Re: [PATCH 2/11] oprofile: arch-independent code for stack trace sampling

[John Levon <levon@movementarian.org>]

On Tue, Nov 09, 2004 at 10:35:48PM +1100, Greg Banks wrote:

> > oprofile is currently doing suspicious things with smp_processor_id() in
> > premptible reasons.  Is this patch compounding things?
> 
> It's not changing the contexts where smp_processor_id() is called,
> just pushing it down one level from a bunch of interrupt handlers
> to the 2 oprofile sampling functions they call.  If it was busted
> before it's no more nor less busted now.
> 
> I presume the perceived problem is that with CONFIG_PREEMPT=y the
> thread can be pre-empted onto another CPU?  If it makes everyone
> happier I can sprinkle a few preempt_disable()s around, but I'd
> prefer to do it in a subsequent patch rather than respin this.

Andrew: basically the warning is false, there is no bug in this code.

we don't want to use preempt_disable(). Instead we want some way to get
a CPU ID and then carry on in pre-emptible fashion. It's only used to
index into an array, and if we get pre-empted onto another CPU it's not
a major deal.

(Yes, this breaks with CPU hotplug, but so does the rest of OProfile and
I've yet to see a sensible API for handling this, that is a ctor/dtor
style API)

regards
john