On 12/15/2017 03:31 PM, Peter Zijlstra wrote:
> 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;
> }
>
Really thanks for kindly pointing out.
jianchao
