Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via IPI from page allocator context

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

[Reinstating the original Cc list]

On 01/19/2012 09:50 PM, Mel Gorman wrote:> 

> On a different x86-64 machines with an intel-specific MCE, I have
> also noted that the value of num_online_cpus() can change while
> stop_machine() is running.


That is expected and intentional right? Meaning, it is during the
stop_machine() thing itself that a CPU is actually taken offline.
And at the same time, it is removed from the cpu_online_mask.

On Intel boxes, essentially, the following gets executed on the dying
CPU, as set up by the stop_machine stuff.

__cpu_disable()
    native_cpu_disable()
        cpu_disable_common()
            remove_cpu_from_maps()
                set_cpu_online(cpu, false)
			^^^^^^
So, set_cpu_online will remove this CPU from the cpu_online_mask.
And all this runs while still under the stop machine context.
And this is exactly what we want right?

> This is sensitive to timing and part of
> the problem seems to be due to cmci_rediscover() running without the
> CPU hotplug mutex held. This is not related to the IPI mess and is
> unrelated to memory pressure but is just to note that CPU hotplug in
> general can be fragile in parts.
> 


For the cmci_rediscover() part, I feel a simple get/put_online_cpus()
around it should work.

Something like the following patch? (It is untested at the moment, but
I will run it later and see if it works well).

I would like the opinion of MCE/Intel maintainers as to whether this is
a proper fix or something else would have been better..

----
From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Subject: [PATCH] x86/intel mce: Fix race with CPU hotplug in cmci_rediscover()

cmci_rediscover() is invoked upon the CPU_POST_DEAD notification, when
the cpu_hotplug lock is no longer held. And cmci_rediscover() iterates
over all the online cpus. So this can race with an ongoing CPU hotplug
operation. Fix this by wrapping the iteration code within the pair
get_online_cpus() / put_online_cpus().

Reported-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/kernel/cpu/mcheck/mce_intel.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce_intel.c b/arch/x86/kernel/cpu/mcheck/mce_intel.c
index 38e49bc..1c30397 100644
--- a/arch/x86/kernel/cpu/mcheck/mce_intel.c
+++ b/arch/x86/kernel/cpu/mcheck/mce_intel.c
@@ -10,6 +10,7 @@
 #include <linux/interrupt.h>
 #include <linux/percpu.h>
 #include <linux/sched.h>
+#include <linux/cpu.h>
 #include <asm/apic.h>
 #include <asm/processor.h>
 #include <asm/msr.h>
@@ -179,6 +180,7 @@ void cmci_rediscover(int dying)
 		return;
 	cpumask_copy(old, &current->cpus_allowed);
 
+	get_online_cpus();
 	for_each_online_cpu(cpu) {
 		if (cpu == dying)
 			continue;
@@ -188,6 +190,7 @@ void cmci_rediscover(int dying)
 		if (cmci_supported(&banks))
 			cmci_discover(banks, 0);
 	}
+	put_online_cpus();
 
 	set_cpus_allowed_ptr(current, old);
 	free_cpumask_var(old);