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Subject[PATCH v2 3/3] 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>
---
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@redhat.com


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