bcache: improve bcache_reboot()

BugLink: https://bugs.launchpad.net/bugs/1784665

This patch tries to release mutex bch_register_lock early, to give
chance to stop cache set and bcache device early.

This patch also expends time out of stopping all bcache device from
2 seconds to 10 seconds, because stopping writeback rate update worker
may delay for 5 seconds, 2 seconds is not enough.

After this patch applied, stopping bcache devices during system reboot
or shutdown is very hard to be observed any more.
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit eb8cbb6d)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/md/bcache/super.c

@@ -2106,10 +2106,19 @@ static int bcache_reboot(struct notifier_block *n, unsigned long code, void *x)
 		list_for_each_entry_safe(dc, tdc, &uncached_devices, list)
 			bcache_device_stop(&dc->disk);
 
+		mutex_unlock(&bch_register_lock);
+
+		/*
+		 * Give an early chance for other kthreads and
+		 * kworkers to stop themselves
+		 */
+		schedule();
+
 		/* What's a condition variable? */
 		while (1) {
-			long timeout = start + 2 * HZ - jiffies;
+			long timeout = start + 10 * HZ - jiffies;
 
+			mutex_lock(&bch_register_lock);
 			stopped = list_empty(&bch_cache_sets) &&
 				list_empty(&uncached_devices);
 
@@ -2121,7 +2130,6 @@ static int bcache_reboot(struct notifier_block *n, unsigned long code, void *x)
 
 			mutex_unlock(&bch_register_lock);
 			schedule_timeout(timeout);
-			mutex_lock(&bch_register_lock);
 		}
 
 		finish_wait(&unregister_wait, &wait);