blk-mq: bitmap tag: fix races in bt_get() function

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@infradead.org>
Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 5 insertions, 8 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 08fc6716d362..c1b92426c95e 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;
 }