Hi Ming,
On 04/04/2019 04:43 PM, Ming Lei wrote:
> Just like aio/io_uring, we need to grab 2 refcount for queuing one
> request, one is for submission, another is for completion.
>
> If the request isn't queued from plug code path, the refcount grabbed
> in generic_make_request() serves for submission. In theroy, this
> refcount should have been released after the sumission(async run queue)
> is done. blk_freeze_queue() works with blk_sync_queue() together
> for avoiding race between cleanup queue and IO submission, given async
> run queue activities are canceled because hctx->run_work is scheduled with
> the refcount held, so it is fine to not hold the refcount when
> running the run queue work function for dispatch IO.
>
> However, if request is staggered into plug list, and finally queued
> from plug code path, the refcount in submission side is actually missed.
> And we may start to run queue after queue is removed because the queue's
> kobject refcount isn't guaranteed to be grabbed in flushing plug list
> context, then kernel oops is triggered, see the following race:
>
> blk_mq_flush_plug_list():
> blk_mq_sched_insert_requests()
> insert requests to sw queue or scheduler queue
> blk_mq_run_hw_queue
>
> Because of concurrent run queue, all requests inserted above may be
> completed before calling the above blk_mq_run_hw_queue. Then queue can
> be freed during the above blk_mq_run_hw_queue().
>
> Fixes the issue by grab .q_usage_counter before calling
> blk_mq_sched_insert_requests() in blk_mq_flush_plug_list(). This way is
> safe because the queue is absolutely alive before inserting request.
>
> Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> Cc: James Smart <james.smart@xxxxxxxxxxxx>
> Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
> Cc: linux-scsi@xxxxxxxxxxxxxxx,
> Cc: Martin K . Petersen <martin.petersen@xxxxxxxxxx>,
> Cc: Christoph Hellwig <hch@xxxxxx>,
> Cc: James E . J . Bottomley <jejb@xxxxxxxxxxxxxxxxxx>,
> Cc: jianchao wang <jianchao.w.wang@xxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> block/blk-mq.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 3ff3d7b49969..5b586affee09 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1728,9 +1728,12 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
> if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx) {
> if (this_hctx) {
> trace_block_unplug(this_q, depth, !from_schedule);
> +
> + percpu_ref_get(&this_q->q_usage_counter);
Sorry to bother but I would just like to double confirm the reason to use
"percpu_ref_get()" here which does not check whether the queue has been frozen.
Is it because there is assumption that any direct/indirect caller of
blk_mq_flush_plug_list() much have already grabbed q_usage_counter, which is
similar to blk_queue_enter_live()?
Thank you very much!
Dongli Zhang
