Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

[Yongji Xie]

On Fri, 30 Nov 2018 at 06:17, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 29, 2018 at 01:34:21PM -0800, Davidlohr Bueso wrote:
> > I messed up something such that waiman was not in the thread. Ccing.
> >
> > > On Thu, 29 Nov 2018, Waiman Long wrote:
> > >
> > > > That can be costly for x86 which will now have 2 locked instructions.
> > >
> > > Yeah, and when used as an actual queue we should really start to notice.
> > > Some users just have a single task in the wake_q because avoiding the cost
> > > of wake_up_process() with locks held is significant.
> > >
> > > How about instead of adding the barrier before the cmpxchg, we do it
> > > in the failed branch, right before we return. This is the uncommon
> > > path.
> > >
> > > Thanks,
> > > Davidlohr
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 091e089063be..0d844a18a9dc 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -408,8 +408,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> > >      * This cmpxchg() executes a full barrier, which pairs with the full
> > >      * barrier executed by the wakeup in wake_up_q().
> > >      */
> > > -   if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
> > > +   if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) {
> > > +           /*
> > > +            * Ensure, that when the cmpxchg() fails, the corresponding
> > > +            * wake_up_q() will observe our prior state.
> > > +            */
> > > +           smp_mb__after_atomic();
> > >             return;
> > > +   }
>
> So wake_up_q() does:
>
>   wake_up_q():
>         node->next = NULL;
>         /* implied smp_mb */
>         wake_up_process();
>
> So per the cross your variables 'rule', this side then should do:
>
>   wake_q_add():
>         /* wake_cond = true */
>         smp_mb()
>         cmpxchg_relaxed(&node->next, ...);
>
> So that the ordering pivots around node->next.
>
> Either we see NULL and win the cmpxchg (in which case we'll do the
> wakeup later) or, when we fail the cmpxchg, we must observe what came
> before the failure.
>
> If it wasn't so damn late, I'd try and write a litmus test for this,
> because now I'm starting to get confused -- also probably because it's
> late.
>

Hi Peter,

Please let me know If there is any progress on this issue. Thank you!

Thanks,
Yongji