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

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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