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

Srivatsa S. Bhat
2014-05-23T15:34:54Z

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).