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

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On Fri, 2018-04-20 at 14:55 +0800, jianchao.wang wrote:
> 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.

Hello Jianchao,

How about using the upper 16 or 32 bits of rq->__deadline to store a generation
number? I don't know any block driver for which the product of (deadline in
seconds) and HZ exceeds (1 << 32).

Thanks,

Bart.







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