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SubjectRe: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context
On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
> On 12/11, Srivatsa S. Bhat wrote:
>>
>> IOW, the hotplug readers just increment/decrement their per-cpu refcounts
>> when no writer is active.
>
> plus cli/sti ;)

Of course, forgot to mention it, again! :)

> and increment/decrement are atomic.
>
> At first glance looks correct to me, but I'll try to read it carefully
> later.
>
> A couple of minor nits,
>
>> +static DEFINE_PER_CPU(bool, writer_signal);
>
> Why it needs to be per-cpu? It can be global and __read_mostly to avoid
> the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
> into a single cacheline...
>

Even I realized this (that we could use a global) after posting out the
series.. But do you think that it would be better to retain the per-cpu
variant itself, due to the cache effects?

>> +void get_online_cpus_atomic(void)
>> +{
>> + unsigned long flags;
>> +
>> + preempt_disable();
>> +
>> + if (cpu_hotplug.active_writer == current)
>> + return;
>> +
>> + local_irq_save(flags);
>
> Yes... this is still needed, we are going to increment reader_percpu_refcnt
> unconditionally and this makes reader_nested_percpu() == T.
>
> But,
>
>> +void put_online_cpus_atomic(void)
>> +{
>> + unsigned long flags;
>> +
>> + if (cpu_hotplug.active_writer == current)
>> + goto out;
>> +
>> + local_irq_save(flags);
>> +
>> + /*
>> + * We never allow heterogeneous nesting of readers. So it is trivial
>> + * to find out the kind of reader we are, and undo the operation
>> + * done by our corresponding get_online_cpus_atomic().
>> + */
>> + if (__this_cpu_read(reader_percpu_refcnt))
>> + __this_cpu_dec(reader_percpu_refcnt);
>> + else
>> + read_unlock(&hotplug_rwlock);
>> +
>> + local_irq_restore(flags);
>> +out:
>> + preempt_enable();
>> +}
>
> Do we really need local_irq_save/restore in put_ ?
>

Hmm.. good point! I don't think we need it.


Regards,
Srivatsa S. Bhat




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