Re: Kernel broken on processors without performance counters

[Andy Lutomirski]

On Wed, Jul 8, 2015 at 1:04 PM, Jason Baron <jasonbaron0@xxxxxxxxx> wrote:
> On 07/08/2015 01:37 PM, Andy Lutomirski wrote:
>> 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.
>
> So the 'static_key_false' is really branch is initially false. We had
> a naming discussion before, but if ppl think its confusing,
> 'static_key_init_false', or 'static_key_default_false' might be better,
> or other ideas.... I agree its confusing.

I think the current naming is almost maximally bad.  The naming would
be less critical if it were typesafe, though.

>
> In terms of getting the type to match so we don't have these
> mismatches, I think we could introduce 'struct static_key_false'
> and 'struct static_key_true' with proper initializers. However,
> 'static_key_slow_inc()/dec()'  would also have to add the
> true/false modifier.

I think that would be okay.

> Or maybe we do:
>
> struct static_key_false {
>     struct static_key key;
> } random_key;
>
> and then the 'static_key_sloc_inc()/dec()' would just take
> a &random_key.key....

That would be okay, too.  Or it could be a macro that has the same effect.

>
> If we were to change this, I don't think it would be too hard to
> introduce the new API, convert subsystems over time and then
> drop the old one.

And we might discover a bug or three while doing it :)

--Andy
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