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SubjectRe: [PATCH v7 21/23] block: Avoid that flushing triggers a lockdep complaint
On Thu, Feb 14, 2019 at 03:00:56PM -0800, Bart Van Assche wrote:
> 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;
> };
>

Hi Bart,

Did you look at the following comment?

https://marc.info/?l=linux-block&m=155014828206209&w=2

There might be lots of blk_flush_queue instance which is allocated
for each hctx, then lots of class key slot may be wasted.

So I suggest to use one nvmet_loop_flush_lock_key for this particular issue,
something like the following patch:

diff --git a/drivers/nvme/target/loop.c b/drivers/nvme/target/loop.c
index 4aac1b4a8112..ec4248c12ed9 100644
--- a/drivers/nvme/target/loop.c
+++ b/drivers/nvme/target/loop.c
@@ -524,7 +524,9 @@ static const struct nvme_ctrl_ops nvme_loop_ctrl_ops = {

static int nvme_loop_create_io_queues(struct nvme_loop_ctrl *ctrl)
{
- int ret;
+ static struct lock_class_key nvme_loop_flush_lock_key;
+ int ret, i;
+ struct blk_mq_hw_ctx *hctx;

ret = nvme_loop_init_io_queues(ctrl);
if (ret)
@@ -553,6 +555,10 @@ static int nvme_loop_create_io_queues(struct nvme_loop_ctrl *ctrl)
goto out_free_tagset;
}

+ queue_for_each_hw_ctx(ctrl->ctrl.connect_q, hctx, i)
+ lockdep_set_class(&hctx->fq->mq_flush_lock,
+ &nvme_loop_flush_lock_key);
+
ret = nvme_loop_connect_io_queues(ctrl);
if (ret)
goto out_cleanup_connect_q;
--
Ming

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