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blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter 

For blk-mq, part_in_flight/rw will invoke blk_mq_in_flight/rw to
account the inflight requests. It will access the queue_hw_ctx and
nr_hw_queues w/o any protection. When updating nr_hw_queues and
blk_mq_in_flight/rw occur concurrently, panic comes up.

Before update nr_hw_queues, the q will be frozen. So we could use
q_usage_counter to avoid the race. percpu_ref_is_zero is used here
so that we will not miss any in-flight request. The access to
nr_hw_queues and queue_hw_ctx in blk_mq_queue_tag_busy_iter are
under rcu critical section, __blk_mq_update_nr_hw_queues could use
synchronize_rcu to ensure the zeroed q_usage_counter to be globally
visible.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jianchao Wang 2018-08-21 15:15:04 +08:00 committed by Jens Axboe
parent d48ece209f
commit f5bbbbe4d6
2 changed files with 17 additions and 1 deletions
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14
block/blk-mq-tag.c
View File

@ -320,6 +320,18 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
struct blk_mq_hw_ctx *hctx;
int i;
/*
* __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.
*/
rcu_read_lock();
if (percpu_ref_is_zero(&q->q_usage_counter)) {
rcu_read_unlock();
return;
}
queue_for_each_hw_ctx(q, hctx, i) {
struct blk_mq_tags *tags = hctx->tags;
@ -335,7 +347,7 @@ 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();
}
static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,

4
block/blk-mq.c
View File

@ -2976,6 +2976,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
list_for_each_entry(q, &set->tag_list, tag_set_list)
blk_mq_freeze_queue(q);
/*
* Sync with blk_mq_queue_tag_busy_iter.
*/
synchronize_rcu();
/*
* Switch IO scheduler to 'none', cleaning up the data associated
* with the previous scheduler. We will switch back once we are done