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

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Date:	Fri, 14 Dec 2012 19:03:45 +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 09:47 PM, Oleg Nesterov wrote:
> > 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?
> >
>
> The comment seems to say that its not safe wrt interrupts. But looking at
> the code in include/linux/percpu.h, IIUC, that is true only about
> this_cpu_read() because it only disables preemption.
>
> However, this_cpu_inc() looks safe wrt interrupts because it wraps the
> increment within raw_local_irqsave()/restore().

You mean _this_cpu_generic_to_op() I guess. So yes, I think you are right,
this_cpu_* should be irq-safe, but __this_cpu_* is not.

Thanks.

At least on x86 there is no difference between this_ and __this_, both do
percpu_add_op() without local_irq_disable/enable. But it seems that most
of architectures use generic code.

Oleg.

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