[PATCH] md/raid5: protect batch_head->bm_seq updates

["Chen Cheng" <chencheng@fnnas.com>]

From: Chen Cheng <chencheng@fnnas.com>

bm_seq means "stripe delay to flush until bm_seq <= seq_write".

do_release_stripe() keeps STRIPE_BIT_DELAY stripes on bitmap_list
when bm_seq >= seq_write.

after raid5d() flushes bitmap update and ++seq_write, and
active_bit_delay() retry to release delayed stripes.

the stripe batch head must carry the newest bm_seq among all
member stripes, because the whole batch later released according
to the batch head state and bm_seq.

race scenario:
===================
1. cpu0 - sh0->bm_seq=101; cpu1 - sh1->bm_seq=102;
2. both cpu0 and cpu1 read batch_head->bm_seq = 100;
3. cpu1 write 102, and cpu0 overwrite with 101;

the point is, if the head has a lower bm_seq than one of its
members, the whole batch could be released before that
member's bitmap is flushed.
and the on-disk bitmap not record sh1's changes.

Signed-off-by: Chen Cheng <chencheng@fnnas.com>
---
 drivers/md/raid5.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index a08230aac711..ee145a7bf9e8 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -980,32 +980,31 @@ static void stripe_add_to_batch_list(struct r5conf *conf,
 		/*
 		 * at this point, head's BATCH_READY could be cleared, but we
 		 * can still add the stripe to batch list
 		 */
 		list_add(&sh->batch_list, &head->batch_list);
-		spin_unlock(&head->batch_head->batch_lock);
 	} else {
 		head->batch_head = head;
 		sh->batch_head = head->batch_head;
 		spin_lock(&head->batch_lock);
 		list_add_tail(&sh->batch_list, &head->batch_list);
-		spin_unlock(&head->batch_lock);
 	}
 
-	if (test_and_clear_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
-		if (atomic_dec_return(&conf->preread_active_stripes)
-		    < IO_THRESHOLD)
-			md_wakeup_thread(conf->mddev->thread);
-
 	if (test_and_clear_bit(STRIPE_BIT_DELAY, &sh->state)) {
 		int seq = sh->bm_seq;
 		if (test_bit(STRIPE_BIT_DELAY, &sh->batch_head->state) &&
 		    sh->batch_head->bm_seq - seq > 0)
 			seq = sh->batch_head->bm_seq;
 		set_bit(STRIPE_BIT_DELAY, &sh->batch_head->state);
 		sh->batch_head->bm_seq = seq;
 	}
+	spin_unlock(&head->batch_head->batch_lock);
+
+	if (test_and_clear_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
+		if (atomic_dec_return(&conf->preread_active_stripes)
+		    < IO_THRESHOLD)
+			md_wakeup_thread(conf->mddev->thread);
 
 	atomic_inc(&sh->count);
 unlock_out:
 	unlock_two_stripes(head, sh);
 out:
-- 
2.54.0