Re: Kernel broken on processors without performance counters

[Andy Lutomirski]

On Wed, Jul 8, 2015 at 9:07 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Jul 08, 2015 at 11:17:38AM -0400, Mikulas Patocka wrote:
>> Hi
>>
>> I found out that the patch a66734297f78707ce39d756b656bfae861d53f62 breaks
>> the kernel on processors without performance counters, such as AMD K6-3.
>> Reverting the patch fixes the problem.
>>
>> The static key rdpmc_always_available somehow gets set (I couldn't really
>> find out what is setting it, the function set_attr_rdpmc is not executed),
>> cr4_set_bits(X86_CR4_PCE) is executed and that results in a crash on boot
>> when attempting to execute init, because the proecssor doesn't support
>> that bit in CR4.
>
> Urgh, the static key trainwreck bites again.
>
> One is not supposed to mix static_key_true() and STATIC_KEY_INIT_FALSE.
>
> Does this make it go again?
>
> ---
>  arch/x86/include/asm/mmu_context.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 5e8daee7c5c9..804a3a6030ca 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -23,7 +23,7 @@ extern struct static_key rdpmc_always_available;
>
>  static inline void load_mm_cr4(struct mm_struct *mm)
>  {
> -       if (static_key_true(&rdpmc_always_available) ||
> +       if (static_key_false(&rdpmc_always_available) ||

In what universe is "static_key_false" a reasonable name for a
function that returns true if a static key is true?

Can we rename that function?  And could we maybe make static keys type
safe?  I.e. there would be a type that starts out true and a type that
starts out false.

--Andy
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