blk-mq: order getting budget and driver tag

BugLink: http://bugs.launchpad.net/bugs/1769723
commit 0bca799b92807ee9be0890690f5dde7d8c6a8e25 upstream.

This patch orders getting budget and driver tag by making sure to acquire
driver tag after budget is got, this way can help to avoid the following
race:

1) before dispatch request from scheduler queue, get one budget first, then
dequeue a request, call it request A.

2) in another IO path for dispatching request B which is from hctx->dispatch,
driver tag is got, then try to get budget in blk_mq_dispatch_rq_list(),
unfortunately the budget is held by request A.

3) meantime blk_mq_dispatch_rq_list() is called for dispatching request
A, and try to get driver tag first, unfortunately no driver tag is
available because the driver tag is held by request B

4) both two IO pathes can't move on, and IO stall is caused.

This issue can be observed when running dbench on USB storage.

This patch fixes this issue by always getting budget before getting
driver tag.

Cc: stable@vger.kernel.org
Fixes: de1482974080ec9e ("blk-mq: introduce .get_budget and .put_budget in blk_mq_ops")
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Omar Sandoval <osandov@fb.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1a80d8c4f3ecffc713741f640b4e0bb3ba44865d..8ed71e35e7c3ffee8dec44a5477253514a67ff84 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1096,7 +1096,12 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
                blk_status_t ret;
 
                rq = list_first_entry(list, struct request, queuelist);
-               if (!blk_mq_get_driver_tag(rq, &hctx, false)) {
+
+               hctx = blk_mq_map_queue(rq->q, rq->mq_ctx->cpu);
+               if (!got_budget && !blk_mq_get_dispatch_budget(hctx))
+                       break;
+
+               if (!blk_mq_get_driver_tag(rq, NULL, false)) {
                        /*
                         * The initial allocation attempt failed, so we need to
                         * rerun the hardware queue when a tag is freed. The
@@ -1105,8 +1110,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
                         * we'll re-run it below.
                         */
                        if (!blk_mq_mark_tag_wait(&hctx, rq)) {
-                               if (got_budget)
-                                       blk_mq_put_dispatch_budget(hctx);
+                               blk_mq_put_dispatch_budget(hctx);
                                /*
                                 * For non-shared tags, the RESTART check
                                 * will suffice.
@@ -1117,11 +1121,6 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
                        }
                }
 
-               if (!got_budget && !blk_mq_get_dispatch_budget(hctx)) {
-                       blk_mq_put_driver_tag(rq);
-                       break;
-               }
-
                list_del_init(&rq->queuelist);
 
                bd.rq = rq;
@@ -1637,11 +1636,11 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
        if (q->elevator)
                goto insert;
 
-       if (!blk_mq_get_driver_tag(rq, NULL, false))
+       if (!blk_mq_get_dispatch_budget(hctx))
                goto insert;
 
-       if (!blk_mq_get_dispatch_budget(hctx)) {
-               blk_mq_put_driver_tag(rq);
+       if (!blk_mq_get_driver_tag(rq, NULL, false)) {
+               blk_mq_put_dispatch_budget(hctx);
                goto insert;
        }