[PATCH 1/2] iosched: fix typo and barrier()

On rmmod path, cfq/as waits to make sure all io-contexts was
freed. However, it's using complete(), not wait_for_completion().

I think barrier() is not enough in here. To avoid the following case,
this patch replaces barrier() with smb_wmb().

	cpu0			visibility			cpu1
	                [ioc_gnone=NULL,ioc_count=1]

ioc_gnone = &all_gone		NULL,ioc_count=1
atomic_read(&ioc_count)		NULL,ioc_count=1
wait_for_completion()		NULL,ioc_count=0	atomic_sub_and_test()
				NULL,ioc_count=0	if ( && ioc_gone)
						    [ioc_gone==NULL,
						    so doesn't call complete()]
			   &all_gone,ioc_count=0
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jens Axboe <axboe@suse.de>

block/as-iosched.c

@@ -1844,9 +1844,10 @@ static void __exit as_exit(void)
 	DECLARE_COMPLETION(all_gone);
 	elv_unregister(&iosched_as);
 	ioc_gone = &all_gone;
-	barrier();
+	/* ioc_gone's update must be visible before reading ioc_count */
+	smp_wmb();
 	if (atomic_read(&ioc_count))
-		complete(ioc_gone);
+		wait_for_completion(ioc_gone);
 	synchronize_rcu();
 	kmem_cache_destroy(arq_pool);
 }

block/cfq-iosched.c

@@ -2439,9 +2439,10 @@ static void __exit cfq_exit(void)
 	DECLARE_COMPLETION(all_gone);
 	elv_unregister(&iosched_cfq);
 	ioc_gone = &all_gone;
-	barrier();
+	/* ioc_gone's update must be visible before reading ioc_count */
+	smp_wmb();
 	if (atomic_read(&ioc_count))
-		complete(ioc_gone);
+		wait_for_completion(ioc_gone);
 	synchronize_rcu();
 	cfq_slab_kill();
 }