[PATCH] rbd: don't hold lock_rwsem while running_list is being drained

[Ilya Dryomov <idryomov@gmail.com>]

Currently rbd_quiesce_lock() holds lock_rwsem for read while blocking
on releasing_wait completion.  On the I/O completion side, each image
request also needs to take lock_rwsem for read.  Because rw_semaphore
implementation doesn't allow new readers after a writer has indicated
interest in the lock, this can result in a deadlock if something that
needs to take lock_rwsem for write gets involved.  For example:

1. watch error occurs
2. rbd_watch_errcb() takes lock_rwsem for write, clears owner_cid and
   releases lock_rwsem
3. after reestablishing the watch, rbd_reregister_watch() takes
   lock_rwsem for write and calls rbd_reacquire_lock()
4. rbd_quiesce_lock() downgrades lock_rwsem to for read and blocks on
   releasing_wait until running_list becomes empty
5. another watch error occurs
6. rbd_watch_errcb() blocks trying to take lock_rwsem for write
7. no in-flight image request can complete and delete itself from
   running_list because lock_rwsem won't be granted anymore

A similar scenario can occur with "lock has been acquired" and "lock
has been released" notification handers which also take lock_rwsem for
write to update owner_cid.

We don't actually get anything useful from sitting on lock_rwsem in
rbd_quiesce_lock() -- owner_cid updates certainly don't need to be
synchronized with.  In fact the whole owner_cid tracking logic could
probably be removed from the kernel client because we don't support
proxied maintenance operations.

Cc: stable@vger.kernel.org # 5.3+
URL: https://tracker.ceph.com/issues/42757
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
---
 drivers/block/rbd.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index a3caecae5d6a..8a6b7997fb1e 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -4100,8 +4100,6 @@ static void rbd_acquire_lock(struct work_struct *work)
 
 static bool rbd_quiesce_lock(struct rbd_device *rbd_dev)
 {
-	bool need_wait;
-
 	dout("%s rbd_dev %p\n", __func__, rbd_dev);
 	lockdep_assert_held_write(&rbd_dev->lock_rwsem);
 
@@ -4113,11 +4111,11 @@ static bool rbd_quiesce_lock(struct rbd_device *rbd_dev)
 	 */
 	rbd_dev->lock_state = RBD_LOCK_STATE_RELEASING;
 	rbd_assert(!completion_done(&rbd_dev->releasing_wait));
-	need_wait = !list_empty(&rbd_dev->running_list);
-	downgrade_write(&rbd_dev->lock_rwsem);
-	if (need_wait)
-		wait_for_completion(&rbd_dev->releasing_wait);
-	up_read(&rbd_dev->lock_rwsem);
+	if (list_empty(&rbd_dev->running_list))
+		return true;
+
+	up_write(&rbd_dev->lock_rwsem);
+	wait_for_completion(&rbd_dev->releasing_wait);
 
 	down_write(&rbd_dev->lock_rwsem);
 	if (rbd_dev->lock_state != RBD_LOCK_STATE_RELEASING)
-- 
2.19.2