linux-kernel - Re: [PATCH] Make smp_call_function{_single} go WARNING and return -EINVAL on !SMP (was Re: [PATCH] i386/x86_64: NMI watchdog: Protect smp_call_function() within CONFIG

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Date:	Thu, 7 Jun 2007 19:51:27 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Satyam Sharma <satyam.sharma@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Alan Cox <alan@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] Make smp_call_function{_single} go WARNING and return -EINVAL on !SMP (was Re: [PATCH] i386/x86_64: NMI watchdog: Protect smp_call_function() within CONFIG_SMP)

> The smp_call_function{_single} functions are used to run
> given function on all {or speicified} *other* CPUs. For
> UP systems, "other" CPUs simply don't exist, so we flag
> such incorrect usage of these functions using a WARNING.

If other cpus don't exist then smp_call_function() should just do
*nothing* (there is no other cpu right?). We don't want to sprinkle
a ton of #ifdef CONFIG_SMP around each smp_call_function().

> Also, -EBUSY is generally returned by arch implementations
> when they find that target_cpu == current_cpu, which is not
> a comparable case to the !SMP case. Use -EINVAL instead,
> similar to what powerpc does for !cpu_online(target), which
> is somewhat more analogous.

No. Current semantics of smp_call_function_single() are that it
returns -EBUSY if called on the *current* cpu. Since on !CONFIG_SMP the
only possible cpu it can be called on is the current one, the only
sane return value is -EBUSY.
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