Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

[Peter Zijlstra]

On Fri, Dec 15, 2017 at 10:12:50AM +0800, jianchao.wang wrote:
> > That only makes it a little better:
> > 
> >     Task-A                                  Worker
> > 
> >     write_seqcount_begin()
> >     blk_mq_rw_update_state(rq, IN_FLIGHT)
> >     blk_add_timer(rq)
> >     <timer>
> >             schedule_work()
> >     </timer>
> >     <context-switch to worker>
> >                                             read_seqcount_begin()
> >                                                     while(seq & 1)
> >                                                             cpu_relax();
> > 
> Hi Peter
> 
> The current seqcount read side is as below:
>       do {
>               start = read_seqcount_begin(&rq->gstate_seq);


static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
        seqcount_lockdep_reader_access(s);
        return raw_read_seqcount_begin(s);
}

static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
        unsigned ret = __read_seqcount_begin(s);
        smp_rmb();
        return ret;
}

static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
        unsigned ret;

repeat:
        ret = READ_ONCE(s->sequence);
        if (unlikely(ret & 1)) {
                cpu_relax();
                goto repeat;
        }
        return ret;
}