* Patch "md/raid1/10: fix potential deadlock" has been added to the 4.4-stable tree
@ 2017-03-24 15:02 gregkh
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From: gregkh @ 2017-03-24 15:02 UTC (permalink / raw)
To: shli, colyli, gregkh, jinpu.wang, neilb; +Cc: stable, stable-commits
This is a note to let you know that I've just added the patch titled
md/raid1/10: fix potential deadlock
to the 4.4-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-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.
>From 61eb2b43b99ebdc9bc6bc83d9792257b243e7cb3 Mon Sep 17 00:00:00 2001
From: Shaohua Li <shli@fb.com>
Date: Tue, 28 Feb 2017 13:00:20 -0800
Subject: md/raid1/10: fix potential deadlock
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 */
Patches currently in stable-queue which might be from shli@fb.com are
queue-4.4/md-raid1-10-fix-potential-deadlock.patch
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2017-03-24 15:02 Patch "md/raid1/10: fix potential deadlock" has been added to the 4.4-stable tree gregkh
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