Date: Sun, 19 Apr 2015 20:01:40 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Rabin Vincent <rabin@rab.in>,
        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: <20150419180140.GA8934@gmail.com>
References: <20150418232325.GA22411@roeck-us.net>
 <20150418234050.GA5987@roeck-us.net>
 <CA+55aFz1934X3wu7FdGerwYMJ_BAkrFsajOQFtU3_ogsUX3_eQ@mail.gmail.com>
 <55330B32.4010907@roeck-us.net>
 <20150419033940.GA25145@debian>
 <CA+55aFw4FSja+VBuCYJ7wLXKVRQZ7w6vOUaUJ4B=FXyBmNkrUg@mail.gmail.com>
 <CA+55aFwTMJ7CAtBaL-uGsgVf9-oyGwqLZwz_pwjX=umbS-2CNQ@mail.gmail.com>
 <20150419093112.GA6139@gmail.com>
 <5533B6D7.9050101@roeck-us.net>
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* Guenter Roeck <linux@roeck-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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