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DateMon, 1 Jun 2009 12:29:55 -0400
FromDave Jones <>
SubjectRe: 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@redhat.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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