From 8ee1b7b9d8761a7b1a0f37a6e596b5315d27d2f2 Mon Sep 17 00:00:00 2001 From: Kosuke Tatsukawa Date: Fri, 9 Oct 2015 00:35:38 +0000 Subject: blk-mq: fix waitqueue_active without memory barrier in block/blk-mq-tag.c blk_mq_tag_update_depth() seems to be missing a memory barrier which might cause the waker to not notice the waiter and fail to send a wake_up as in the following figure. blk_mq_tag_update_depth bt_get ------------------------------------------------------------------------ if (waitqueue_active(&bs->wait)) /* The CPU might reorder the test for the waitqueue up here, before prior writes complete */ prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE); tag = __bt_get(hctx, bt, last_tag, tags); /* Value set in bt_update_count not visible yet */ bt_update_count(&tags->bitmap_tags, tdepth); /* blk_mq_tag_wakeup_all(tags, false); */ bt = &tags->bitmap_tags; wake_index = atomic_read(&bt->wake_index); ... io_schedule(); ------------------------------------------------------------------------ This patch adds the missing memory barrier. I found this issue when I was looking through the linux source code for places calling waitqueue_active() before wake_up*(), but without preceding memory barriers, after sending a patch to fix a similar issue in drivers/tty/n_tty.c (Details about the original issue can be found here: https://lkml.org/lkml/2015/9/28/849). Signed-off-by: Kosuke Tatsukawa Signed-off-by: Jens Axboe --- block/blk-mq-tag.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'block/blk-mq-tag.c') diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index ed96474d75cb..7a6b6e27fc26 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -75,6 +75,10 @@ void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool include_reserve) struct blk_mq_bitmap_tags *bt; int i, wake_index; + /* + * Make sure all changes prior to this are visible from other CPUs. + */ + smp_mb(); bt = &tags->bitmap_tags; wake_index = atomic_read(&bt->wake_index); for (i = 0; i < BT_WAIT_QUEUES; i++) { -- cgit v1.2.3