[PATCH 4.4 22/30] md/raid1/10: fix potential deadlock

Greg Kroah-Hartman
2017-03-24T18:12:07Z

From: Shaohua Li <shli@fb.com>

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.

Cc: Coly Li <colyli@suse.de>
Suggested-by: NeilBrown <neilb@suse.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1477,7 +1477,25 @@ static void make_request(struct mddev *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 */