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

In the Linux kernel, the following vulnerability has been resolved:

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 b248371628aa). 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.