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SubjectRe: [PATCH] rcu/nocb: Fix misordered rcu_barrier() while (de-)offloading
On Mon, Oct 18, 2021 at 11:50:46PM +0200, Frederic Weisbecker wrote:
> On Mon, Oct 18, 2021 at 11:36:04AM -0700, Paul E. McKenney wrote:
> > On Mon, Oct 18, 2021 at 07:42:42PM +0200, Frederic Weisbecker wrote:
> > > I think you're right. The real issue is something I wanted to
> > > fix next: RCU_SEGCBLIST_RCU_CORE isn't cleared when nocb is enabled on
> > > boot so rcu_core() always run concurrently with nocb kthreads in TREE04,
> > > without holding rcu_barrier mutex of course (I mean with the latest patchset).
> >
> > That would do it!
> >
> > > Ok forget this patch, I'm testing again with simply clearing
> > > RCU_SEGCBLIST_RCU_CORE on boot.
> >
> > Sounds good, looking forward to it!
>
> So yes that was indeed the real issue. I've pushed an "rcu/rt-v3" branch
> which is the same as your dev.2021.10.07a branch but the first patch
> "rcu/nocb: Prepare state machine for a new step" has this added:
>
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 3b470113ae38..2461fe8d0c23 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1171,8 +1171,8 @@ void __init rcu_init_nohz(void)
> if (rcu_segcblist_empty(&rdp->cblist))
> rcu_segcblist_init(&rdp->cblist);
> rcu_segcblist_offload(&rdp->cblist, true);
> - rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB);
> - rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
> + rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
> + rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_RCU_CORE);
> }
> rcu_organize_nocb_kthreads();
> }
>
>
> I still see the oom though, hopefully my series just makes an existing
> problem more likely to happen.

Yes, I have been seeing that without your patch, at least unless I bump
up memory a bit.

Looking forward to the new series!

Thanx, Paul

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