Re: FAILED: patch "[PATCH] ALSA: timer: Fix mutex deadlock at releasing card" failed to apply to 4.19-stable tree

[Sasha Levin <sashal@kernel.org>]

On Mon, Nov 04, 2019 at 11:42:14AM +0100, Takashi Iwai wrote:
>On Mon, 04 Nov 2019 11:30:20 +0100,
>Sasha Levin wrote:
>>
>> On Sun, Nov 03, 2019 at 06:40:59PM +0100, gregkh@linuxfoundation.org wrote:
>> >
>> >The patch below does not apply to the 4.19-stable tree.
>> >If someone wants it applied there, or to any other stable or longterm
>> >tree, then please email the backport, including the original git commit
>> >id to <stable@vger.kernel.org>.
>> >
>> >thanks,
>> >
>> >greg k-h
>> >
>> >------------------ original commit in Linus's tree ------------------
>> >
>> >From a39331867335d4a94b6165e306265c9e24aca073 Mon Sep 17 00:00:00 2001
>> >From: Takashi Iwai <tiwai@suse.de>
>> >Date: Wed, 30 Oct 2019 22:42:57 +0100
>> >Subject: [PATCH] ALSA: timer: Fix mutex deadlock at releasing card
>> >
>> >When a card is disconnected while in use, the system waits until all
>> >opened files are closed then releases the card.  This is done via
>> >put_device() of the card device in each device release code.
>> >
>> >The recently reported mutex deadlock bug happens in this code path;
>> >snd_timer_close() for the timer device deals with the global
>> >register_mutex and it calls put_device() there.  When this timer
>> >device is the last one, the card gets freed and it eventually calls
>> >snd_timer_free(), which has again the protection with the global
>> >register_mutex -- boom.
>> >
>> >Basically put_device() call itself is race-free, so a relative simple
>> >workaround is to move this put_device() call out of the mutex.  For
>> >achieving that, in this patch, snd_timer_close_locked() got a new
>> >argument to store the card device pointer in return, and each caller
>> >invokes put_device() with the returned object after the mutex unlock.
>> >
>> >Reported-and-tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> >Cc: <stable@vger.kernel.org>
>> >Signed-off-by: Takashi Iwai <tiwai@suse.de>
>>
>> Looks like this was introduced by 41672c0c24a6 ("ALSA: timer: Simplify
>> error path in snd_timer_open()"), which means it's not needed on 4.19 or
>> older.
>
>We'd still need a similar fix, as the code path in question is about
>closing, not opening the device.  If backporting the commit
>41672c0c24a6 makes the fix cleanly applicable, it'd be worth to
>backport both.
>
>If not, I can submit a modified 4.19.y patch, too.

Yeah, it works for 4.19 and 4.14, I've queued it up.

The 4.9 backport requires two more commits:

9b7d869ee5a7 ("ALSA: timer: Limit max instances per timer")
988563929d5b ("ALSA: timer: Follow standard EXPORT_SYMBOL() declarations")

Does it makes sense to take them?

-- 
Thanks,
Sasha