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?
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