linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Date:	Thu, 13 Dec 2012 17:17:09 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU
	offline from atomic context

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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