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 17:12:52 +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 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.

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