From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
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 bfd21af7aa38b..ea3b0e055657f 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1137,27 +1137,6 @@ 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);
@@ -1244,11 +1223,20 @@ 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;
}
/*
@@ -1263,11 +1251,11 @@ static int loop_clr_fd(struct loop_device *lo)
*/
if (atomic_read(&lo->lo_refcnt) > 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;
--
2.30.2
