[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@xxxxxxxxxxxxxxxxxx>
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@xxxxxxx>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---
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, ¤t->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);
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