lkml.org 
[lkml]   [2017]   [Dec]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme
On Thu, Dec 14, 2017 at 06:51:11PM +0000, Bart Van Assche wrote:
> On Tue, 2017-12-12 at 11:01 -0800, Tejun Heo wrote:
> > + write_seqcount_begin(&rq->gstate_seq);
> > + blk_mq_rq_update_state(rq, MQ_RQ_IN_FLIGHT);
> > + blk_add_timer(rq);
> > + write_seqcount_end(&rq->gstate_seq);
>
> My understanding is that both write_seqcount_begin() and write_seqcount_end()
> trigger a write memory barrier. Is a seqcount really faster than a spinlock?

Yes lots, no atomic operations and no waiting.

The only constraint for write_seqlock is that there must not be any
concurrency.

But now that I look at this again, TJ, why can't the below happen?

write_seqlock_begin();
blk_mq_rq_update_state(rq, IN_FLIGHT);
blk_add_timer(rq);
<timer-irq>
read_seqcount_begin()
while (seq & 1)
cpurelax();
// life-lock
</timer-irq>
write_seqlock_end();

\
 
 \ /
  Last update: 2017-12-14 21:21    [W:0.127 / U:0.416 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site