Messages in this thread | | | From | Yongji Xie <> | Date | Mon, 10 Dec 2018 23:12:52 +0800 | Subject | Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil |
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On Fri, 30 Nov 2018 at 06:17, Peter Zijlstra <peterz@infradead.org> 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
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