Commit 9fc75b71 authored by Ilya Dryomov's avatar Ilya Dryomov
Browse files

rbd: eliminate a race in lock_dwork draining on unmap 



Given how rbd_lock_add_request() and rbd_img_exclusive_lock() are
written, lock_dwork may be (re)queued more than it's actually needed:
for example in case a new I/O request comes in while we are in the
middle of rbd_acquire_lock() on behalf of another I/O request.  This is
expected and with rbd_release_lock() preemptively canceling lock_dwork
is benign under normal operation.

A more problematic example is maybe_kick_acquire():

    if (have_requests || delayed_work_pending(&rbd_dev->lock_dwork)) {
            dout("%s rbd_dev %p kicking lock_dwork\n", __func__, rbd_dev);
            mod_delayed_work(rbd_dev->task_wq, &rbd_dev->lock_dwork, 0);
    }

It's not unrealistic for lock_dwork to get canceled right after
delayed_work_pending() returns true and for mod_delayed_work() to
requeue it right there anyway.  This is a classic TOCTOU race.

When it comes to unmapping the image, there is an implicit assumption
of no self-initiated exclusive lock activity past the point of return
from rbd_dev_image_unlock() which unlocks the lock if it happens to be
held.  This unlock is assumed to be final and lock_dwork (as well as
all other exclusive lock tasks, really) isn't expected to get queued
again.  However, lock_dwork is canceled only in cancel_tasks_sync()
(i.e. later in the unmap sequence) and on top of that the cancellation
can get in effect nullified by maybe_kick_acquire().  This may result
in rbd_acquire_lock() executing after rbd_dev_device_release() and
rbd_dev_image_release() run and free and/or reset a bunch of things.
One of the possible failure modes then is a violated

    rbd_assert(rbd_image_format_valid(rbd_dev->image_format));

in rbd_dev_header_info() which is called via rbd_dev_refresh() from
rbd_post_acquire_action().

Redo exclusive lock task draining to provide saner semantics and try
to meet the assumptions around rbd_dev_image_unlock().

Cc: stable@vger.kernel.org
Signed-off-by: default avatarIlya Dryomov <idryomov@gmail.com>
Reviewed-by: default avatarViacheslav Dubeyko <Slava.Dubeyko@ibm.com>
parent 5200f5f4

drivers/block/rbd.c

+8 −12
Original line number Diff line number Diff line
@@ -4565,24 +4565,12 @@ static int rbd_register_watch(struct rbd_device *rbd_dev) 
	return ret;

}



static void cancel_tasks_sync(struct rbd_device *rbd_dev)

{

	dout("%s rbd_dev %p\n", __func__, rbd_dev);



	cancel_work_sync(&rbd_dev->acquired_lock_work);

	cancel_work_sync(&rbd_dev->released_lock_work);

	cancel_delayed_work_sync(&rbd_dev->lock_dwork);

	cancel_work_sync(&rbd_dev->unlock_work);

}



/*

 * header_rwsem must not be held to avoid a deadlock with

 * rbd_dev_refresh() when flushing notifies.

 */

static void rbd_unregister_watch(struct rbd_device *rbd_dev)

{

	cancel_tasks_sync(rbd_dev);



	mutex_lock(&rbd_dev->watch_mutex);

	if (rbd_dev->watch_state == RBD_WATCH_STATE_REGISTERED)

		__rbd_unregister_watch(rbd_dev);
@@ -6548,10 +6536,18 @@ static int rbd_add_parse_args(const char *buf, 


static void rbd_dev_image_unlock(struct rbd_device *rbd_dev)

{

	dout("%s rbd_dev %p\n", __func__, rbd_dev);



	disable_delayed_work_sync(&rbd_dev->lock_dwork);

	disable_work_sync(&rbd_dev->unlock_work);



	down_write(&rbd_dev->lock_rwsem);

	if (__rbd_is_lock_owner(rbd_dev))

		__rbd_release_lock(rbd_dev);

	up_write(&rbd_dev->lock_rwsem);



	flush_work(&rbd_dev->acquired_lock_work);

	flush_work(&rbd_dev->released_lock_work);

}



/*