Re: Kernel broken on processors without performance counters

[Mikulas Patocka]

On Wed, 8 Jul 2015, Peter Zijlstra 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?

Yes, this patch fixes the problem.

Mikulas

> ---
>  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) ||
>  	    atomic_read(&mm->context.perf_rdpmc_allowed))
>  		cr4_set_bits(X86_CR4_PCE);
>  	else
> 
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