linux-kernel - [PATCH] x86: fix boot crash in NMI watchdog with CONFIG_CPUMASK

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Message-Id: <200904211600.16348.rusty@rustcorp.com.au>
Date:	Tue, 21 Apr 2009 16:00:15 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] x86: fix boot crash in NMI watchdog with CONFIG_CPUMASK_OFFSTACK=y and flat APIC

fcef8576d8a64fc603e719c97d423f9f6d4e0e8b converted backtrace_mask to a
cpumask_var_t, and assumed check_nmi_watchdog was called before
nmi_watchdog_tick was ever called.  Steven's oops shows I was wrong.

This is something of a bandaid: I'm not sure we *should* be calling
nmi_watchdog_tick before check_nmi_watchdog.  Note that gcc eliminates
this test for the CONFIG_CPUMASK_OFFSTACK=n case.

LKML Message-ID: <alpine.DEB.2.00.0904202113520.10097@...dalf.stny.rr.com>
Reported-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
---
 arch/x86/kernel/apic/nmi.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/nmi.c b/arch/x86/kernel/apic/nmi.c
--- a/arch/x86/kernel/apic/nmi.c
+++ b/arch/x86/kernel/apic/nmi.c
@@ -414,7 +414,8 @@ nmi_watchdog_tick(struct pt_regs *regs, 
 		touched = 1;
 	}
 
-	if (cpumask_test_cpu(cpu, backtrace_mask)) {
+	/* We can be called before check_nmi_watchdog, hence NULL check. */
+	if (backtrace_mask != NULL && cpumask_test_cpu(cpu, backtrace_mask)) {
 		static DEFINE_SPINLOCK(lock);	/* Serialise the printks */
 
 		spin_lock(&lock);
--
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