Date: Mon, 20 Apr 2015 17:41:48 +0200
From: Rabin Vincent <rabin@rab.in>
To: Ingo Molnar <mingo@kernel.org>
Cc: Guenter Roeck <linux@roeck-us.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@us.ibm.com>
Subject: Re: qemu:arm test failure due to commit 8053871d0f7f (smp: Fix
 smp_call_function_single_async() locking)
Message-ID: <20150420154148.GA18104@debian>
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 <55330B32.4010907@roeck-us.net>
 <20150419033940.GA25145@debian>
 <CA+55aFw4FSja+VBuCYJ7wLXKVRQZ7w6vOUaUJ4B=FXyBmNkrUg@mail.gmail.com>
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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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