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SubjectRe: [PATCH 2/2] consolidate writes in smp_call_funtion_interrupt
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On Tue, 2011-01-18 at 15:06 -0600, Milton Miller wrote:

> Index: common/kernel/smp.c
> ===================================================================
> --- common.orig/kernel/smp.c 2011-01-17 20:16:18.000000000 -0600
> +++ common/kernel/smp.c 2011-01-17 20:17:50.000000000 -0600
> @@ -193,6 +193,7 @@ void generic_smp_call_function_interrupt
> */
> list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
> int refs;
> + void (*func) (void *info);
>
> /*
> * Since we walk the list without any locks, we might
> @@ -212,24 +213,32 @@ void generic_smp_call_function_interrupt
> if (atomic_read(&data->refs) == 0)
> continue;
>
> + func = data->csd.func; /* for later warn */
> data->csd.func(data->csd.info);
>
> + /*
> + * If the cpu mask is not still set then it enabled interrupts,
> + * we took another smp interrupt, and executed the function
> + * twice on this cpu. In theory that copy decremented refs.
> + */
> + if (!cpumask_test_and_clear_cpu(cpu, data->cpumask)) {
> + WARN(1, "%pS enabled interrupts and double executed\n",
> + func);
> + continue;
> + }
> +
> refs = atomic_dec_return(&data->refs);
> WARN_ON(refs < 0);
>
> if (refs)
> continue;
>
> + WARN_ON(!cpumask_empty(data->cpumask));
> +
> + raw_spin_lock(&call_function.lock);
> + list_del_rcu(&data->csd.list);
> + raw_spin_unlock(&call_function.lock);
> +
> csd_unlock(&data->csd);
> }
>


So after this we have:

list_for_each_entry_rcu()
rbd
!->cpumask ->cpumask =
rmb wmb
!->refs ->refs =
->func() wmb
mb list_add_rcu()
->refs--
if (!->refs)
list_del_rcu()
So even if we see it as an old-ref, when we see a valid cpumask, valid
ref, we execute the function clear our cpumask bit and decrement the ref
and delete the entry, even though it might not yet be added?


(old-ref)
->cpumask =
if (!->cpumask)
->refs =
if (!->refs)
->func()
->refs--
if (!->refs)
list_del_rcu()
list_add_rcu()
Then what happens?


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