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

Srivatsa S. Bhat
2014-05-23T15:19:24Z

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.