Hello,
On Thu, Apr 12, 2018 at 06:43:45AM +0800, Ming Lei wrote:
> On Wed, Apr 11, 2018 at 02:30:07PM -0700, Tejun Heo wrote:
> > Hello, Ming.
> >
> > On Thu, Apr 12, 2018 at 04:55:29AM +0800, Ming Lei wrote:
> > ...
> > > + spin_lock_irqsave(req->q->queue_lock, flags);
> > > + if (blk_mq_rq_state(req) != MQ_RQ_COMPLETE_IN_RESET) {
> > > + blk_mq_rq_update_aborted_gstate(req, 0);
> > > + blk_add_timer(req);
> >
> > Nothing prevents the above blk_add_timer() racing against the next
> > recycle instance of the request, so this still leaves a small race
> > window.
>
> OK.
>
> But this small race window can be avoided by running blk_add_timer(req)
> before blk_mq_rq_update_aborted_gstate(req, 0), can't it?
Not really because aborted_gstate right now doesn't have any memory
barrier around it, so nothing ensures blk_add_timer() actually appears
before. We can either add the matching barriers in aborted_gstate
update and when it's read in the normal completion path, or we can
wait for the update to be visible everywhere by waiting for rcu grace
period (because the reader is rcu protected).
Thanks.
--
tejun
