md/r5cache: assign conf->log before r5l_load_log()

r5l_load_log() calls functions that requires a proper conf->log,
for example, r5c_is_writeback(). Therefore, we should set
conf->log before calling r5l_load_log(). If r5l_load_log() fails,
conf->log is set back to NULL.
Signed-off-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/raid5-cache.c

@@ -2633,14 +2633,16 @@ int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
 	spin_lock_init(&log->stripe_in_journal_lock);
 	atomic_set(&log->stripe_in_journal_count, 0);
 
+	rcu_assign_pointer(conf->log, log);
+
 	if (r5l_load_log(log))
 		goto error;
 
-	rcu_assign_pointer(conf->log, log);
 	set_bit(MD_HAS_JOURNAL, &conf->mddev->flags);
 	return 0;
 
 error:
+	rcu_assign_pointer(conf->log, NULL);
 	md_unregister_thread(&log->reclaim_thread);
 reclaim_thread:
 	mempool_destroy(log->meta_pool);