[PATCH] Remove indirect variable usage at arch/x86/kernel/tsc.c

Subrata Modak
2009-05-19T18:03:28Z

Hi,

The following warning is generated on compilation:

CC arch/x86/kernel/tsc.o
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

However, there seems to be no practical usage of variable 'dummy'
in the following piece of code:

630 static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
631 void *data)
632 {
633 struct cpufreq_freqs *freq = data;
634 unsigned long *lpj, dummy;
635
636 if (cpu_has(&cpu_data(freq->cpu), X86_FEATURE_CONSTANT_TSC))
637 return 0;
638
639 lpj = &dummy;
640 if (!(freq->flags & CPUFREQ_CONST_LOOPS))
641 #ifdef CONFIG_SMP
642 lpj = &cpu_data(freq->cpu).loops_per_jiffy;
643 #else
644 lpj = &boot_cpu_data.loops_per_jiffy;
645 #endif
646

'lpj' probably will get to point to some address after this if() statement.

647 if (!ref_freq) {
648 ref_freq = freq->old;
649 loops_per_jiffy_ref = *lpj;

And, if it does, then "loops_per_jiffy_ref" will have a proper value,
else, even with "lpj = &dummy" will not gurantee "loops_per_jiffy_ref = *lpj"
to have the expected value.

650 tsc_khz_ref = tsc_khz;
651 }
652 if ((val == CPUFREQ_PRECHANGE && freq->old < freq->new) ||
653 (val == CPUFREQ_POSTCHANGE && freq->old > freq->new) ||
654 (val == CPUFREQ_RESUMECHANGE)) {
655 *lpj = cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new);
656
657 tsc_khz = cpufreq_scale(tsc_khz_ref, ref_freq, freq->new);
658 if (!(freq->flags & CPUFREQ_CONST_LOOPS))
659 mark_tsc_unstable("cpufreq changes");
660 }
661
662 set_cyc2ns_scale(tsc_khz, freq->cpu);
663
664 return 0;
665 }

Is there any specific reason for 'dummy' to exist in this function ?
If not, i would like to propose the following fix.

Signed-off-by: Subrata Modak <subrata@linux.vnet.ibm.com>
To: <x86@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Sachin P Sant <sachinp@linux.vnet.ibm.com>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Subject: [PATCH] Remove indirect variable usage at arch/x86/kernel/tsc.c
---

--- a/arch/x86/kernel/tsc.c 2009-05-19 00:57:26.000000000 +0530
+++ b/arch/x86/kernel/tsc.c 2009-05-19 21:45:46.000000000 +0530
@@ -631,12 +631,11 @@ static int time_cpufreq_notifier(struct
void *data)
{
struct cpufreq_freqs *freq = data;
- unsigned long *lpj, dummy;
+ unsigned long *lpj = NULL;

if (cpu_has(&cpu_data(freq->cpu), X86_FEATURE_CONSTANT_TSC))
return 0;

- lpj = &dummy;
if (!(freq->flags & CPUFREQ_CONST_LOOPS))
#ifdef CONFIG_SMP
lpj = &cpu_data(freq->cpu).loops_per_jiffy;

---
Regards--
Subrata