Messages in this thread | | | Date | Fri, 23 May 2014 20:48:07 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH v6 2/3] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU" |
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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.
Regards, Srivatsa S. Bhat
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