linux-kernel - Re: fishy code in arch/x86/kernel/tsc.c:time_cpufreq

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Message-ID: <20090601162955.GA15237@redhat.com>
Date:	Mon, 1 Jun 2009 12:29:55 -0400
From:	Dave Jones <davej@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: fishy code in arch/x86/kernel/tsc.c:time_cpufreq_notifier()

On Mon, Jun 01, 2009 at 10:21:04AM -0400, Christoph Hellwig wrote:
 > Just notice the following error from gcc 4.4:
 > 
 > arch/x86/kernel/tsc.c: In function 'time_cpufreq_notifier':
 > arch/x86/kernel/tsc.c:634: warning: 'dummy' may be used uninitialized in this function
 > 
 > where we do have CONFIG_SMP set, freq->flags & CPUFREQ_CONST_LOOPS is
 > true and ref_freq is false.
 > 
 > Can that case actually happen?
 
I think you're right, though the circumstances for hitting it are really
low. Nearly all SMP capable cpufreq drivers set CPUFREQ_CONST_LOOPS.
powernow-k8 is really the only exception. The older CPUs were typically
only ever UP. (powernow-k7 never supported SMP for eg)

It's probably worth fixing.

I think the patch below is closer to the actual intent, with the bonus
of being a lot more readable.

Sanity check?

	Dave

Fix possible uninitialized use of dummy, by just removing it,
and making the setting of lpj more obvious.

Signed-off-by: Dave Jones <davej@...hat.com>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index d57de05..78c54ea 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -631,17 +631,15 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
 				void *data)
 {
 	struct cpufreq_freqs *freq = data;
-	unsigned long *lpj, dummy;
+	unsigned long *lpj;
 
 	if (cpu_has(&cpu_data(freq->cpu), X86_FEATURE_CONSTANT_TSC))
 		return 0;
 
-	lpj = &dummy;
-	if (!(freq->flags & CPUFREQ_CONST_LOOPS))
+	lpj = &boot_cpu_data.loops_per_jiffy;
 #ifdef CONFIG_SMP
+	if (!(freq->flags & CPUFREQ_CONST_LOOPS))
 		lpj = &cpu_data(freq->cpu).loops_per_jiffy;
-#else
-	lpj = &boot_cpu_data.loops_per_jiffy;
 #endif
 
 	if (!ref_freq) {

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