linux-kernel - Re: [PATCH] rcu/nocb: Fix misordered rcu

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Message-ID: <20211018215046.GA453817@lothringen>
Date:   Mon, 18 Oct 2021 23:50:46 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Fix misordered rcu_barrier() while
 (de-)offloading

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.

Thanks.