[PATCH v2 3/3] blk-mq: bitmap tag: fix races in bt_get() function

[Alexander Gordeev]

This update fixes few issues in bt_get() function:

- list_empty(&wait.task_list) check is not protected;

- 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;

- 'bt_wait_state::wait_cnt' counter update is redundant, since
  it also gets reset in bt_clear_tag() function;

Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Ming Lei <tom.leiming@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxx>
---
 block/blk-mq-tag.c |   13 +++++--------
 1 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 08fc671..c1b9242 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -248,18 +248,12 @@ static int bt_get(struct blk_mq_alloc_data *data,
 
 	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;
 
-		if (was_empty)
-			atomic_set(&bs->wait_cnt, bt->wake_cnt);
-
 		blk_mq_put_ctx(data->ctx);
 
 		io_schedule();
@@ -519,10 +513,13 @@ static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
 		return -ENOMEM;
 	}
 
-	for (i = 0; i < BT_WAIT_QUEUES; i++)
+	bt_update_count(bt, depth);
+
+	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_update_count(bt, depth);
 	return 0;
 }
 
-- 
1.7.7.6

-- 
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx
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