Re: [PATCH v6 2/3] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU"

[Srivatsa S. Bhat]

On 05/23/2014 09:18 PM, Peter Zijlstra wrote:
> On Fri, May 23, 2014 at 09:07:18PM +0530, Srivatsa S. Bhat wrote:
>> On 05/23/2014 09:03 PM, Srivatsa S. Bhat wrote:
>>> On 05/23/2014 09:01 PM, Peter Zijlstra wrote:
>>>> On Fri, May 23, 2014 at 08:48:07PM +0530, Srivatsa S. Bhat wrote:
>>>>> On 05/23/2014 08:42 PM, Peter Zijlstra wrote:
>>>>>> On Fri, May 23, 2014 at 08:15:35PM +0530, Srivatsa S. Bhat wrote:
>>>>>>>>> +		 * During CPU offline, we don't want the other CPUs to send
>>>>>>>>> +		 * IPIs to the active_cpu (the outgoing CPU) *after* it has
>>>>>>>>> +		 * disabled interrupts (because, then it will notice the IPIs
>>>>>>>>> +		 * only after it has gone offline). We can prevent this by
>>>>>>>>> +		 * making the other CPUs disable their interrupts first - that
>>>>>>>>> +		 * way, they will run the stop-machine code with interrupts
>>>>>>>>> +		 * disabled, and hence won't send IPIs after that point.
>>>>>>
>>>>>> That's complete nonsense, you can send IPIs all you want with interrupts
>>>>>> disabled.
>>>>>>
>>>>>
>>>>> True, but that's not what the comment says. It says "you can't send IPIs
>>>>> because you are running the *stop-machine* loop, because the stop-machine loop
>>>>> doesn't send IPIs itself! The only possibility of sending IPIs from within
>>>>> stop-machine is if that CPU can takes an interrupt and the *interrupt handler*
>>>>> sends the IPI (like what the block layer used to do) - and we precisely avoid
>>>>> that possibility by disabling interrupts. So no IPIs will be sent beyond
>>>>> this point.
>>>>
>>>> but one of those CPUs is running the stop machine function, which calls
>>>> CPU_DYING which runs all kinds of nonsense and therefore can send IPIs
>>>> all it wants, right?
>>>>
>>>
>>> Yes, but that CPU certainly won't IPI itself! (We are trying to avoid getting
>>> IPIs on precisely that CPU - the one which is about to go offline).
>>>
>>
>> And the comment makes that distinction between the "active-cpu" and "other CPUs"
>> (where active-cpu is the one which runs the stop-machine function and eventually
>> goes offline). Thus "other CPUs" won't send IPIs after that point, because they
>> are running the stop-machine loop with interrupts disabled. This ensures that
>> the "active-cpu" doesn't get any IPIs - which is what we want.
> 
> OK, so clearly I'm having trouble reading today :/ Makes sense now.
> 
> But yes, its unlikely for CPU_DYING to self-IPI, although if you really
> want, I can do ;-)
>

Haha :-)
 
> And I guess the one extra state doesn't hurt too bad for
> stop_two_cpus().
> 

Ok, that's good then.

Regards,
Srivatsa S. Bhat

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