tree 9085feda04eae0d109ff291000abfb9779fe19e9
parent b86c3b9c6b902761c97b0c54c72523245ad04f44
author Takashi Iwai <tiwai@suse.de> 1572471777 +0100
committer Todd Kjos <tkjos@google.com> 1586197439 -0700

ALSA: timer: Fix mutex deadlock at releasing card

[ Upstream commit a39331867335d4a94b6165e306265c9e24aca073 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>