loop: avoid loop_validate_mutex/lo_mutex in ->release

loop: avoid loop_validate_mutex/lo_mutex in ->release

Since ->release is called with disk->open_mutex held, and __loop_clr_fd()
 from lo_release() is called via ->release when disk_openers() == 0, we are
guaranteed that "struct file" which will be passed to loop_validate_file()
via fget() cannot be the loop device __loop_clr_fd(lo, true) will clear.
Thus, there is no need to hold loop_validate_mutex from __loop_clr_fd()
if release == true.

When I made commit 3ce6e1f662a91097 ("loop: reintroduce global lock for
safe loop_validate_file() traversal"), I wrote "It is acceptable for
loop_validate_file() to succeed, for actual clear operation has not started
yet.". But now I came to feel why it is acceptable to succeed.

It seems that the loop driver was added in Linux 1.3.68, and

  if (lo->lo_refcnt > 1)
    return -EBUSY;

check in loop_clr_fd() was there from the beginning. The intent of this
check was unclear. But now I think that current

  disk_openers(lo->lo_disk) > 1

form is there for three reasons.

(1) Avoid I/O errors when some process which opens and reads from this
    loop device in response to uevent notification (e.g. systemd-udevd),
    as described in commit a1ecac3b0656a682 ("loop: Make explicit loop
    device destruction lazy"). This opener is short-lived because it is
    likely that the file descriptor used by that process is closed soon.

(2) Avoid I/O errors caused by underlying layer of stacked loop devices
    (i.e. ioctl(some_loop_fd, LOOP_SET_FD, other_loop_fd)) being suddenly
    disappeared. This opener is long-lived because this reference is
    associated with not a file descriptor but lo->lo_backing_file.

(3) Avoid I/O errors caused by underlying layer of mounted loop device
    (i.e. mount(some_loop_device, some_mount_point)) being suddenly
    disappeared. This opener is long-lived because this reference is
    associated with not a file descriptor but mount.

While race in (1) might be acceptable, (2) and (3) should be checked
racelessly. That is, make sure that __loop_clr_fd() will not run if
loop_validate_file() succeeds, by doing refcount check with global lock
held when explicit loop device destruction is requested.

As a result of no longer waiting for lo->lo_mutex after setting Lo_rundown,
we can remove pointless BUG_ON(lo->lo_state != Lo_rundown) check.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220330052917.2566582-14-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>