Re: [RFC 2/3] x86: Switch all C consumers of kernel_stack to this_cpu_sp0

[Denys Vlasenko]

On 02/27/2015 08:56 PM, Andy Lutomirski wrote:
> On Feb 27, 2015 8:13 AM, "Denys Vlasenko" <dvlasenk@xxxxxxxxxx> wrote:
>>
>> On 02/27/2015 01:07 AM, Andy Lutomirski wrote:
>>> This will make modifying the semantics of kernel_stack easier.
>>>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>>> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
>>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>>> ---
>>>  arch/x86/include/asm/thread_info.h | 3 +--
>>>  arch/x86/kernel/traps.c            | 2 +-
>>>  2 files changed, 2 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
>>> index e82e95abc92b..92549053d86d 100644
>>> --- a/arch/x86/include/asm/thread_info.h
>>> +++ b/arch/x86/include/asm/thread_info.h
>>> @@ -163,8 +163,7 @@ DECLARE_PER_CPU(unsigned long, kernel_stack);
>>>  static inline struct thread_info *current_thread_info(void)
>>>  {
>>>       struct thread_info *ti;
>>> -     ti = (void *)(this_cpu_read_stable(kernel_stack) +
>>> -                   KERNEL_STACK_OFFSET - THREAD_SIZE);
>>> +     ti = (void *)(this_cpu_sp0() - THREAD_SIZE);
>>>       return ti;
>>>  }
>>>
>>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>>> index c74f2f5652da..d287ea779728 100644
>>> --- a/arch/x86/kernel/traps.c
>>> +++ b/arch/x86/kernel/traps.c
>>> @@ -174,7 +174,7 @@ void ist_begin_non_atomic(struct pt_regs *regs)
>>>        * will catch asm bugs and any attempt to use ist_preempt_enable
>>>        * from double_fault.
>>>        */
>>> -     BUG_ON(((current_stack_pointer() ^ this_cpu_read_stable(kernel_stack))
>>> +     BUG_ON(((current_stack_pointer() ^ (this_cpu_sp0() - 1))
>>>               & ~(THREAD_SIZE - 1)) != 0);
>>
>> While we are at it, I propose a more readable version of this check:
>>
>> BUG_ON(this_cpu_sp0() - current_stack_pointer() >= THREAD_SIZE);
>>
>> Yes, I am aware that it is not equivalent to the existing condition
>> - it uses the fact that this_cpu_sp0(), previous check
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Oops...
I meant to say "...the fact that this_cpu_sp0() points to the very top
of the stack, previous check ..."

>> wasn't making that assumption. But that assumption is true,
>> so shouldn't be a problem.
> 
> You're missing an absolute value in here, though.  This isn't a check
> for overflow; it's a check that we aren't on an IST or other per cpu
> stack.

Yes, that's exactly what the condition checks for. It reads

"is current stack pointer below task's kernel stack by no more
than THREAD_SIZE?"

which is only possible if we are on task's kernel stack:

If current_stack_pointer() is elsewhere, it is either
(a) much smaller than this_cpu_sp0(), and BUG_ON condition
    obviously triggers; or
(b) it is somewhere above this_cpu_sp0(), in which case subtraction
    overflows and condition triggers too.

How would you write this condition so that it's easily readable?
Evidently, my version isn't as readable sa I hoped...
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