Re: [PATCH] fix rcu vs hotplug race

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxx> wrote:

> > Ingo,
> > 
> > I believe Gautham's fix at http://lkml.org/lkml/2008/6/27/9 is 
> > better and also explains it better.
> 
> ah, indeed - picked that one up instead.

this is the patch i picked up:

-------------------------->
Subject: rcu: fix hotplug vs rcu race
From: Gautham R Shenoy <ego@xxxxxxxxxx>
Date: Fri, 27 Jun 2008 10:17:38 +0530

Dhaval Giani reported this warning during cpu hotplug stress-tests:

| On running kernel compiles in parallel with cpu hotplug:
|
| WARNING: at arch/x86/kernel/smp.c:118
| native_smp_send_reschedule+0x21/0x36()
| Modules linked in:
| Pid: 27483, comm: cc1 Not tainted 2.6.26-rc7 #1
| [...]
|  [<c0110355>] native_smp_send_reschedule+0x21/0x36
|  [<c014fe8f>] force_quiescent_state+0x47/0x57
|  [<c014fef0>] call_rcu+0x51/0x6d
|  [<c01713b3>] __fput+0x130/0x158
|  [<c0171231>] fput+0x17/0x19
|  [<c016fd99>] filp_close+0x4d/0x57
|  [<c016fdff>] sys_close+0x5c/0x97

IMHO the warning is a spurious one.

cpu_online_map is updated by the _cpu_down() using stop_machine_run().
Since force_quiescent_state is invoked from irqs disabled section,
stop_machine_run() won't be executing while a cpu is executing
force_quiescent_state(). Hence the cpu_online_map is stable while we're
in the irq disabled section.

However, a cpu might have been offlined _just_ before we disabled irqs
while entering force_quiescent_state(). And rcu subsystem might not yet
have handled the CPU_DEAD notification, leading to the offlined cpu's
bit being set in the rcp->cpumask.

Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent sending
smp_reschedule() to an offlined CPU.

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!
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