Re: [PATCH 15/14] loop: avoid loop_validate_mutex/lo_mutex in ->release
From: Jan Kara <jack@suse.cz>
Date: 2022-03-30 08:14:18
On Wed 30-03-22 00:36:49, Tetsuo Handa wrote:
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>Yeah, the patch makes sense to me. Feel free to add: Reviewed-by: Jan Kara <jack@suse.cz> Honza
quoted hunk ↗ jump to hunk
--- drivers/block/loop.c | 38 +++++++++++++------------------------- 1 file changed, 13 insertions(+), 25 deletions(-)diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 2506193a4fd1..6b813c592159 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c@@ -1135,27 +1135,6 @@ static void __loop_clr_fd(struct loop_device *lo, bool release) struct file *filp; gfp_t gfp = lo->old_gfp_mask; - /* - * Flush loop_configure() and loop_change_fd(). It is acceptable for - * loop_validate_file() to succeed, for actual clear operation has not - * started yet. - */ - mutex_lock(&loop_validate_mutex); - mutex_unlock(&loop_validate_mutex); - /* - * loop_validate_file() now fails because l->lo_state != Lo_bound - * became visible. - */ - - /* - * Since this function is called upon "ioctl(LOOP_CLR_FD)" xor "close() - * after ioctl(LOOP_CLR_FD)", it is a sign of something going wrong if - * lo->lo_state has changed while waiting for lo->lo_mutex. - */ - mutex_lock(&lo->lo_mutex); - BUG_ON(lo->lo_state != Lo_rundown); - mutex_unlock(&lo->lo_mutex); - if (test_bit(QUEUE_FLAG_WC, &lo->lo_queue->queue_flags)) blk_queue_write_cache(lo->lo_queue, false, false);@@ -1238,11 +1217,20 @@ static int loop_clr_fd(struct loop_device *lo) { int err; - err = mutex_lock_killable(&lo->lo_mutex); + /* + * Since lo_ioctl() is called without locks held, it is possible that + * loop_configure()/loop_change_fd() and loop_clr_fd() run in parallel. + * + * Therefore, use global lock when setting Lo_rundown state in order to + * make sure that loop_validate_file() will fail if the "struct file" + * which loop_configure()/loop_change_fd() found via fget() was this + * loop device. + */ + err = loop_global_lock_killable(lo, true); if (err) return err; if (lo->lo_state != Lo_bound) { - mutex_unlock(&lo->lo_mutex); + loop_global_unlock(lo, true); return -ENXIO; } /*@@ -1257,11 +1245,11 @@ static int loop_clr_fd(struct loop_device *lo) */ if (disk_openers(lo->lo_disk) > 1) { lo->lo_flags |= LO_FLAGS_AUTOCLEAR; - mutex_unlock(&lo->lo_mutex); + loop_global_unlock(lo, true); return 0; } lo->lo_state = Lo_rundown; - mutex_unlock(&lo->lo_mutex); + loop_global_unlock(lo, true); __loop_clr_fd(lo, false); return 0;-- 2.32.0
-- Jan Kara [off-list ref] SUSE Labs, CR