Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPUoffline from atomic context

[Oleg Nesterov]

On 12/13, Srivatsa S. Bhat wrote:
>
> On 12/13/2012 12:42 AM, Srivatsa S. Bhat wrote:
> >
> > Even I don't spot anything wrong with it. But I'll give it some more
> > thought..
>
> Since an interrupt handler can also run get_online_cpus_atomic(), we
> cannot use the __this_cpu_* versions for modifying reader_percpu_refcnt,
> right?

Hmm. I thought that __this_cpu_* must be safe under preempt_disable().
IOW, I thought that, say, this_cpu_inc() is "equal" to preempt_disable +
__this_cpu_inc() correctness-wise.

And. I thought that this_cpu_inc() is safe wrt interrupt, like local_t.

But when I try to read the comments percpu.h, I am starting to think that
even this_cpu_inc() is not safe if irq handler can do the same?

Confused...

I am shy to ask... will, say, DEFINE_PER_CPU(local_t) and
local_inc(__this_cpu_ptr(...)) work??

> But still, this scheme is better, because the reader doesn't have to spin
> on the read_lock() with interrupts disabled.

Yes, but my main concern is that irq_disable/enable itself is not that cheap.

Oleg.

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