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

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Message-ID: <50CA0317.90501@linux.vnet.ibm.com>
Date:	Thu, 13 Dec 2012 22:02:23 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.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 12/13/2012 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().

> 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.
> 

Regards,
Srivatsa S. Bhat

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