Re: [PATCH V2 1/3] blk-mq: add new API of blk_mq_hctx_set_fq_lock_class

[Hannes Reinecke <hare@suse.de>]

On 12/3/20 2:26 AM, Ming Lei wrote:
> flush_end_io() may be called recursively from some driver, such as
> nvme-loop, so lockdep may complain 'possible recursive locking'.
> Commit b3c6a5997541("block: Fix a lockdep complaint triggered by
> request queue flushing") tried to address this issue by assigning
> dynamically allocated per-flush-queue lock class. This solution
> adds synchronize_rcu() for each hctx's release handler, and causes
> horrible SCSI MQ probe delay(more than half an hour on megaraid sas).
> 
> Add new API of blk_mq_hctx_set_fq_lock_class() for these drivers, so
> we just need to use driver specific lock class for avoiding the
> lockdep warning of 'possible recursive locking'.
> 
> Tested-by: Kashyap Desai <kashyap.desai@broadcom.com>
> Reported-by: Qian Cai <cai@redhat.com>
> Cc: Sumit Saxena <sumit.saxena@broadcom.com>
> Cc: John Garry <john.garry@huawei.com>
> Cc: Kashyap Desai <kashyap.desai@broadcom.com>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   block/blk-flush.c      | 25 +++++++++++++++++++++++++
>   include/linux/blk-mq.h |  3 +++
>   2 files changed, 28 insertions(+)
> 
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index 9507dcdd5881..bf51588762d8 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -490,3 +490,28 @@ void blk_free_flush_queue(struct blk_flush_queue *fq)
>   	kfree(fq->flush_rq);
>   	kfree(fq);
>   }
> +
> +/*
> + * Allow driver to set its own lock class to fq->mq_flush_lock for
> + * avoiding lockdep complaint.
> + *
> + * flush_end_io() may be called recursively from some driver, such as
> + * nvme-loop, so lockdep may complain 'possible recursive locking' because
> + * all 'struct blk_flush_queue' instance share same mq_flush_lock lock class
> + * key. We need to assign different lock class for these driver's
> + * fq->mq_flush_lock for avoiding the lockdep warning.
> + *
> + * Use dynamically allocated lock class key for each 'blk_flush_queue'
> + * instance is over-kill, and more worse it introduces horrible boot delay
> + * issue because synchronize_rcu() is implied in lockdep_unregister_key which
> + * is called for each hctx release. SCSI probing may synchronously create and
> + * destroy lots of MQ request_queues for non-existent devices, and some robot
> + * test kernel always enable lockdep option. It is observed that more than half
> + * an hour is taken during SCSI MQ probe with per-fq lock class.
> + */
> +void blk_mq_hctx_set_fq_lock_class(struct blk_mq_hw_ctx *hctx,
> +		struct lock_class_key *key)
> +{
> +	lockdep_set_class(&hctx->fq->mq_flush_lock, key);
> +}
> +EXPORT_SYMBOL_GPL(blk_mq_hctx_set_fq_lock_class);
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 794b2a33a2c3..5f639240760e 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -5,6 +5,7 @@
>   #include <linux/blkdev.h>
>   #include <linux/sbitmap.h>
>   #include <linux/srcu.h>
> +#include <linux/lockdep.h>
>   
>   struct blk_mq_tags;
>   struct blk_flush_queue;
> @@ -594,5 +595,7 @@ static inline void blk_mq_cleanup_rq(struct request *rq)
>   }
>   
>   blk_qc_t blk_mq_submit_bio(struct bio *bio);
> +void blk_mq_hctx_set_fq_lock_class(struct blk_mq_hw_ctx *hctx,
> +		struct lock_class_key *key);
>   
>   #endif
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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