Re: Additional compiler barrier required in sched_preempt_enable_no_resched?

[Vikram Mulukutla <markivx@codeaurora.org>]

On 5/14/2016 8:39 AM, Thomas Gleixner wrote:
> On Fri, 13 May 2016, Vikram Mulukutla wrote:
>> On 5/13/2016 7:58 AM, Peter Zijlstra wrote:
>>> diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
>>> index 5d8ffa3e6f8c..c1cde3577551 100644
>>> --- a/include/asm-generic/preempt.h
>>> +++ b/include/asm-generic/preempt.h
>>> @@ -7,10 +7,10 @@
>>>
>>>    static __always_inline int preempt_count(void)
>>>    {
>>> -	return current_thread_info()->preempt_count;
>>> +	return READ_ONCE(current_thread_info()->preempt_count);
>>>    }
>>>
>>> -static __always_inline int *preempt_count_ptr(void)
>>> +static __always_inline volatile int *preempt_count_ptr(void)
>>>    {
>>>    	return &current_thread_info()->preempt_count;
>>>    }
>>>
>>
>> Thanks Peter, this patch worked for me. The compiler no longer optimizes out
>> the increment/decrement of the preempt_count.
>
> I have a hard time to understand why the compiler optimizes out stuff w/o that
> patch.
>
> We already have:
>
> #define preempt_disable() \
> do { \
>          preempt_count_inc(); \
>          barrier(); \
> } while (0)
>
> #define sched_preempt_enable_no_resched() \
> do { \
>          barrier(); \
>          preempt_count_dec(); \
> } while (0)
>
> #define preempt_enable() \
> do { \
>          barrier(); \
>          if (unlikely(preempt_count_dec_and_test())) \
>                  __preempt_schedule(); \
> } while (0)
>
> So the barriers already forbid that the compiler reorders code. How on earth
> is the compiler supposed to optimize the dec/inc out?
>

While I cannot claim more than a very rudimentary knowledge of 
compilers, this was the initial reaction that I had as well. But then 
the barriers are in the wrong spots for the way the code was used in the 
driver in question. preempt_disable has the barrier() _after_ the 
increment, and sched_preempt_enable_no_resched has it _before_ the 
decrement. Therefore, if one were to use preempt_enable_no_resched 
followed by preempt_disable, there is no compiler barrier between the 
decrement and the increment statements. Whether this should translate to 
such a seemingly aggressive optimization is something I'm not completely 
sure of.

> There is more code than the preempt stuff depending on barrier() and expecting
> that the compiler does not optimize out things at will. Are we supposed to
> hunt all occurences and amend them with READ_ONCE or whatever one by one?
>

I think the barrier is working as intended for the sequence of 
preempt_disable followed by preempt_enable_no_resched.

> Thanks,
>
> 	tglx
>
>
>

As a simple experiment I used gcc on x86 on the following C program. 
This was really to convince myself rather than you and Peter!

#include <stdio.h>

#define barrier() __asm__ __volatile__("": : :"memory")

struct thread_info {
         int pc;
};

#define preempt_count() \
         ti_ptr->pc

#define preempt_disable() \
do \
{ \
         preempt_count() += 1; \
         barrier(); \
} \
while (0)

#define preempt_enable() \
do \
{ \
         barrier(); \
         preempt_count() -=  1; \
} \
while (0)

struct thread_info *ti_ptr;

int main(void)
{
         struct thread_info ti;
         ti_ptr = &ti;
         int b;

         preempt_enable();
         b = b + 500;
         preempt_disable();

         printf("b = %d\n", b);

         return 0;
}

With gcc -O2 I get this (the ARM compiler behaves similarly):

0000000000400470 <main>:
   400470:       48 83 ec 18             sub    $0x18,%rsp
   400474:       48 89 25 cd 0b 20 00    mov    %rsp,0x200bcd(%rip)
   40047b:       ba f4 01 00 00          mov    $0x1f4,%edx
   400480:       be 14 06 40 00          mov    $0x400614,%esi
   400485:       bf 01 00 00 00          mov    $0x1,%edi
   40048a:       31 c0                   xor    %eax,%eax
   40048c:       e8 cf ff ff ff          callq  400460 <__printf_chk@plt>
   400491:       31 c0                   xor    %eax,%eax
   400493:       48 83 c4 18             add    $0x18,%rsp
   400497:       c3                      retq

If I swap preempt_enable and preempt_disable I get this:

0000000000400470 <main>:
   400470:       48 83 ec 18             sub    $0x18,%rsp
   400474:       48 89 25 cd 0b 20 00    mov    %rsp,0x200bcd(%rip)
   40047b:       83 04 24 01             addl   $0x1,(%rsp)
   40047f:       48 8b 05 c2 0b 20 00    mov    0x200bc2(%rip),%rax
   400486:       ba f4 01 00 00          mov    $0x1f4,%edx
   40048b:       be 14 06 40 00          mov    $0x400614,%esi
   400490:       bf 01 00 00 00          mov    $0x1,%edi
   400495:       83 28 01                subl   $0x1,(%rax)
   400498:       31 c0                   xor    %eax,%eax
   40049a:       e8 c1 ff ff ff          callq  400460 <__printf_chk@plt>
   40049f:       31 c0                   xor    %eax,%eax
   4004a1:       48 83 c4 18             add    $0x18,%rsp
   4004a5:       c3                      retq

Note: If I place ti_ptr on the stack, the ordering/barriers no longer 
matter, the inc/dec is always optimized out. I suppose the compiler does 
treat current_thread_info as coming from a different memory location 
rather than the current stack frame. In any case, IMHO the volatile 
preempt_count patch is necessary.

Thanks,
Vikram

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