On Tue, Apr 10, 2018 at 03:59:30PM +0800, jianchao.wang wrote: > Hi Bart > > On 04/10/2018 09:34 AM, Bart Van Assche wrote: > > If a completion occurs after blk_mq_rq_timed_out() has reset > > rq->aborted_gstate and the request is again in flight when the timeout > > expires then a request will be completed twice: a first time by the > > timeout handler and a second time when the regular completion occurs > > Would you please detail more here about why the request could be completed twice ? > > Is it the scenario you described as below in https://marc.info/?l=linux-block&m=151796816127318 > > The following race can occur between the code that resets the timer > and completion handling: > - The code that handles BLK_EH_RESET_TIMER resets aborted_gstate. > - A completion occurs and blk_mq_complete_request() calls > __blk_mq_complete_request(). > - The timeout code calls blk_add_timer() and that function sets the > request deadline and adjusts the timer. > - __blk_mq_complete_request() frees the request tag. > - The timer fires and the timeout handler gets called for a freed > request. > If yes, how does the timeout handler get the freed request when the tag has been freed ? Thinking of this patch further. The issue may not be a double completion issue, and it may be the following behaviour which breaks NVMe or other drivers easily: 1) there is long delay(synchronize_rcu()) between setting rq->aborted_gstate and handling the timeout by blk_mq_rq_timed_out(). 2) during the long delay, the rq may be completed by hardware, then if the following timeout is handled as BLK_EH_RESET_TIMER, it is driver's bug, and driver's .timeout() may be confused about this behaviour, I guess. In theory this behaviour should exist in all these approaches, but just easier to trigger if long delay is introduced before handling timeout. Thanks, Ming
