Re: [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 24 mag 2021, alle ore 12:04, Jan Kara <jack@suse.cz> ha scritto:
> 
> Lockdep complains about lock inversion between ioc->lock and bfqd->lock:
> 
> bfqd -> ioc:
> put_io_context+0x33/0x90 -> ioc->lock grabbed
> blk_mq_free_request+0x51/0x140
> blk_put_request+0xe/0x10
> blk_attempt_req_merge+0x1d/0x30
> elv_attempt_insert_merge+0x56/0xa0
> blk_mq_sched_try_insert_merge+0x4b/0x60
> bfq_insert_requests+0x9e/0x18c0 -> bfqd->lock grabbed
> blk_mq_sched_insert_requests+0xd6/0x2b0
> blk_mq_flush_plug_list+0x154/0x280
> blk_finish_plug+0x40/0x60
> ext4_writepages+0x696/0x1320
> do_writepages+0x1c/0x80
> __filemap_fdatawrite_range+0xd7/0x120
> sync_file_range+0xac/0xf0
> 
> ioc->bfqd:
> bfq_exit_icq+0xa3/0xe0 -> bfqd->lock grabbed
> put_io_context_active+0x78/0xb0 -> ioc->lock grabbed
> exit_io_context+0x48/0x50
> do_exit+0x7e9/0xdd0
> do_group_exit+0x54/0xc0
> 
> To avoid this inversion we change blk_mq_sched_try_insert_merge() to not
> free the merged request but rather leave that upto the caller similarly
> to blk_mq_sched_try_merge(). And in bfq_insert_requests() we make sure
> to free all the merged requests after dropping bfqd->lock.
> 

I see you added a (short) loop. Apart from that,
Acked-by: Paolo Valente <paolo.valente@linaro.org>

