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blk-mq: Allow blocking queue tag iter callbacks

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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: f5bbbbe4d6 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
Acked-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Keith Busch 2018-09-25 10:36:20 -06:00 committed by Jens Axboe
parent b57e99b4b8
commit 530ca2c9bd
1 changed files with 4 additions and 9 deletions
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13
block/blk-mq-tag.c
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@ -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,