linux/pull/3215

Thanks and have a nice week-end.
-Pierre