md/raid1/10: fix potential deadlock
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> Cc: stable@vger.kernel.org (v3.14+, only the raid10 part) Suggested-by:NeilBrown <neilb@suse.com> Reviewed-by:
Jack Wang <jinpu.wang@profitbricks.com> Signed-off-by:
Shaohua Li <shli@fb.com>
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