Messages in this thread | | | Subject | Re: [PATCH 5/7] blk-mq: remove REQ_ATOM_COMPLETE usages from blk-mq | From | "jianchao.wang" <> | Date | Fri, 22 Dec 2017 12:02:20 +0800 |
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Sorry for my non-detailed description.
On 12/21/2017 09:50 PM, Tejun Heo wrote: > Hello, > > On Thu, Dec 21, 2017 at 11:56:49AM +0800, jianchao.wang wrote: >> It's worrying that even though the blk_mark_rq_complete() here is intended to synchronize with >> timeout path, but it indeed give the blk_mq_complete_request() the capability to exclude with There could be scenario where the driver itself stop a request itself with blk_mq_complete_request() or some other interface that will invoke it, races with the normal completion path where a same request comes. For example: a reset could be triggered through sysfs on nvme-rdma Then the driver will cancel all the reqs, including in-flight ones. nvme_rdma_reset_ctrl_work() nvme_rdma_shutdown_ctrl() >>>> if (ctrl->ctrl.queue_count > 1) { nvme_stop_queues(&ctrl->ctrl); //quiesce the queue blk_mq_tagset_busy_iter(&ctrl->tag_set, nvme_cancel_request, &ctrl->ctrl); //invoke blk_mq_complete_request() nvme_rdma_destroy_io_queues(ctrl, shutdown); } >>>>
These operations could race with the normal completion path of in-flight ones. It should drain all the in-flight ones first here. But there maybe some other places similar with this. >> itself. Maybe this capability should be reserved. > > Can you explain further how that'd help? The problem there is that if > you have two competing completions, where one isn't a timeout, there's > nothing synchronizing the reuse of the request. IOW, the losing on > can easily complete the next recycle instance. The atomic bitops > might feel like more protection but it's just feels.
In above case, the request may simultaneously enter requeue and end path.
Thanks Jianchao
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