Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPUoffline from atomic context
From: Oleg Nesterov
Date: Tue Dec 18 2012 - 14:43:58 EST
On 12/18, Srivatsa S. Bhat wrote:
>
> So now that we can't avoid disabling and enabling interrupts,
Still I think it would be better to not use local_irq_save/restore
directly. And,
> I was
> wondering if we could exploit this to avoid the smp_mb()..
>
> Maybe this is a stupid question, but I'll shoot it anyway...
> Does local_irq_disable()/enable provide any ordering guarantees by any chance?
Oh, I do not know.
But please look at the comment above prepare_to_wait(). It assumes
that even spin_unlock_irqrestore() is not the full barrier.
In any case. get_online_cpus_atomic() has to use irq_restore, not
irq_enable. And _restore does nothing "special" if irqs were already
disabled, so I think we can't rely on sti.
> I tried thinking about other ways to avoid that smp_mb() in the reader,
Just in case, I think there is no way to avoid mb() in _get (although
perhaps it can be "implicit").
The writer changes cpu_online_mask and drops the lock. We need to ensure
that the reader sees the change in cpu_online_mask after _get returns.
> but was unsuccessful. So if the above assumption is wrong, I guess we'll
> just have to go with the version that uses synchronize_sched() at the
> writer-side.
In this case we can also convert get_online_cpus() to use percpu_rwsem
and avoid mutex_lock(&cpu_hotplug.lock), but this is minor I guess.
I do not think get_online_cpus() is called too often.
Oleg.
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