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

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Date:	Mon, 26 Jan 2009 17:34:00 -0800
From:	Ed Swierk <eswierk@...stanetworks.com>
To:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	rml@...h9.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix BUG: using smp_processor_id() in preemptible code
 in print_fatal_signal()

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. 

> 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.

Signed-off-by: Ed Swierk <eswierk@...stanetworks.com>

---
Index: linux-2.6.27.4/kernel/signal.c
===================================================================
--- linux-2.6.27.4.orig/kernel/signal.c
+++ linux-2.6.27.4/kernel/signal.c
@@ -890,7 +890,9 @@ static void print_fatal_signal(struct pt
 	}
 #endif
 	printk("\n");
-	show_regs(regs);
+	preempt_disable();
+	show_regs(regs); /* calls smp_processor_id(), preemption not allowed */
+	preempt_enable();
 }
 
 static int __init setup_print_fatal_signals(char *str)


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