Re: [PATCH v6] blk-mq: Avoid that a completion can be ignored for BLK_EH_RESET_TIMER

["jianchao.wang" <jianchao.w.wang@xxxxxxxxxx>]

Hi Bart

On 04/20/2018 12:43 AM, Bart Van Assche wrote:
> Use the deadline instead of the request generation to detect whether
>   or not a request timer fired after reinitialization of a request

Maybe use deadline to do this is not suitable.

Let's think of the following scenario.

T1/T2 times in nano seconds
J1/J2 times in jiffies

rq is started at T1,J1
blk_mq_timeout_work
  -> blk_mq_check_expired
    -> save rq->__deadline (J1 + MQ_RQ_IN_FLIGHT)

                                             rq is completed and freed 
                                             rq is allocated and started again on T2
                                             rq->__deadline = J2 + MQ_RQ_IN_FLIGHT
  -> synchronize srcu/rcu
  -> blk_mq_terminate_expired
    -> rq->__deadline (J2 + MQ_RQ_IN_FLIGHT)

1. deadline = rq->__deadline & ~RQ_STATE_MASK (0x3)
   if J2-J1 < 4 jiffies, we will get the same deadline value.

2. even if we do some bit shift when save and get deadline
   if T2 - T1 < time of 1 jiffies, we sill get the same deadline.

Thanks
Jianchao