[PATCH v3] md/raid10: fix deadlock with check operation and nowait requests

When an array check is running it will raise the barrier at which point
normal requests will become blocked and increment the nr_pending value to
signal there is work pending inside of wait_barrier(). NOWAIT requests
do not block and so will return immediately with an error, and additionally
do not increment nr_pending in wait_barrier(). Upstream change
43806c3d5b9b ("raid10: cleanup memleak at raid10_make_request") added a
call to raid_end_bio_io() to fix a memory leak when NOWAIT requests hit
this condition. raid_end_bio_io() eventually calls allow_barrier() and
it will unconditionally do an atomic_dec_and_test(&conf->nr_pending) even
though the corresponding increment on nr_pending didn't happen in the
NOWAIT case.

This can be easily seen by starting a check operation while an application is
doing nowait IO on the same array. This results in a deadlocked state due to
nr_pending value underflowing and so the md resync thread gets stuck waiting
for nr_pending to == 0.

Output of r10conf state of the array when we hit this condition:

crash> struct r10conf.barrier,nr_pending,nr_waiting,nr_queued <addr of r10conf>
	barrier = 1,
        nr_pending = {
          counter = -41
        },
        nr_waiting = 15,
        nr_queued = 0,

Example of md_sync thread stuck waiting on raise_barrier() and other requests
stuck in wait_barrier():

md1_resync
[<0>] raise_barrier+0xce/0x1c0
[<0>] raid10_sync_request+0x1ca/0x1ed0
[<0>] md_do_sync+0x779/0x1110
[<0>] md_thread+0x90/0x160
[<0>] kthread+0xbe/0xf0
[<0>] ret_from_fork+0x34/0x50
[<0>] ret_from_fork_asm+0x1a/0x30

kworker/u1040:2+flush-253:4
[<0>] wait_barrier+0x1de/0x220
[<0>] regular_request_wait+0x30/0x180
[<0>] raid10_make_request+0x261/0x1000
[<0>] md_handle_request+0x13b/0x230
[<0>] __submit_bio+0x107/0x1f0
[<0>] submit_bio_noacct_nocheck+0x16f/0x390
[<0>] ext4_io_submit+0x24/0x40
[<0>] ext4_do_writepages+0x254/0xc80
[<0>] ext4_writepages+0x84/0x120
[<0>] do_writepages+0x7a/0x260
[<0>] __writeback_single_inode+0x3d/0x300
[<0>] writeback_sb_inodes+0x1dd/0x470
[<0>] __writeback_inodes_wb+0x4c/0xe0
[<0>] wb_writeback+0x18b/0x2d0
[<0>] wb_workfn+0x2a1/0x400
[<0>] process_one_work+0x149/0x330
[<0>] worker_thread+0x2d2/0x410
[<0>] kthread+0xbe/0xf0
[<0>] ret_from_fork+0x34/0x50
[<0>] ret_from_fork_asm+0x1a/0x30

Fixes: 43806c3d5b9b ("raid10: cleanup memleak at raid10_make_request")
Cc: [email protected]
Signed-off-by: Josh Hunt <[email protected]>
---
v3:
  * Call free_r10bio() as per Yu Kuai's suggestion
---
 drivers/md/raid10.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 9debb20cf129..b4892c5d571c 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1184,7 +1184,7 @@ static void raid10_read_request(struct mddev *mddev, struct bio *bio,
 	}
 
 	if (!regular_request_wait(mddev, conf, bio, r10_bio->sectors)) {
-		raid_end_bio_io(r10_bio);
+		free_r10bio(r10_bio);
 		return;
 	}
 
@@ -1372,7 +1372,7 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,
 
 	sectors = r10_bio->sectors;
 	if (!regular_request_wait(mddev, conf, bio, sectors)) {
-		raid_end_bio_io(r10_bio);
+		free_r10bio(r10_bio);
 		return;
 	}
 
-- 
2.34.1