linux-kernel - Re: [PATCH] Fix BUG: using smp_processor_id() in preemptible code in print_fatal

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Date:	Tue, 27 Jan 2009 13:46:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ed Swierk <eswierk@...stanetworks.com>, rml@...h9.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix BUG: using smp_processor_id() in preemptible code
	in print_fatal_signal()


* Oleg Nesterov <oleg@...hat.com> wrote:

> On 01/26, Ed Swierk wrote:
> >
> > On Tue, 2009-01-27 at 01:41 +0100, Oleg Nesterov wrote:
> > > Ed, Ingo, but isn't it better to just use raw_smp_processor_id() in
> > > __show_regs() ? This is only debug info, the printed CPU doesn't
> > > have the "exact" meaning.
> >
> > I guess it doesn't really matter which CPU the signal handling thread 
> > happened to be running on, but are there other situations where 
> > show_regs() is always expected to print the correct CPU (and if not, 
> > why bother printing the CPU at all)?  Disabling preemption here seems 
> > the safest approach and doesn't add much overhead.
> 
> OK.
> 
> > > And, without the comment, it is not easy to see why print_fatal_signal()
> > > disables preeemption before show_regs().
> >
> > Agreed; here's an updated patch.
> 
> Actually, now I think show_regs() has other reasons to run with the 
> preemption disabled, __show_regs() does read_crX()/etc, I guess it is 
> better to stay on the same CPU throughout.
> 
> So, Ed, I am sorry for noise.

another reason why it's good to run it with preemption disabled is that 
whatever context does show_regs() ought to be non-preemptible as it deals 
with CPU local details.

In the fatal-signals case we indeed have a "it does not really matter" 
boundary case, but in most of the other uses we want to be non-preemptible 
in debug contexts, and want a constant reminder in terms of 
smp_processor_id() warnings if that expectation is not met.

	Ingo
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