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