md/raid5-cache: fix a deadlock in r5l_exit_log()

Commit b13015af ("md/raid5-cache: Clear conf->log after finishing
work") introduce a new problem:

// caller hold reconfig_mutex
r5l_exit_log
 flush_work(&log->disable_writeback_work)
			r5c_disable_writeback_async
			 wait_event
			  /*
			   * conf->log is not NULL, and mddev_trylock()
			   * will fail, wait_event() can never pass.
			   */
 conf->log = NULL

Fix this problem by setting 'config->log' to NULL before wake_up() as it
used to be, so that wait_event() from r5c_disable_writeback_async() can
exist. In the meantime, move forward md_unregister_thread() so that
null-ptr-deref this commit fixed can still be fixed.

Fixes: b13015af ("md/raid5-cache: Clear conf->log after finishing work")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20230708091727.1417894-1-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/raid5-cache.c

@@ -3168,12 +3168,15 @@ void r5l_exit_log(struct r5conf *conf)
 {
 	struct r5l_log *log = conf->log;
 
-	/* Ensure disable_writeback_work wakes up and exits */
-	wake_up(&conf->mddev->sb_wait);
-	flush_work(&log->disable_writeback_work);
 	md_unregister_thread(&log->reclaim_thread);
 
+	/*
+	 * 'reconfig_mutex' is held by caller, set 'confg->log' to NULL to
+	 * ensure disable_writeback_work wakes up and exits.
+	 */
 	conf->log = NULL;
+	wake_up(&conf->mddev->sb_wait);
+	flush_work(&log->disable_writeback_work);
 
 	mempool_exit(&log->meta_pool);
 	bioset_exit(&log->bs);