linux-kernel - Re: [PATCH] [14/58] x86_64: Add on_cpu

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Date:	Thu, 19 Jul 2007 16:39:20 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Andi Kleen" <ak@...e.de>
Cc:	patches@...-64.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [14/58] x86_64: Add on_cpu_single

Hi Andi,

On 7/19/07, Andi Kleen <ak@...e.de> wrote:
>
> Call a function on a target CPU but do the right thing when
> we're already on that CPU. That's the main difference from
> smp_call_function_single
> which does the wrong thing in this case (erroring out)

I think this is no longer the case, is it? With KVM updates already
merged in latest mainline -git, that modified smp_call_function_single()
behaviour ...

> +#ifdef CONFIG_SMP
> +/* Similar to smp_call_function_single, but DTRT when we're already
> +   on the right CPU. */
> +static inline void on_cpu_single(int cpu, void (*func)(void *), void *info)
> +{
> +       int me = get_cpu();
> +       if (cpu == me) {
> +               func(info);
> +               put_cpu();
> +       } else {
> +               put_cpu();
> +               /* wait is forced on because the me==cpu case above will always wait */
> +               smp_call_function_single(cpu, func, info, 0, 1);

In any case, this is unsafe. smp_call_function_single() -- with the old
semantics, which is what this patch assumes, obviously -- is quite
pointless without its _caller_ disabling preemption around it. So the
put_cpu() must come after the smp_call_function_single, otherwise
you won't even detect the error that might happen, seeing you're
ignoring its return and this wrapper being void-returning.

> +       }
> +}
> +#else
> +static inline void on_cpu_single(int cpu, void (*func)(void *), void *info)
> +{

WARN_ON(irqs_disabled());
local_irq_disable();

> +       func(info);

local_irq_restore();

> +}
> +#endif

... for the sake of API / behaviour consistency.

But probably you should just drop this ... with smp_call_function_single's
new semantics, I don't see this function growing any users.

Satyam
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