linux-kernel - Re: [PATCH] blk-mq: fix waitqueue_active without memory barrier in block/blk-mq-tag.c

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Date:	Fri, 9 Oct 2015 09:11:16 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Kosuke Tatsukawa <tatsu@...jp.nec.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blk-mq: fix waitqueue_active without memory barrier in
 block/blk-mq-tag.c

On 10/08/2015 06:35 PM, Kosuke Tatsukawa wrote:
> 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 <tatsu@...jp.nec.com>
> ---
>   block/blk-mq-tag.c |    4 ++++
>   1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index ed96474..7a6b6e2 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++) {
> 

Thanks, after looking at this, I think this patch is fine. It's not a
super hot path, so not worth it to further optimize this or look into
ways to avoid the barrier. I do wonder if there are archs where
atomic_read() is a memory barrier, in that case we need not do it at
all. And perhaps we have some weird smp_before_bla variant that could be
used here instead fo improve upon that case.

-- 
Jens Axboe

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