Re: [PATCH v4] blk-mq: Fix race conditions in request timeout handling

[Ming Lei <ming.lei@redhat.com>]

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