linux-kernel - Re: [PATCH 32/35] clockevents: Fix cpu down race for hrtimer based broadcasting

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Date:	Tue, 17 Feb 2015 11:39:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, rjw@...ysocki.net,
	tglx@...utronix.de
Subject: Re: [PATCH 32/35] clockevents: Fix cpu down race for hrtimer based
 broadcasting

On Tue, Feb 17, 2015 at 09:33:45AM +0530, Preeti U Murthy wrote:
> On 02/16/2015 05:45 PM, Peter Zijlstra wrote:
> 
> >> From: Thomas Gleixner <tglx@...utronix.de>
> 
> >> @@ -428,7 +428,7 @@ static int __ref _cpu_down(unsigned int
> >> 	__cpu_die(cpu);
> >>
> >> 	/* CPU is completely dead: tell everyone.  Too late to complain. */
> >>-	tick_cleanup_dead_cpu(cpu);
> >>+	tick_takeover(cpu);
> 
> Why is tick_handover() called after __cpu_die()?

See: [PATCH 11/35] clockevents: Cleanup dead cpu explicitely
it used to be a CPU_DEAD notifier.

But, I think, the actual reason would be that you cannot be sure its not
still ticking until its actually proper dead and buried, so trying to
take over a tick from a cpu that's still ticking is... well, suspect.

> And the function tick_takeover()
> is not introduced until the next patch. tick_broadcast_takeover_bc() is the
> function used instead in this patch.

Indeed so; let me correct that for bisection's sake.
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