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

Jens Axboe
2018-01-10T20:30:07Z

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