Thanks,
Paolo

> Fixes: aee69d78dec0 ("block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler")
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> block/bfq-iosched.c      |  6 ++++--
> block/blk-merge.c        | 19 ++++++++-----------
> block/blk-mq-sched.c     |  5 +++--
> block/blk-mq-sched.h     |  3 ++-
> block/blk-mq.h           | 11 +++++++++++
> block/blk.h              |  2 +-
> block/elevator.c         | 11 ++++++++---
> block/mq-deadline.c      |  5 ++++-
> include/linux/elevator.h |  3 ++-
> 9 files changed, 43 insertions(+), 22 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 50a29fdf51da..5e076396b588 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2317,9 +2317,9 @@ static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
> 
> 	ret = blk_mq_sched_try_merge(q, bio, nr_segs, &free);
> 
> +	spin_unlock_irq(&bfqd->lock);
> 	if (free)
> 		blk_mq_free_request(free);
> -	spin_unlock_irq(&bfqd->lock);
> 
> 	return ret;
> }
> @@ -5933,14 +5933,16 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> 	struct bfq_queue *bfqq;
> 	bool idle_timer_disabled = false;
> 	unsigned int cmd_flags;
> +	LIST_HEAD(free);
> 
> #ifdef CONFIG_BFQ_GROUP_IOSCHED
> 	if (!cgroup_subsys_on_dfl(io_cgrp_subsys) && rq->bio)
> 		bfqg_stats_update_legacy_io(q, rq);
> #endif
> 	spin_lock_irq(&bfqd->lock);
> -	if (blk_mq_sched_try_insert_merge(q, rq)) {
> +	if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
> 		spin_unlock_irq(&bfqd->lock);
> +		blk_mq_free_requests(&free);
> 		return;
> 	}
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 4d97fb6dd226..1398b52a24b4 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -846,18 +846,15 @@ static struct request *attempt_front_merge(struct request_queue *q,
> 	return NULL;
> }
> 
> -int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
> -			  struct request *next)
> +/*
> + * Try to merge 'next' into 'rq'. Return true if the merge happened, false
> + * otherwise. The caller is responsible for freeing 'next' if the merge
> + * happened.
> + */
> +bool blk_attempt_req_merge(struct request_queue *q, struct request *rq,
> +			   struct request *next)
> {
> -	struct request *free;
> -
> -	free = attempt_merge(q, rq, next);
> -	if (free) {
> -		blk_put_request(free);
> -		return 1;
> -	}
> -
> -	return 0;
> +	return attempt_merge(q, rq, next);
> }
> 
> bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 714e678f516a..bf0a3dec8226 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -400,9 +400,10 @@ bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
> 	return ret;
> }
> 
> -bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
> +bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq,
> +				   struct list_head *free)
> {
> -	return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
> +	return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq, free);
> }
> EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);
> 
> diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
> index 5b18ab915c65..8b70de4b8d23 100644
> --- a/block/blk-mq-sched.h
> +++ b/block/blk-mq-sched.h
> @@ -11,7 +11,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
> 		unsigned int nr_segs, struct request **merged_request);
> bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
> 		unsigned int nr_segs);
> -bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq);
> +bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq,
> +				   struct list_head *free);
> void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx);
> void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx);
> 
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index 81a775171be7..20ef743a3ff6 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -301,6 +301,17 @@ static inline struct blk_plug *blk_mq_plug(struct request_queue *q,
> 	return NULL;
> }
> 
> +/* Free all requests on the list */
> +static inline void blk_mq_free_requests(struct list_head *list)
> +{
> +	while (!list_empty(list)) {
> +		struct request *rq = list_entry_rq(list->next);
> +
> +		list_del_init(&rq->queuelist);
> +		blk_mq_free_request(rq);
> +	}
> +}
> +
> /*
>  * For shared tag users, we track the number of currently active users
>  * and attempt to provide a fair share of the tag depth for each of them.
> diff --git a/block/blk.h b/block/blk.h
> index 8b3591aee0a5..99ef4f7e7a70 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -225,7 +225,7 @@ ssize_t part_timeout_store(struct device *, struct device_attribute *,
> void __blk_queue_split(struct bio **bio, unsigned int *nr_segs);
> int ll_back_merge_fn(struct request *req, struct bio *bio,
> 		unsigned int nr_segs);
> -int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
> +bool blk_attempt_req_merge(struct request_queue *q, struct request *rq,
> 				struct request *next);
> unsigned int blk_recalc_rq_segments(struct request *rq);
> void blk_rq_set_mixed_merge(struct request *rq);
> diff --git a/block/elevator.c b/block/elevator.c
> index 440699c28119..62e9c672da7c 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -350,9 +350,11 @@ enum elv_merge elv_merge(struct request_queue *q, struct request **req,
>  * we can append 'rq' to an existing request, so we can throw 'rq' away
>  * afterwards.
>  *
> - * Returns true if we merged, false otherwise
> + * Returns true if we merged, false otherwise. 'free' will contain all
> + * requests that need to be freed.
>  */
> -bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
> +bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq,
> +			      struct list_head *free)
> {
> 	struct request *__rq;
> 	bool ret;
> @@ -363,8 +365,10 @@ bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
> 	/*
> 	 * First try one-hit cache.
> 	 */
> -	if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
> +	if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq)) {
> +		list_add(&rq->queuelist, free);
> 		return true;
> +	}
> 
> 	if (blk_queue_noxmerges(q))
> 		return false;
> @@ -378,6 +382,7 @@ bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
> 		if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
> 			break;
> 
> +		list_add(&rq->queuelist, free);
> 		/* The merged request could be merged with others, try again */
> 		ret = true;
> 		rq = __rq;
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 8eea2cbf2bf4..7136262819f1 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -487,6 +487,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> 	struct request_queue *q = hctx->queue;
> 	struct deadline_data *dd = q->elevator->elevator_data;
> 	const int data_dir = rq_data_dir(rq);
> +	LIST_HEAD(free);
> 
> 	/*
> 	 * This may be a requeue of a write request that has locked its
> @@ -494,8 +495,10 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> 	 */
> 	blk_req_zone_write_unlock(rq);
> 
> -	if (blk_mq_sched_try_insert_merge(q, rq))
> +	if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
> +		blk_mq_free_requests(&free);
> 		return;
> +	}
> 
> 	trace_block_rq_insert(rq);
> 
> diff --git a/include/linux/elevator.h b/include/linux/elevator.h
> index dcb2f9022c1d..1a5965174f5b 100644
> --- a/include/linux/elevator.h
> +++ b/include/linux/elevator.h
> @@ -117,7 +117,8 @@ extern void elv_merge_requests(struct request_queue *, struct request *,
> 			       struct request *);
> extern void elv_merged_request(struct request_queue *, struct request *,
> 		enum elv_merge);
> -extern bool elv_attempt_insert_merge(struct request_queue *, struct request *);
> +extern bool elv_attempt_insert_merge(struct request_queue *, struct request *,
> +				     struct list_head *);
> extern struct request *elv_former_request(struct request_queue *, struct request *);
> extern struct request *elv_latter_request(struct request_queue *, struct request *);
> 
> -- 
> 2.26.2
>