md/raid5: simplfy delaying of writes while metadata is updated.

If a device fails during a write, we must ensure the failure is
recorded in the metadata before the completion of the write is
acknowleged.

Commit c3cce6cd ("md/raid5: ensure device failure recorded before
write request returns.")  added code for this, but it was
unnecessarily complicated.  We already had similar functionality for
handling updates to the bad-block-list, thanks to Commit de393cde
("md: make it easier to wait for bad blocks to be acknowledged.")

So revert most of the former commit, and instead avoid collecting
completed writes if MD_CHANGE_PENDING is set.  raid5d() will then flush
the metadata and retry the stripe_head.
As this change can leave a stripe_head ready for handling immediately
after handle_active_stripes() returns, we change raid5_do_work() to
pause when MD_CHANGE_PENDING is set, so that it doesn't spin.

We check MD_CHANGE_PENDING *after* analyse_stripe() as it could be set
asynchronously.  After analyse_stripe(), we have collected stable data
about the state of devices, which will be used to make decisions.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5.c

@@ -4691,7 +4691,8 @@ static void handle_stripe(struct stripe_head *sh)
 	if (test_bit(STRIPE_LOG_TRAPPED, &sh->state))
 		goto finish;
 
-	if (s.handle_bad_blocks) {
+	if (s.handle_bad_blocks ||
+	    test_bit(MD_SB_CHANGE_PENDING, &conf->mddev->sb_flags)) {
 		set_bit(STRIPE_HANDLE, &sh->state);
 		goto finish;
 	}
@@ -5021,15 +5022,8 @@ static void handle_stripe(struct stripe_head *sh)
 			md_wakeup_thread(conf->mddev->thread);
 	}
 
-	if (!bio_list_empty(&s.return_bi)) {
-		if (test_bit(MD_SB_CHANGE_PENDING, &conf->mddev->sb_flags)) {
-			spin_lock_irq(&conf->device_lock);
-			bio_list_merge(&conf->return_bi, &s.return_bi);
-			spin_unlock_irq(&conf->device_lock);
-			md_wakeup_thread(conf->mddev->thread);
-		} else
+	if (!bio_list_empty(&s.return_bi))
 		return_io(&s.return_bi);
-	}
 
 	clear_bit_unlock(STRIPE_ACTIVE, &sh->state);
 }
@@ -6226,6 +6220,7 @@ static void raid5_do_work(struct work_struct *work)
 	struct r5worker *worker = container_of(work, struct r5worker, work);
 	struct r5worker_group *group = worker->group;
 	struct r5conf *conf = group->conf;
+	struct mddev *mddev = conf->mddev;
 	int group_id = group - conf->worker_groups;
 	int handled;
 	struct blk_plug plug;
@@ -6246,6 +6241,9 @@ static void raid5_do_work(struct work_struct *work)
 		if (!batch_size && !released)
 			break;
 		handled += batch_size;
+		wait_event_lock_irq(mddev->sb_wait,
+			!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags),
+			conf->device_lock);
 	}
 	pr_debug("%d stripes handled\n", handled);
 
@@ -6273,18 +6271,6 @@ static void raid5d(struct md_thread *thread)
 
 	md_check_recovery(mddev);
 
-	if (!bio_list_empty(&conf->return_bi) &&
-	    !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags)) {
-		struct bio_list tmp = BIO_EMPTY_LIST;
-		spin_lock_irq(&conf->device_lock);
-		if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags)) {
-			bio_list_merge(&tmp, &conf->return_bi);
-			bio_list_init(&conf->return_bi);
-		}
-		spin_unlock_irq(&conf->device_lock);
-		return_io(&tmp);
-	}
-
 	blk_start_plug(&plug);
 	handled = 0;
 	spin_lock_irq(&conf->device_lock);
@@ -6936,7 +6922,6 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	INIT_LIST_HEAD(&conf->hold_list);
 	INIT_LIST_HEAD(&conf->delayed_list);
 	INIT_LIST_HEAD(&conf->bitmap_list);
-	bio_list_init(&conf->return_bi);
 	init_llist_head(&conf->released_stripes);
 	atomic_set(&conf->active_stripes, 0);
 	atomic_set(&conf->preread_active_stripes, 0);

drivers/md/raid5.h

@@ -638,9 +638,6 @@ struct r5conf {
 	int			skip_copy; /* Don't copy data from bio to stripe cache */
 	struct list_head	*last_hold; /* detect hold_list promotions */
 
-	/* bios to have bi_end_io called after metadata is synced */
-	struct bio_list		return_bi;
-
 	atomic_t		reshape_stripes; /* stripes with pending writes for reshape */
 	/* unfortunately we need two cache names as we temporarily have
 	 * two caches.