On 2018年04月10日 18:04, Ming Lei wrote:
On Tue, Apr 10, 2018 at 03:59:30PM +0800, jianchao.wang wrote:
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
Is it the scenario you described as below in
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
- 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
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
Or think it as below?
C C C
S T F E
Request life time line:
T: timed out
F: found (by timer), inflight but timed out because of a long delay
E: completed by timeout handler
C: regular completion
normal request completion time range: (S, T)
completion in (T, F) is fine since it's not inflight anymore
race window time range: (F, E)
if the delayed request is completed in (F, E) range then it could be
twice by the regular completion first and then the timeout handler.