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Subject[PATCH 3/3] blk-mq: bitmap tag: clean-up bt_get() function
Date
The tag waiting loop in bt_get() function is a mystery for me:

do {
bool was_empty;

1. was_empty = list_empty(&wait.task_list);
prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);

tag = __bt_get(hctx, bt, last_tag);
if (tag != -1)
break;

2. if (was_empty)
3. atomic_set(&bs->wait_cnt, bt->wake_cnt);

io_schedule();
} while (1);

[1] list_empty(&wait.task_list) check is not protected;
[2] was_empty check is always true which results in *every* thread
entering the loop resets bt_wait_state::wait_cnt counter rather
than every bt->wake_cnt'th thread;
[3] 'bt_wait_state::wait_cnt' counter update seems redundant anyway,
since it is also gets reset in bt_clear_tag() function;

Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
---
block/blk-mq-tag.c | 27 +++++++--------------------
1 files changed, 7 insertions(+), 20 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 5579fae..dc1f684 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -224,7 +224,6 @@ static int bt_get(struct blk_mq_bitmap_tags *bt, struct blk_mq_hw_ctx *hctx,
unsigned int *last_tag, gfp_t gfp)
{
struct bt_wait_state *bs;
- DEFINE_WAIT(wait);
int tag;

tag = __bt_get(hctx, bt, last_tag);
@@ -235,23 +234,9 @@ static int bt_get(struct blk_mq_bitmap_tags *bt, struct blk_mq_hw_ctx *hctx,
return -1;

bs = bt_wait_ptr(bt, hctx);
- do {
- bool was_empty;
-
- was_empty = list_empty(&wait.task_list);
- prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
-
- tag = __bt_get(hctx, bt, last_tag);
- if (tag != -1)
- break;
+ ___wait_event(bs->wait, (tag = __bt_get(hctx, bt, last_tag)) != -1,
+ TASK_UNINTERRUPTIBLE, 0, 0, io_schedule());

- if (was_empty)
- atomic_set(&bs->wait_cnt, bt->wake_cnt);
-
- io_schedule();
- } while (1);
-
- finish_wait(&bs->wait, &wait);
return tag;
}

@@ -477,13 +462,15 @@ static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
return -ENOMEM;
}

- for (i = 0; i < BT_WAIT_QUEUES; i++)
- init_waitqueue_head(&bt->bs[i].wait);
-
bt->wake_cnt = BT_WAIT_BATCH;
if (bt->wake_cnt > depth / 4)
bt->wake_cnt = max(1U, depth / 4);

+ for (i = 0; i < BT_WAIT_QUEUES; i++) {
+ init_waitqueue_head(&bt->bs[i].wait);
+ atomic_set(&bt->bs[i].wait_cnt, bt->wake_cnt);
+ }
+
bt->depth = depth;
return 0;
}
--
1.7.7.6


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