On Mon, 2 Aug 2010 16:37:54 -0400
Justin Bronder [off-list ref] wrote:

On 02/08/10 12:58 +1000, Neil Brown wrote:

quoted

On Mon, 2 Aug 2010 12:29:49 +1000
Neil Brown [off-list ref] wrote:

quoted

Ahhhh.... I see the problem.  Because a 'generic_make_request' is already
active, the once called by raid10::make_request just queues the request until
the top level one completes.   This results in a deadlock.

I'll have to ponder a bit to figure out the best way to fix this.

So, one good strong cup of tea later I think I have a good solution.

Would you be able to test with this patch and confirm that you cannot
reproduce the hang?

I've been running with this patch on 2.6.34.1 all day and have yet to cause
the hang.  Given it took under 5 minutes earlier, feel free to add:

Tested-by:  Justin Bronder <redacted>

I really appreciate you taking care of this.  Thanks.

And thank you for testing.  I've queued this up now and will send it to Linus
and -stable shortly.

NeilBrown

quoted

Thanks.

NeilBrown

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 42e64e4..d1d6891 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -825,11 +825,29 @@ static int make_request(mddev_t *mddev, struct bio * bio)
 		 */
 		bp = bio_split(bio,
 			       chunk_sects - (bio->bi_sector & (chunk_sects - 1)) );
+
+		/* Each of these 'make_request' calls will call 'wait_barrier'.
+		 * If the first succeeds but the second blocks due to the resync
+		 * thread raising the barrier, we will deadlock because the
+		 * IO to the underlying device will be queued in generic_make_request
+		 * and will never complete, so will never reduce nr_pending.
+		 * So increment nr_waiting here so no new raise_barriers will
+		 * succeed, and so the second wait_barrier cannot block.
+		 */
+		spin_lock_irq(&conf->resync_lock);
+		conf->nr_waiting++;
+		spin_unlock_irq(&conf->resync_lock);
+
 		if (make_request(mddev, &bp->bio1))
 			generic_make_request(&bp->bio1);
 		if (make_request(mddev, &bp->bio2))
 			generic_make_request(&bp->bio2);
 
+		spin_lock_irq(&conf->resync_lock);
+		conf->nr_waiting--;
+		wake_up(&conf->wait_barrier);
+		spin_unlock_irq(&conf->resync_lock);
+
 		bio_pair_release(bp);
 		return 0;
 	bad_map:
--

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