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