blk-mq: don't allocate driver tag upfront for flush rq

The idea behind it is simple:

1) for none scheduler, driver tag has to be borrowed for flush rq,
   otherwise we may run out of tag, and that causes an IO hang. And
   get/put driver tag is actually noop for none, so reordering tags
   isn't necessary at all.

2) for a real I/O scheduler, we need not allocate a driver tag upfront
   for flush rq. It works just fine to follow the same approach as
   normal requests: allocate driver tag for each rq just before calling
   ->queue_rq().

One driver visible change is that the driver tag isn't shared in the
flush request sequence. That won't be a problem, since we always do that
in legacy path.

Then flush rq need not be treated specially wrt. get/put driver tag.
This cleans up the code - for instance, reorder_tags_to_front() can be
removed, and we needn't worry about request ordering in dispatch list
for avoiding I/O deadlock.

Also we have to put the driver tag before requeueing.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 5 insertions, 37 deletions

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index e7094f44afaf..01a43fed6b8c 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -356,29 +356,12 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
                 return true;
         }
 
-        if (has_sched) {
+        if (has_sched)
                 rq->rq_flags |= RQF_SORTED;
-                WARN_ON(rq->tag != -1);
-        }
 
         return false;
 }
 
-/*
- * Add flush/fua to the queue. If we fail getting a driver tag, then
- * punt to the requeue list. Requeue will re-invoke us from a context
- * that's safe to block from.
- */
-static void blk_mq_sched_insert_flush(struct blk_mq_hw_ctx *hctx,
-                                      struct request *rq, bool can_block)
-{
-        if (blk_mq_get_driver_tag(rq, &hctx, can_block)) {
-                blk_insert_flush(rq);
-                blk_mq_run_hw_queue(hctx, true);
-        } else
-                blk_mq_add_to_requeue_list(rq, false, true);
-}
-
 void blk_mq_sched_insert_request(struct request *rq, bool at_head,
                                  bool run_queue, bool async, bool can_block)
 {
@@ -389,10 +372,12 @@ void blk_mq_sched_insert_request(struct request *rq, bool at_head,
 
         /* flush rq in flush machinery need to be dispatched directly */
         if (!(rq->rq_flags & RQF_FLUSH_SEQ) && op_is_flush(rq->cmd_flags)) {
-                blk_mq_sched_insert_flush(hctx, rq, can_block);
-                return;
+                blk_insert_flush(rq);
+                goto run;
         }
 
+        WARN_ON(e && (rq->tag != -1));
+
         if (blk_mq_sched_bypass_insert(hctx, !!e, rq))
                 goto run;
 
@@ -419,23 +404,6 @@ void blk_mq_sched_insert_requests(struct request_queue *q,
         struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
         struct elevator_queue *e = hctx->queue->elevator;
 
-        if (e) {
-                struct request *rq, *next;
-
-                /*
-                 * We bypass requests that already have a driver tag assigned,
-                 * which should only be flushes. Flushes are only ever inserted
-                 * as single requests, so we shouldn't ever hit the
-                 * WARN_ON_ONCE() below (but let's handle it just in case).
-                 */
-                list_for_each_entry_safe(rq, next, list, queuelist) {
-                        if (WARN_ON_ONCE(rq->tag != -1)) {
-                                list_del_init(&rq->queuelist);
-                                blk_mq_sched_bypass_insert(hctx, true, rq);
-                        }
-                }
-        }
-
         if (e && e->type->ops.mq.insert_requests)
                 e->type->ops.mq.insert_requests(hctx, list, false);
         else