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

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Message-ID: <20150419180140.GA8934@gmail.com>
Date:	Sun, 19 Apr 2015 20:01:40 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rabin Vincent <rabin@....in>,
	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)


* Guenter Roeck <linux@...ck-us.net> wrote:

> > I _think_ most such callers calling an SMP function call for 
> > offline or out of range CPUs are at minimum racy.
>
> Not really; at least the online cpu part is an absolutely normal use 
> case for qemu-arm.

The problem is that an IPI is attempted to be sent to a non-existent 
CPU AFAICS, right?

> Sure, you can argue that "this isn't the real system", and that 
> qemu-arm should be "fixed", but there are reasons - the emulation is 
> (much) slower if the number of CPUs is set to 4, and not everyone 
> who wants to use qemu has a system with as many CPUs as the emulated 
> system would normally have.

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.

Thanks,

	Ingo
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