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). Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
