On Wed, Oct 28, 2020 at 03:54:28PM +0100, Peter Zijlstra wrote:
> On Wed, Oct 28, 2020 at 12:07:13PM +0100, Peter Zijlstra wrote:
> > AFAICT we only need/use irq_work_queue_on() on remote CPUs, since we
> > can directly access local state. So avoid the IRQ_WORK dependency and
> > use the unconditionally available irq_work_queue_remote().
> >
> > This survives a number of TREE01 runs.
>
> OK, Paul mentioned on IRC that while it is extremely unlikely, this code
> does not indeed guarantee it will not try to IPI self.
>
> I'll try again.
This is the best I could come up with.. :/
---
Subject: rcu/tree: Use irq_work_queue_remote()
From: Peter Zijlstra <pet...@infradead.org>
Date: Wed Oct 28 11:53:40 CET 2020
All sites that consume rcu_iw_gp_seq seem to have rcu_node lock held,
so setting it probably should too. Also the effect of self-IPI here
would be setting rcu_iw_gp_seq to the value we just set it to
(pointless) and clearing rcu_iw_pending, which we just set, so don't
set it.
Passes TREE01.
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
kernel/rcu/tree.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1308,14 +1308,16 @@ static int rcu_implicit_dynticks_qs(stru
resched_cpu(rdp->cpu);
WRITE_ONCE(rdp->last_fqs_resched, jiffies);
}
-#ifdef CONFIG_IRQ_WORK
+ raw_spin_lock_rcu_node(rnp);
if (!rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
(rnp->ffmask & rdp->grpmask)) {
- rdp->rcu_iw_pending = true;
rdp->rcu_iw_gp_seq = rnp->gp_seq;
- irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
+ if (likely(rdp->cpu != smp_processor_id())) {
+ rdp->rcu_iw_pending = true;
+ irq_work_queue_remote(rdp->cpu, &rdp->rcu_iw);
+ }
}
-#endif
+ raw_spin_unlock_rcu_node(rnp);
}
return 0;
