blk-mq: fix schedule-under-preempt for blocking drivers

Commit a4d907b6a33b unified the single and multi queue request handlers,
but in the process, it also screwed up the locking balance and calls
blk_mq_try_issue_directly() with the ctx preempt lock held. This is a
problem for drivers that have set BLK_MQ_F_BLOCKING, since now they
can't reliably sleep.

While in there, protect against similar issues in the future, by adding
a might_sleep() trigger in the BLOCKING path for direct issue or queue
run.

Reported-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Josef Bacik <josef@toxicpanda.com>
Fixes: a4d907b6a33b ("blk-mq: streamline blk_mq_make_request")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ef63367077ad8c7c08c45f958e4791f40fa84220..061fc2cc88d3d54c1244e4f911c0b0990163da51 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1121,6 +1121,8 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
                blk_mq_sched_dispatch_requests(hctx);
                rcu_read_unlock();
        } else {
+               might_sleep();
+
                srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
                blk_mq_sched_dispatch_requests(hctx);
                srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
@@ -1495,7 +1497,11 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
                __blk_mq_try_issue_directly(rq, cookie, false);
                rcu_read_unlock();
        } else {
-               unsigned int srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
+               unsigned int srcu_idx;
+
+               might_sleep();
+
+               srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
                __blk_mq_try_issue_directly(rq, cookie, true);
                srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
        }
@@ -1595,18 +1601,23 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
                        list_del_init(&same_queue_rq->queuelist);
                list_add_tail(&rq->queuelist, &plug->mq_list);
 
+               blk_mq_put_ctx(data.ctx);
+
                if (same_queue_rq)
                        blk_mq_try_issue_directly(data.hctx, same_queue_rq,
                                        &cookie);
+
+               return cookie;
        } else if (q->nr_hw_queues > 1 && is_sync) {
+               blk_mq_put_ctx(data.ctx);
                blk_mq_bio_to_request(rq, bio);
                blk_mq_try_issue_directly(data.hctx, rq, &cookie);
+               return cookie;
        } else if (q->elevator) {
                blk_mq_bio_to_request(rq, bio);
                blk_mq_sched_insert_request(rq, false, true, true, true);
-       } else if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio)) {
+       } else if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio))
                blk_mq_run_hw_queue(data.hctx, true);
-       }
 
        blk_mq_put_ctx(data.ctx);
        return cookie;