Subject: Re: linux-next: build warnings after merge of the access_once tree

From: Christian Borntraeger <[email-masked]>
Date: Thu, 26 Mar 2015 20:42:03 +0100

Am 26.03.2015 um 18:23 schrieb Christian Borntraeger:
> Am 26.03.2015 um 18:07 schrieb Peter Zijlstra:
>> On Thu, Mar 26, 2015 at 09:45:07AM -0700, Linus Torvalds wrote:
>>
>>> Stop this idiocy.
>>
>> Yeah, clearly I can type faster than I can think straight :/
>>
>>
>> In any case, I've the below patch; do you want to take it now or do you
>> want me to route it through tip/locking/urgent or something like that?
> 
> Its not urgent. Current upstream has a broken check (gcc will not emit the
> warning if the function is static). I just fixed the check in my next tree
> but I can certainly drop that tree.
> 

Thinking more about that, the removal of the ifdef for 64bit data might be
a reason to schedule that for 4.0.

>>
>> ---
>> Subject: kernel: Remove atomicy checks from {READ,WRITE}_ONCE
>> From: Peter Zijlstra <[email-masked]>
>> Date: Thu, 26 Mar 2015 17:45:37 +0100
>>
>> The fact that volatile allows for atomic load/stores is a special case
>> not a requirement for {READ,WRITE}_ONCE(). Their primary purpose is to
>> force the compiler to emit load/stores _once_.
>>
>> So remove the warning as it is correct behaviour. This also implies that
>> the u64 case is not 64bit only, so remove the #ifdef so we can generate
>> better code in that case.
>>
>> Cc: Stephen Rothwell <[email-masked]>
>> Cc: Christian Borntraeger <[email-masked]>
>> Cc: Thomas Gleixner <[email-masked]>
>> Cc: Ingo Molnar <[email-masked]>
>> Cc: "H. Peter Anvin" <[email-masked]>
>> Cc: Davidlohr Bueso <[email-masked]>
>> Cc: Paul McKenney <[email-masked]>
>> Cc: Will Deacon <[email-masked]>
>> Acked-by: Linus Torvalds <[email-masked]>
>> Signed-off-by: Peter Zijlstra (Intel) <[email-masked]>
> 
> Acked-by: Christian Borntraeger <[email-masked]>
> 
> 
>> ---
>>  include/linux/compiler.h | 16 ----------------
>>  1 file changed, 16 deletions(-)
>>
>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>> index 1b45e4a0519b..0e41ca0e5927 100644
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -192,29 +192,16 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>>
>>  #include <uapi/linux/types.h>
>>
>> -static __always_inline void data_access_exceeds_word_size(void)
>> -#ifdef __compiletime_warning
>> -__compiletime_warning("data access exceeds word size and won't be atomic")
>> -#endif
>> -;
>> -
>> -static __always_inline void data_access_exceeds_word_size(void)
>> -{
>> -}
>> -
>>  static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
>>  {
>>  	switch (size) {
>>  	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
>>  	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
>>  	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
>> -#ifdef CONFIG_64BIT
>>  	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
>> -#endif
>>  	default:
>>  		barrier();
>>  		__builtin_memcpy((void *)res, (const void *)p, size);
>> -		data_access_exceeds_word_size();
>>  		barrier();
>>  	}
>>  }
>> @@ -225,13 +212,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>>  	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
>>  	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
>>  	case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
>> -#ifdef CONFIG_64BIT
>>  	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
>> -#endif
>>  	default:
>>  		barrier();
>>  		__builtin_memcpy((void *)p, (const void *)res, size);
>> -		data_access_exceeds_word_size();
>>  		barrier();
>>  	}
>>  }
>>
>