ALSA: pcm: Fix potential AB/BA lock with buffer_mutex and mmap_lock

syzbot caught a potential deadlock between the PCM
runtime->buffer_mutex and the mm->mmap_lock.  It was brought by the
recent fix to cover the racy read/write and other ioctls, and in that
commit, I overlooked a (hopefully only) corner case that may take the
revert lock, namely, the OSS mmap.  The OSS mmap operation
exceptionally allows to re-configure the parameters inside the OSS
mmap syscall, where mm->mmap_mutex is already held.  Meanwhile, the
copy_from/to_user calls at read/write operations also take the
mm->mmap_lock internally, hence it may lead to a AB/BA deadlock.

A similar problem was already seen in the past and we fixed it with a
refcount (in commit b2483716

).  The former fix covered only the
call paths with OSS read/write and OSS ioctls, while we need to cover
the concurrent access via both ALSA and OSS APIs now.

This patch addresses the problem above by replacing the buffer_mutex
lock in the read/write operations with a refcount similar as we've
used for OSS.  The new field, runtime->buffer_accessing, keeps the
number of concurrent read/write operations.  Unlike the former
buffer_mutex protection, this protects only around the
copy_from/to_user() calls; the other codes are basically protected by
the PCM stream lock.  The refcount can be a negative, meaning blocked
by the ioctls.  If a negative value is seen, the read/write aborts
with -EBUSY.  In the ioctl side, OTOH, they check this refcount, too,
and set to a negative value for blocking unless it's already being
accessed.

Reported-by:  <syzbot+6e5c88838328e99c7e1c@syzkaller.appspotmail.com>
Fixes: dca947d4

 ("ALSA: pcm: Fix races among concurrent read/write and buffer changes")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000381a0d05db622a81@google.com
Link: https://lore.kernel.org/r/20220330120903.4738-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>