On Thu, Jan 12, 2012 at 5:08 PM, Peter Zijlstra [off-list ref] wrote:

On Thu, 2012-01-12 at 16:51 +0200, Gilad Ben-Yossef wrote:

quoted

What I can't figure out is why we don't need  get/put_online_cpus()
pair around each and every call
to on_each_cpu everywhere? and if we do, perhaps making it a part of
on_each_cpu is the way to go?

Something like:

diff --git a/kernel/smp.c b/kernel/smp.c
index f66a1b2..cfa3882 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c

@@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void

*info, int wait)
 {
        unsigned long flags;

+       BUG_ON(in_atomic());
+
+       get_online_cpus();
        preempt_disable();

Your preempt_disable() here serializes against hotplug..

I'm probably daft, but why didn't it work for the page allocator then?

Mel's description reads: "Part of the problem is the page allocator is
sending IPIs using
on_each_cpu() without calling get_online_cpus() to prevent changes
to the online cpumask."

on_each_cpu() disables preemption in master as well...

Gilad




Thanks,
Gilad

-- 
Gilad Ben-Yossef
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gilad@benyossef.com
Israel Cell: +972-52-8260388
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"Unfortunately, cache misses are an equal opportunity pain provider."
-- Mike Galbraith, LKML

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