linux-kernel - Re: [PATCH] fix rcu vs hotplug race

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Message-ID: <20080627051855.GD26167@in.ibm.com>
Date:	Fri, 27 Jun 2008 10:48:55 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>, laijs@...fujitsu.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	lkml <linux-kernel@...r.kernel.org>,
	Rusty Russel <rusty@...tcorp.com.au>
Subject: Re: [PATCH] fix rcu vs hotplug race

On Fri, Jun 27, 2008 at 10:17:38AM +0530, Gautham R Shenoy wrote:
> IMHO the warning is a spurious one.
> Here's the timeline.
> CPU_A						 CPU_B
> --------------------------------------------------------------
> cpu_down():					.
> .					   	.
> .						.
> stop_machine(): /* disables preemption,		.
> 		 * and irqs */			.
> .						.
> .						.
> take_cpu_down();				.
> .						.
> .						.
> .						.
> cpu_disable(); /*this removes cpu 		.
> 		*from cpu_online_map 		.
> 		*/				.
> .						.
> .						.
> restart_machine(); /* enables irqs */		.
> ------WINDOW DURING WHICH rcp->cpumask is stale ---------------
> .						call_rcu();
> .						/* disables irqs here */
> .						.force_quiescent_state();
> .CPU_DEAD:					.for_each_cpu(rcp->cpumask)
> .						.   smp_send_reschedule();
> .						.
> .						.   WARN_ON() for offlined CPU!
> .

Exactly. The call_rcu()s are coming from a different subsystem
and can happen anytime during the CPU hotplug path. So, RCU subsystem
doesn't have anything to do to keep rcu->cpumask consistent.
It is *safe* even if we miss poking a cpu or two while
forcing quiescent state in all CPUs. The worst that can happen
is a delay in grace period. No correctness problem here.

Thanks
Dipankar
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