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Subject[PATCH v7 21/23] block: Avoid that flushing triggers a lockdep complaint
Date
Avoid that running test nvme/012 from the blktests suite triggers the
following false positive lockdep complaint:

============================================
WARNING: possible recursive locking detected
5.0.0-rc3-xfstests-00015-g1236f7d60242 #841 Not tainted
--------------------------------------------
ksoftirqd/1/16 is trying to acquire lock:
000000000282032e (&(&fq->mq_flush_lock)->rlock){..-.}, at: flush_end_io+0x4e/0x1d0

but task is already holding lock:
00000000cbadcbc2 (&(&fq->mq_flush_lock)->rlock){..-.}, at: flush_end_io+0x4e/0x1d0

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&(&fq->mq_flush_lock)->rlock);
lock(&(&fq->mq_flush_lock)->rlock);

*** DEADLOCK ***

May be due to missing lock nesting notation

1 lock held by ksoftirqd/1/16:
#0: 00000000cbadcbc2 (&(&fq->mq_flush_lock)->rlock){..-.}, at: flush_end_io+0x4e/0x1d0

stack backtrace:
CPU: 1 PID: 16 Comm: ksoftirqd/1 Not tainted 5.0.0-rc3-xfstests-00015-g1236f7d60242 #841
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
dump_stack+0x67/0x90
__lock_acquire.cold.45+0x2b4/0x313
lock_acquire+0x98/0x160
_raw_spin_lock_irqsave+0x3b/0x80
flush_end_io+0x4e/0x1d0
blk_mq_complete_request+0x76/0x110
nvmet_req_complete+0x15/0x110 [nvmet]
nvmet_bio_done+0x27/0x50 [nvmet]
blk_update_request+0xd7/0x2d0
blk_mq_end_request+0x1a/0x100
blk_flush_complete_seq+0xe5/0x350
flush_end_io+0x12f/0x1d0
blk_done_softirq+0x9f/0xd0
__do_softirq+0xca/0x440
run_ksoftirqd+0x24/0x50
smpboot_thread_fn+0x113/0x1e0
kthread+0x121/0x140
ret_from_fork+0x3a/0x50

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
block/blk-flush.c | 5 ++++-
block/blk.h | 1 +
2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 6e0f2d97fc6d..86c86c76c087 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -70,6 +70,7 @@
#include <linux/blkdev.h>
#include <linux/gfp.h>
#include <linux/blk-mq.h>
+#include <linux/lockdep.h>

#include "blk.h"
#include "blk-mq.h"
@@ -472,7 +473,8 @@ struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q,
if (!fq)
goto fail;

- spin_lock_init(&fq->mq_flush_lock);
+ lockdep_register_key(&fq->key);
+ spin_lock_init_key(&fq->mq_flush_lock, &fq->key);

rq_sz = round_up(rq_sz + cmd_size, cache_line_size());
fq->flush_rq = kzalloc_node(rq_sz, flags, node);
@@ -497,6 +499,7 @@ void blk_free_flush_queue(struct blk_flush_queue *fq)
if (!fq)
return;

+ lockdep_unregister_key(&fq->key);
kfree(fq->flush_rq);
kfree(fq);
}
diff --git a/block/blk.h b/block/blk.h
index 848278c52030..10f5e19aa4a1 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -28,6 +28,7 @@ struct blk_flush_queue {
* at the same time
*/
struct request *orig_rq;
+ struct lock_class_key key;
spinlock_t mq_flush_lock;
};

--
2.21.0.rc0.258.g878e2cd30e-goog
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