linux-kernel - Re: [patch] i386/x86_64: smp_call

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Date:	Fri, 9 Feb 2007 09:42:21 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	akpm@...l.org, mingo@...e.hu, ak@...e.de, jan.glauber@...ibm.com,
	schwidefsky@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency

On Thu, Feb 08, 2007 at 12:43:28PM -0800, David Miller wrote:
> From: Heiko Carstens <heiko.carstens@...ibm.com>
> Date: Thu, 8 Feb 2007 21:32:10 +0100
>  
> > So either all spin_lock_bh's should be converted to spin_lock,
> > which would limit smp_call_function()/smp_call_function_single()
> > to process context & irqs enabled.
> > Or the spin_lock's could be converted to spin_lock_bh which would
> > make it possible to call these two functions even if in softirq
> > context. AFAICS this should be safe.
> [...]
> In short, it's a mess :-)
>  
> I think it's logically simpler if we disallow smp_call_function*()
> from any kind of asynchronous context.  But I'm sure your driver
> has a true need for this for some reason.

I just want to avoid that s390 has different semantics for
smp_call_functiom*() than any other architecture. But then again it
will probably not hurt since we allow more.
Another thing that comes into my mind is smp_call_function together
with cpu hotplug. Who is responsible that preemption and with that
cpu hotplug is disabled?
Is it the caller or smp_call_function itself?
If it's smp_call_function then s390 would be broken, since
then we would have
int cpus = num_online_cpus()-1;
in preemptible context... I agree: what a mess :)
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