linux-kernel - Re: [PATCH v6 2/3] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU"

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Date:	Fri, 23 May 2014 17:48:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>, tglx@...utronix.de,
	mingo@...nel.org, tj@...nel.org, rusty@...tcorp.com.au,
	akpm@...ux-foundation.org, hch@...radead.org, mgorman@...e.de,
	riel@...hat.com, bp@...e.de, rostedt@...dmis.org,
	mgalbraith@...e.de, ego@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, oleg@...hat.com, rjw@...ysocki.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/3] CPU hotplug, stop-machine: Plug race-window that
 leads to "IPI-to-offline-CPU"

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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