Commit 157f377b authored Sep 11, 2017 by Jens Axboe

block: directly insert blk-mq request from blk_insert_cloned_request()



A NULL pointer crash was reported for the case of having the BFQ IO
scheduler attached to the underlying blk-mq paths of a DM multipath
device.  The crash occured in blk_mq_sched_insert_request()'s call to
e->type->ops.mq.insert_requests().

Paolo Valente correctly summarized why the crash occured with:
"the call chain (dm_mq_queue_rq -> map_request -> setup_clone ->
blk_rq_prep_clone) creates a cloned request without invoking
e->type->ops.mq.prepare_request for the target elevator e.  The cloned
request is therefore not initialized for the scheduler, but it is
however inserted into the scheduler by blk_mq_sched_insert_request."

All said, a request-based DM multipath device's IO scheduler should be
the only one used -- when the original requests are issued to the
underlying paths as cloned requests they are inserted directly in the
underlying dispatch queue(s) rather than through an additional elevator.

But commit bd166ef1 ("blk-mq-sched: add framework for MQ capable IO
schedulers") switched blk_insert_cloned_request() from using
blk_mq_insert_request() to blk_mq_sched_insert_request().  Which
incorrectly added elevator machinery into a call chain that isn't
supposed to have any.

To fix this introduce a blk-mq private blk_mq_request_bypass_insert()
that blk_insert_cloned_request() calls to insert the request without
involving any elevator that may be attached to the cloned request's
request_queue.

Fixes: bd166ef1 ("blk-mq-sched: add framework for MQ capable IO schedulers")
Cc: stable@vger.kernel.org
Reported-by: Bart Van Assche <Bart.VanAssche@wdc.com>
Tested-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent be1c7043

block/blk-core.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -2342,7 +2342,12 @@ blk_status_t blk_insert_cloned_request(struct request_queue q, struct request
		if (q->mq_ops) {
		if (blk_queue_io_stat(q))
		blk_account_io_start(rq, true);
		blk_mq_sched_insert_request(rq, false, true, false, false);
		/*
		* Since we have a scheduler attached on the top device,
		* bypass a potential scheduler on the bottom device for
		* insert.
		*/
		blk_mq_request_bypass_insert(rq);
		return BLK_STS_OK;
		}

block/blk-mq.c

+16 −0

Original line number	Diff line number	Diff line
		@@ -1401,6 +1401,22 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx hctx, struct request rq,
		blk_mq_hctx_mark_pending(hctx, ctx);
		}

		/*
		* Should only be used carefully, when the caller knows we want to
		* bypass a potential IO scheduler on the target device.
		*/
		void blk_mq_request_bypass_insert(struct request *rq)
		{
		struct blk_mq_ctx *ctx = rq->mq_ctx;
		struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(rq->q, ctx->cpu);

		spin_lock(&hctx->lock);
		list_add_tail(&rq->queuelist, &hctx->dispatch);
		spin_unlock(&hctx->lock);

		blk_mq_run_hw_queue(hctx, false);
		}

		void blk_mq_insert_requests(struct blk_mq_hw_ctx hctx, struct blk_mq_ctx ctx,
		struct list_head *list)

block/blk-mq.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -54,6 +54,7 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set set, struct blk_mq_tags tags,
		*/
		void __blk_mq_insert_request(struct blk_mq_hw_ctx hctx, struct request rq,
		bool at_head);
		void blk_mq_request_bypass_insert(struct request *rq);
		void blk_mq_insert_requests(struct blk_mq_hw_ctx hctx, struct blk_mq_ctx ctx,
		struct list_head *list);

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