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

Pavel Machek
2009-05-24T19:43:22Z

Hi!

> 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.

But that's a bug to be fixed, I'd say? ... actually I believe you are
introducing a bug here. Yes, old code would put random numbers in
loops_per_jiffy_ref for !CPUFREQ_CONST_LOOPS, but you are introducing
oops there.

Have you tested the code?

> @@ -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;
>

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html