Re: [PATCH 1/2] bfq: Remove merged request already in bfq_requests_merged()

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

> Il giorno 24 mag 2021, alle ore 12:04, Jan Kara <jack@suse.cz> ha scritto:
> 
> Currently, bfq does very little in bfq_requests_merged() and handles all
> the request cleanup in bfq_finish_requeue_request() called from
> blk_mq_free_request(). That is currently safe only because
> blk_mq_free_request() is called shortly after bfq_requests_merged()
> while bfqd->lock is still held. However to fix a lock inversion between
> bfqd->lock and ioc->lock, we need to call blk_mq_free_request() after
> dropping bfqd->lock. That would mean that already merged request could
> be seen by other processes inside bfq queues and possibly dispatched to
> the device which is wrong. So move cleanup of the request from
> bfq_finish_requeue_request() to bfq_requests_merged().
> 

I didn't even remember any longer why I had to handle that deferred
removal in bfq_finish_requeue_request() :)

Your solution seems very clean to me.

Acked-by: Paolo Valente <paolo.valente@linaro.org>

> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> block/bfq-iosched.c | 41 +++++++++++++----------------------------
> 1 file changed, 13 insertions(+), 28 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index acd1f881273e..50a29fdf51da 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2405,7 +2405,7 @@ static void bfq_requests_merged(struct request_queue *q, struct request *rq,
> 		*next_bfqq = bfq_init_rq(next);
> 
> 	if (!bfqq)
> -		return;
> +		goto remove;
> 
> 	/*
> 	 * If next and rq belong to the same bfq_queue and next is older
> @@ -2428,6 +2428,14 @@ static void bfq_requests_merged(struct request_queue *q, struct request *rq,
> 		bfqq->next_rq = rq;
> 
> 	bfqg_stats_update_io_merged(bfqq_group(bfqq), next->cmd_flags);
> +remove:
> +	/* Merged request may be in the IO scheduler. Remove it. */
> +	if (!RB_EMPTY_NODE(&next->rb_node)) {
> +		bfq_remove_request(next->q, next);
> +		if (next_bfqq)
> +			bfqg_stats_update_io_remove(bfqq_group(next_bfqq),
> +						    next->cmd_flags);
> +	}
> }
> 
> /* Must be called with bfqq != NULL */
> @@ -6376,6 +6384,7 @@ static void bfq_finish_requeue_request(struct request *rq)
> {
> 	struct bfq_queue *bfqq = RQ_BFQQ(rq);
> 	struct bfq_data *bfqd;
> +	unsigned long flags;
> 
> 	/*
> 	 * rq either is not associated with any icq, or is an already
> @@ -6393,39 +6402,15 @@ static void bfq_finish_requeue_request(struct request *rq)
> 					     rq->io_start_time_ns,
> 					     rq->cmd_flags);
> 
> +	spin_lock_irqsave(&bfqd->lock, flags);
> 	if (likely(rq->rq_flags & RQF_STARTED)) {
> -		unsigned long flags;
> -
> -		spin_lock_irqsave(&bfqd->lock, flags);
> -
> 		if (rq == bfqd->waited_rq)
> 			bfq_update_inject_limit(bfqd, bfqq);
> 
> 		bfq_completed_request(bfqq, bfqd);
> -		bfq_finish_requeue_request_body(bfqq);
> -
> -		spin_unlock_irqrestore(&bfqd->lock, flags);
> -	} else {
> -		/*
> -		 * Request rq may be still/already in the scheduler,
> -		 * in which case we need to remove it (this should
> -		 * never happen in case of requeue). And we cannot
> -		 * defer such a check and removal, to avoid
> -		 * inconsistencies in the time interval from the end
> -		 * of this function to the start of the deferred work.
> -		 * This situation seems to occur only in process
> -		 * context, as a consequence of a merge. In the
> -		 * current version of the code, this implies that the
> -		 * lock is held.
> -		 */
> -
> -		if (!RB_EMPTY_NODE(&rq->rb_node)) {
> -			bfq_remove_request(rq->q, rq);
> -			bfqg_stats_update_io_remove(bfqq_group(bfqq),
> -						    rq->cmd_flags);
> -		}
> -		bfq_finish_requeue_request_body(bfqq);
> 	}
> +	bfq_finish_requeue_request_body(bfqq);
> +	spin_unlock_irqrestore(&bfqd->lock, flags);
> 
> 	/*
> 	 * Reset private fields. In case of a requeue, this allows
> -- 
> 2.26.2
>