Re: [PATCH] fix rcu vs hotplug race

[Ingo Molnar <mingo@elte.hu>]

* Gautham R Shenoy <ego@in.ibm.com> wrote:

> So, it's the design of stop_machine() that's preventing someone from 
> updating the cpu_online_map while force_quiescent_state() is 
> performing the cpu_is_online() check. Becase we always call 
> force_quiescent_state() with irqs disabled :)

Paul, do you concur? I'll apply the commit in the form below to 
tip/core/urgent if Paul agrees.

	Ingo

----------------------------------->
Subject: cpu-hotplug + rcu: fix spurious warning
From: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Date: Mon, 23 Jun 2008 16:07:00 +0530

On running kernel compiles in parallel with cpu hotplug,

------------[ cut here ]------------
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
 [<c01217d9>] warn_on_slowpath+0x41/0x5d
 [<c01515b7>] ? generic_file_aio_read+0x10f/0x137
 [<c0151340>] ? file_read_actor+0x0/0xf7
 [<c013ae4c>] ? validate_chain+0xaa/0x29c
 [<c013c854>] ? __lock_acquire+0x612/0x666
 [<c013c854>] ? __lock_acquire+0x612/0x666
 [<c013ae4c>] ? validate_chain+0xaa/0x29c
 [<c01715d3>] ? file_kill+0x2d/0x30
 [<c013cbd7>] ? __lock_release+0x4b/0x51
 [<c01715d3>] ? file_kill+0x2d/0x30
 [<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
 [<c0103861>] sysenter_past_esp+0x6a/0xb1
 =======================
---[ end trace aa35f3913ddf2d06 ]---

This is because a reschedule is sent to a CPU which is offline.
Just ensure that the CPU we send the smp_send_reschedule is actually
online.

It's the design of stop_machine() that's preventing someone from
updating the cpu_online_map while force_quiescent_state() is
performing the cpu_is_online() check. Becase we always call
force_quiescent_state() with irqs disabled :)

Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Acked-by: Gautham R Shenoy <ego@in.ibm.com>
Cc: paulmck@linux.vnet.ibm.com
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/rcuclassic.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: tip/kernel/rcuclassic.c
===================================================================
--- tip.orig/kernel/rcuclassic.c
+++ tip/kernel/rcuclassic.c
@@ -93,7 +93,8 @@ static void force_quiescent_state(struct
 		cpumask = rcp->cpumask;
 		cpu_clear(rdp->cpu, cpumask);
 		for_each_cpu_mask(cpu, cpumask)
-			smp_send_reschedule(cpu);
+			if (cpu_online(cpu))
+				smp_send_reschedule(cpu);
 	}
 }
 #else