linux-kernel - Re: qemu:arm test failure due to commit 8053871d0f7f (smp: Fix smp_call_function_single

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Message-ID: <20150420154148.GA18104@debian>
Date:	Mon, 20 Apr 2015 17:41:48 +0200
From:	Rabin Vincent <rabin@....in>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: qemu:arm test failure due to commit 8053871d0f7f (smp: Fix
 smp_call_function_single_async() locking)

On Sun, Apr 19, 2015 at 08:01:40PM +0200, Ingo Molnar wrote:
> That's all fine and good, but why is an IPI sent to a non-existent 
> CPU? It's not like we don't know which CPU is up and down.

The perf events code is trying to call smp_call_function_single() on the
non-existent CPU in perf_event_exit_cpu_context() while handling the
CPU_UP_CANCELED notification.  perf_cpu_notify() handles CPU_UP_CANCELED
and CPU_DOWN_PREPARE in the same way.

(cpu_up() is tried for the non-existing CPUs because in this case what
 is specified in the device tree does not match reality.)
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