Messages in this thread | | | Date | Fri, 23 May 2014 21:03:39 +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" |
| |
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
| |