block: Fix a race between request queue removal and the block cgroup controller
Avoid that the following race can occur:
blk_cleanup_queue() blkcg_print_blkgs()
spin_lock_irq(lock) (1) spin_lock_irq(blkg->q->queue_lock) (2,5)
q->queue_lock = &q->__queue_lock (3)
spin_unlock_irq(lock) (4)
spin_unlock_irq(blkg->q->queue_lock) (6)
(1) take driver lock;
(2) busy loop for driver lock;
(3) override driver lock with internal lock;
(4) unlock driver lock;
(5) can take driver lock now;
(6) but unlock internal lock.
This change is safe because only the SCSI core and the NVME core keep
a reference on a request queue after having called blk_cleanup_queue().
Neither driver accesses any of the removed data structures between its
blk_cleanup_queue() and blk_put_queue() calls.
Reported-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Jan Kara <jack@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-core.c b/block/blk-core.c
index 41c74b3..6febc69 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -719,6 +719,37 @@ void blk_cleanup_queue(struct request_queue *q)
del_timer_sync(&q->backing_dev_info->laptop_mode_wb_timer);
blk_sync_queue(q);
+ /*
+ * I/O scheduler exit is only safe after the sysfs scheduler attribute
+ * has been removed.
+ */
+ WARN_ON_ONCE(q->kobj.state_in_sysfs);
+
+ /*
+ * Since the I/O scheduler exit code may access cgroup information,
+ * perform I/O scheduler exit before disassociating from the block
+ * cgroup controller.
+ */
+ if (q->elevator) {
+ ioc_clear_queue(q);
+ elevator_exit(q, q->elevator);
+ q->elevator = NULL;
+ }
+
+ /*
+ * Remove all references to @q from the block cgroup controller before
+ * restoring @q->queue_lock to avoid that restoring this pointer causes
+ * e.g. blkcg_print_blkgs() to crash.
+ */
+ blkcg_exit_queue(q);
+
+ /*
+ * Since the cgroup code may dereference the @q->backing_dev_info
+ * pointer, only decrease its reference count after having removed the
+ * association with the block cgroup controller.
+ */
+ bdi_put(q->backing_dev_info);
+
if (q->mq_ops)
blk_mq_free_queue(q);
percpu_ref_exit(&q->q_usage_counter);
