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

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

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.

Regards,
Srivatsa S. Bhat