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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
Date: Thu, 15 Oct 2020 10:23:18 -0700	[thread overview]
Message-ID: <20201015172318.GA3705@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20201015161501.GV3249@paulmck-ThinkPad-P72>

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

  reply	other threads:[~2020-10-15 17:23 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <160223032121.7002.1269740091547117869.tip-bot2@tip-bot2>
2020-10-09 13:41 ` [tip: locking/core] lockdep: Fix lockdep recursion Qian Cai
2020-10-09 13:58   ` Paul E. McKenney
2020-10-09 15:30     ` Qian Cai
2020-10-09 16:11       ` Paul E. McKenney
2020-10-09 16:23     ` Peter Zijlstra
2020-10-09 16:37       ` Paul E. McKenney
2020-10-09 17:36       ` Qian Cai
2020-10-09 17:50         ` Paul E. McKenney
2020-10-09 17:54         ` Qian Cai
2020-10-09 18:21           ` Paul E. McKenney
2020-10-12  3:11   ` Boqun Feng
2020-10-12 14:14     ` Qian Cai
2020-10-12 21:28     ` Paul E. McKenney
2020-10-13 10:34       ` Peter Zijlstra
2020-10-13 10:44         ` Peter Zijlstra
2020-10-13 11:25           ` Peter Zijlstra
2020-10-13 16:26             ` Paul E. McKenney
2020-10-13 19:30               ` Paul E. McKenney
2020-10-14 18:34                 ` Paul E. McKenney
2020-10-14 21:53                   ` Peter Zijlstra
2020-10-14 22:11                     ` Paul E. McKenney
2020-10-14 22:39                       ` Peter Zijlstra
2020-10-14 23:55                         ` Paul E. McKenney
2020-10-15  3:41                           ` Paul E. McKenney
2020-10-15  9:49                             ` Peter Zijlstra
2020-10-15  9:50                               ` Peter Zijlstra
2020-10-15 16:15                                 ` Paul E. McKenney
2020-10-15  9:52                               ` Peter Zijlstra
2020-10-15 16:20                                 ` Paul E. McKenney
2020-10-15 16:15                               ` Paul E. McKenney
2020-10-15 17:23                                 ` Paul E. McKenney [this message]
2020-10-13 16:15           ` Paul E. McKenney
2020-10-13 10:27     ` Peter Zijlstra
2020-10-13 16:24       ` Boqun Feng
2020-10-27 19:31     ` Qian Cai
2020-10-28  3:01       ` Paul E. McKenney
2020-10-28 14:39         ` Qian Cai
2020-10-28 15:53           ` Paul E. McKenney
2020-10-28 20:08             ` Qian Cai
2020-10-28 21:02               ` Paul E. McKenney

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