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

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Message-ID: <20090126233732.GA23128@elte.hu>
Date:	Tue, 27 Jan 2009 00:37:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Ed Swierk <eswierk@...stanetworks.com>
Cc:	Oleg Nesterov <oleg@...hat.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()


* Ed Swierk <eswierk@...stanetworks.com> wrote:

> On Tue, 2009-01-27 at 00:15 +0100, Ingo Molnar wrote:
> > This trades a (harmless) debug warning against a potential deadlock or 
> > even a crash, because print_fatal_signal() can do this:
> > 
> >                         __get_user(insn, (unsigned char *)(regs->ip + i));
> > 
> > which will work without a fault most of the time but might also generate a 
> > pagefault and schedule away from atomic context.
> 
> Ouch!
> 
> > So please add preempt_disable()+preempt_enable() calls around the 
> > show_regs() call instead.
> 
> Take 2:
> 
> With print-fatal-signals=1 on a kernel with CONFIG_PREEMPT=y, sending an
> unexpected signal to a process causes a BUG: using smp_processor_id() in
> preemptible code.
> 
> get_signal_to_deliver() releases the siglock before calling
> print_fatal_signal(), which calls show_regs(), which calls
> smp_processor_id(), which is not supposed to be called from a
> preemptible thread.
> 
> Signed-off-by: Ed Swierk <eswierk@...stanetworks.com>

applied to tip/core/urgent, thanks Ed!

You can track/test your fix via the -tip tree's tip/master branch:

  http://people.redhat.com/mingo/tip.git/README

	Ingo
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