Re: [PATCH v2 1/2] blk-mq: Reduce the number of if-statements in blk_mq_mark_tag_wait()

[Jens Axboe]

On 1/10/18 12:39 PM, Bart Van Assche wrote:
> This patch does not change any functionality but makes the
> blk_mq_mark_tag_wait() code slightly easier to read.

I agree it could do with a cleanup, but how about something like the
below? I think that's easier to read.


diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8000ba6db07d..afccd0848d6f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1104,58 +1104,59 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx 
**hctx,
                                 struct request *rq)
 {
        struct blk_mq_hw_ctx *this_hctx = *hctx;
-       bool shared_tags = (this_hctx->flags & BLK_MQ_F_TAG_SHARED) != 0;
        struct sbq_wait_state *ws;
        wait_queue_entry_t *wait;
        bool ret;
 
-       if (!shared_tags) {
+       if (!(this_hctx->flags & BLK_MQ_F_TAG_SHARED)) {
                if (!test_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state))
                        set_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state);
-       } else {
-               wait = &this_hctx->dispatch_wait;
-               if (!list_empty_careful(&wait->entry))
-                       return false;
 
-               spin_lock(&this_hctx->lock);
-               if (!list_empty(&wait->entry)) {
-                       spin_unlock(&this_hctx->lock);
-                       return false;
-               }
+               /*
+                * It's possible that a tag was freed in the window between the
+                * allocation failure and adding the hardware queue to the wait
+                * queue.
+                *
+                * Don't clear RESTART here, someone else could have set it.
+                * At most this will cost an extra queue run.
+                */
+               return blk_mq_get_driver_tag(rq, hctx, false);
+       }
+
+       wait = &this_hctx->dispatch_wait;
+       if (!list_empty_careful(&wait->entry))
+               return false;
 
-               ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
-               add_wait_queue(&ws->wait, wait);
+       spin_lock(&this_hctx->lock);
+       if (!list_empty(&wait->entry)) {
+               spin_unlock(&this_hctx->lock);
+               return false;
        }
 
+       ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
+       add_wait_queue(&ws->wait, wait);
+
        /*
         * It's possible that a tag was freed in the window between the
         * allocation failure and adding the hardware queue to the wait
         * queue.
         */
        ret = blk_mq_get_driver_tag(rq, hctx, false);
-
-       if (!shared_tags) {
-               /*
-                * Don't clear RESTART here, someone else could have set it.
-                * At most this will cost an extra queue run.
-                */
-               return ret;
-       } else {
-               if (!ret) {
-                       spin_unlock(&this_hctx->lock);
-                       return false;
-               }
-
-               /*
-                * We got a tag, remove ourselves from the wait queue to ensure
-                * someone else gets the wakeup.
-                */
-               spin_lock_irq(&ws->wait.lock);
-               list_del_init(&wait->entry);
-               spin_unlock_irq(&ws->wait.lock);
+       if (!ret) {
                spin_unlock(&this_hctx->lock);
-               return true;
+               return false;
        }
+
+       /*
+        * We got a tag, remove ourselves from the wait queue to ensure
+        * someone else gets the wakeup.
+        */
+       spin_lock_irq(&ws->wait.lock);
+       list_del_init(&wait->entry);
+       spin_unlock_irq(&ws->wait.lock);
+       spin_unlock(&this_hctx->lock);
+
+       return true;
 }
 
 bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,

-- 
Jens Axboe