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SubjectRe: [RFC PATCH 02/10] smp, cpu hotplug: Fix smp_call_function_*() to prevent CPU offline properly
On Tue, 04 Dec 2012 14:24:28 +0530
"Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> wrote:

> From: Michael Wang <wangyun@linux.vnet.ibm.com>
>
> With stop_machine() gone from the CPU offline path, we can't depend on
> preempt_disable() to prevent CPUs from going offline from under us.
>
> Use the get/put_online_cpus_stable_atomic() APIs to prevent CPUs from going
> offline, while invoking from atomic context.
>
> ...
>
> */
> - this_cpu = get_cpu();
> + get_online_cpus_stable_atomic();
> + this_cpu = smp_processor_id();

I wonder if get_online_cpus_stable_atomic() should return the local CPU
ID. Just as a little convenience thing. Time will tell.

> /*
> * Can deadlock when called with interrupts disabled.
>
> ...
>
> @@ -380,15 +383,15 @@ int smp_call_function_any(const struct cpumask *mask,
> nodemask = cpumask_of_node(cpu_to_node(cpu));
> for (cpu = cpumask_first_and(nodemask, mask); cpu < nr_cpu_ids;
> cpu = cpumask_next_and(cpu, nodemask, mask)) {
> - if (cpu_online(cpu))
> + if (cpu_online_stable(cpu))
> goto call;
> }
>
> /* Any online will do: smp_call_function_single handles nr_cpu_ids. */
> - cpu = cpumask_any_and(mask, cpu_online_mask);
> + cpu = cpumask_any_and(mask, cpu_online_stable_mask);
> call:
> ret = smp_call_function_single(cpu, func, info, wait);
> - put_cpu();
> + put_online_cpus_stable_atomic();
> return ret;
> }
> EXPORT_SYMBOL_GPL(smp_call_function_any);

So smp_call_function_any() has no synchronization against CPUs coming
online. Hence callers of smp_call_function_any() are responsible for
ensuring that CPUs which are concurrently coming online will adopt the
required state?

I guess that has always been the case...


>
> ...
>



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