Hi Keith
On 09/26/2018 12:36 AM, Keith Busch wrote:
> A recent commit runs tag iterator callbacks under the rcu read lock,
> but existing callbacks do not satisfy the non-blocking requirement.
> The commit intended to prevent an iterator from accessing a queue that's
> being modified. This patch fixes the original issue by taking a queue
> reference instead of reading it, which allows callbacks to make blocking
> calls.
>
> Fixes: f5bbbbe4d6357 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
> Cc: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
> Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
> ---
> v1 -> v2:
>
> Leave the iterator interface as-is, making this change transparent
> to the callers. This will increment the queue usage counter in the
> timeout path twice, which is harmless.
>
This is OK to me.
Acked-by: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
> Changelog.
>
> block/blk-mq-tag.c | 13 ++++---------
> 1 file changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 94e1ed667b6e..41317c50a446 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -322,16 +322,11 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
>
> /*
> * __blk_mq_update_nr_hw_queues will update the nr_hw_queues and
> - * queue_hw_ctx after freeze the queue. So we could use q_usage_counter
> - * to avoid race with it. __blk_mq_update_nr_hw_queues will users
> - * synchronize_rcu to ensure all of the users go out of the critical
> - * section below and see zeroed q_usage_counter.
> + * queue_hw_ctx after freeze the queue, so we use q_usage_counter
> + * to avoid race with it.
> */
> - rcu_read_lock();
> - if (percpu_ref_is_zero(&q->q_usage_counter)) {
> - rcu_read_unlock();
> + if (!percpu_ref_tryget(&q->q_usage_counter))
> return;
> - }
>
> queue_for_each_hw_ctx(q, hctx, i) {
> struct blk_mq_tags *tags = hctx->tags;
> @@ -347,7 +342,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
> bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
> bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
> }
> - rcu_read_unlock();
> + blk_queue_exit(q);
> }
>
> static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
>
