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Date:   Thu, 15 Oct 2020 10:23:18 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Boqun Feng <boqun.feng@gmail.com>, Qian Cai <cai@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>, x86 <x86@kernel.org>,
        linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [tip: locking/core] lockdep: Fix lockdep recursion
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On Thu, Oct 15, 2020 at 09:15:01AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 15, 2020 at 11:49:26AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 14, 2020 at 08:41:28PM -0700, Paul E. McKenney wrote:

[ . . . ]

> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1764,8 +1764,7 @@ static bool rcu_gp_init(void)
> >  		smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values.
> >  		firstseq = READ_ONCE(rnp->ofl_seq);
> >  		if (firstseq & 0x1)
> > -			while (firstseq == smp_load_acquire(&rnp->ofl_seq))
> > -				schedule_timeout_idle(1);  // Can't wake unless RCU is watching.
> > +			smp_cond_load_relaxed(&rnp->ofl_seq, VAL == firstseq);
> >  		smp_mb(); // Pair with barriers used when updating ->ofl_seq to even values.
> >  		raw_spin_lock(&rcu_state.ofl_lock);
> >  		raw_spin_lock_irq_rcu_node(rnp);
> 
> This would work, and would be absolutely necessary if grace periods
> took only (say) 500 nanoseconds to complete.  But given that they take
> multiple milliseconds at best, and given that this race is extremely
> unlikely, and given the heavy use of virtualization, I have to stick
> with the schedule_timeout_idle().
> 
> In fact, I have on my list to force this race to happen on the grounds
> that if it ain't tested, it don't work...

And it only too about 1000 seconds of TREE03 to make this happen, so we
should be good just relying on rcutorture.  ;-)

							Thanx, Paul