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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
