Patch "md/raid1/10: fix potential deadlock" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

md/raid1/10: fix potential deadlock

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
md-raid1-10-fix-potential-deadlock.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Shaohua Li <[email protected]>
Date: Tue, 28 Feb 2017 13:00:20 -0800
Subject: md/raid1/10: fix potential deadlock

From: Shaohua Li <[email protected]>

commit 61eb2b43b99ebdc9bc6bc83d9792257b243e7cb3 upstream.

Neil Brown pointed out a potential deadlock in raid 10 code with bio_split/chain. The raid1 code could have the same issue, but recent barrier rework makes it less likely to happen. The deadlock happens in below sequence:

1. generic_make_request(bio), this will set current->bio_list
2. raid10_make_request will split bio to bio1 and bio2
3. __make_request(bio1), wait_barrer, add underlayer disk bio to current->bio_list
4. __make_request(bio2), wait_barrer

If raise_barrier happens between 3 & 4, since wait_barrier runs at 3, raise_barrier waits for IO completion from 3. And since raise_barrier sets barrier, 4 waits for raise_barrier. But IO from 3 can't be dispatched because raid10_make_request() doesn't finished yet.

The solution is to adjust the IO ordering. Quotes from Neil:
"
It is much safer to:

    if (need to split) {
        split = bio_split(bio, ...)
        bio_chain(...)
        make_request_fn(split);
        generic_make_request(bio);
   } else
        make_request_fn(mddev, bio);

This way we first process the initial section of the bio (in 'split') which will queue some requests to the underlying devices. These requests will be queued in generic_make_request.
Then we queue the remainder of the bio, which will be added to the end of the generic_make_request queue.
Then we return.
generic_make_request() will pop the lower-level device requests off the queue and handle them first. Then it will process the remainder of the original bio once the first section has been fully processed. "

Note, this only happens in read path. In write path, the bio is flushed to underlaying disks either by blk flush (from schedule) or offladed to raid1/10d. It's queued in current->bio_list.

---
 drivers/md/raid10.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

@@ -1470,7 +1470,25 @@ static void raid10_make_request(struct m
 			split = bio;
 		}
 
+		/*
+		 * If a bio is splitted, the first part of bio will pass
+		 * barrier but the bio is queued in current->bio_list (see
+		 * generic_make_request). If there is a raise_barrier() called
+		 * here, the second part of bio can't pass barrier. But since
+		 * the first part bio isn't dispatched to underlaying disks
+		 * yet, the barrier is never released, hence raise_barrier will
+		 * alays wait. We have a deadlock.
+		 * Note, this only happens in read path. For write path, the
+		 * first part of bio is dispatched in a schedule() call
+		 * (because of blk plug) or offloaded to raid10d.
+		 * Quitting from the function immediately can change the bio
+		 * order queued in bio_list and avoid the deadlock.
+		 */
 		__make_request(mddev, split);
+		if (split != bio && bio_data_dir(bio) == READ) {
+			generic_make_request(bio);
+			break;
+		}
 	} while (split != bio);
 
 	/* In case raid10d snuck in to freeze_array */

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/md-raid1-10-fix-potential-deadlock.patch