linux-kernel - [PATCH 4.19 21/24] blk-mq: dont hold q->sysfs_lock in blk_mq_map

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Date:   Thu, 11 Feb 2021 16:02:55 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Hannes Reinecke <hare@...e.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Bart Van Assche <bvanassche@....org>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Jack Wang <jinpu.wang@...ud.ionos.com>
Subject: [PATCH 4.19 21/24] blk-mq: dont hold q->sysfs_lock in blk_mq_map_swqueue

From: Ming Lei <ming.lei@...hat.com>

commit c6ba933358f0d7a6a042b894dba20cc70396a6d3 upstream.

blk_mq_map_swqueue() is called from blk_mq_init_allocated_queue()
and blk_mq_update_nr_hw_queues(). For the former caller, the kobject
isn't exposed to userspace yet. For the latter caller, hctx sysfs entries
and debugfs are un-registered before updating nr_hw_queues.

On the other hand, commit 2f8f1336a48b ("blk-mq: always free hctx after
request queue is freed") moves freeing hctx into queue's release
handler, so there won't be race with queue release path too.

So don't hold q->sysfs_lock in blk_mq_map_swqueue().

Cc: Christoph Hellwig <hch@...radead.org>
Cc: Hannes Reinecke <hare@...e.com>
Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Mike Snitzer <snitzer@...hat.com>
Cc: Bart Van Assche <bvanassche@....org>
Reviewed-by: Bart Van Assche <bvanassche@....org>
Signed-off-by: Ming Lei <ming.lei@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Jack Wang <jinpu.wang@...ud.ionos.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 block/blk-mq.c |    7 -------
 1 file changed, 7 deletions(-)

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2324,11 +2324,6 @@ static void blk_mq_map_swqueue(struct re
 	struct blk_mq_ctx *ctx;
 	struct blk_mq_tag_set *set = q->tag_set;
 
-	/*
-	 * Avoid others reading imcomplete hctx->cpumask through sysfs
-	 */
-	mutex_lock(&q->sysfs_lock);
-
 	queue_for_each_hw_ctx(q, hctx, i) {
 		cpumask_clear(hctx->cpumask);
 		hctx->nr_ctx = 0;
@@ -2362,8 +2357,6 @@ static void blk_mq_map_swqueue(struct re
 		hctx->ctxs[hctx->nr_ctx++] = ctx;
 	}
 
-	mutex_unlock(&q->sysfs_lock);
-
 	queue_for_each_hw_ctx(q, hctx, i) {
 		/*
 		 * If no software queues are mapped to this hardware queue,