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SubjectRe: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction
Ingo Molnar wrote:
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>
>> On Fri, 20 Jun 2008, Jeremy Fitzhardinge wrote:
>>
>>> Blows up on "gcc version 3.4.4 20050314 (prerelease) (Debian 3.4.3-13)":
>>>
>> Yeah, I was a bit worried about that. Gcc sometimes does insane
>> things.
>>
>> We literally just tested that the asm should only _ever_ be generated
>> with a constant value, but if some gcc dead-code removal thing doesn't
>> work, it will then screw up and try to generate the asm even for a
>> non-constant thing.
>>
>> The fairly trivial fix is probably to just change the "i" to "ir",
>> safe in the knowledge that any _sane_ case will never use the "r"
>> possibility. I suspect even your insane case will end up then killing
>> the bad choice later.
>>
>
> okay - Jeremy, could you try the fix below? (or tip/master, i just
> pushed this out)
>
>

Hm. On 32-bit I get these, but they're warnings. On 64-bit they're errors.

{standard input}: Assembler messages:
{standard input}:1609: Warning: using `%dl' instead of `%edx' due to `b'
suffix

vs

{standard input}: Assembler messages:
{standard input}:20511: Error: Incorrect register `%eax' used with `b'
suffix


> (i dont use gcc 3.x myself to build the kernel, had way too many
> miscompilations in randconfig tests in the past.)
>

I do it mainly to pick up these kinds of problems.


> Ingo
>
> -------------->
> commit b68b80b8ab39c42707dc126c41e87d46edc97c27
> Author: Ingo Molnar <mingo@elte.hu>
> Date: Fri Jun 20 21:50:20 2008 +0200
>
> x86, bitops: make constant-bit set/clear_bit ops faster, gcc workaround
>
> Jeremy Fitzhardinge reported this compiler bug:
>
> Suggestion from Linus: add "r" to the input constraint of the
> set_bit()/clear_bit()'s constant 'nr' branch:
>
> Blows up on "gcc version 3.4.4 20050314 (prerelease) (Debian 3.4.3-13)":
>
> CC init/main.o
> include2/asm/bitops.h: In function `start_kernel':
> include2/asm/bitops.h:59: warning: asm operand 1 probably doesn't match constraints
> include2/asm/bitops.h:59: warning: asm operand 1 probably doesn't match constraints
> include2/asm/bitops.h:59: warning: asm operand 1 probably doesn't match constraints
> include2/asm/bitops.h:59: error: impossible constraint in `asm'
> include2/asm/bitops.h:59: error: impossible constraint in `asm'
> include2/asm/bitops.h:59: error: impossible constraint in `asm'
>
> Reported-by: Jeremy Fitzhardinge <jeremy@goop.org>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
> diff --git a/include/asm-x86/bitops.h b/include/asm-x86/bitops.h
> index 6c50548..4575de4 100644
> --- a/include/asm-x86/bitops.h
> +++ b/include/asm-x86/bitops.h
> @@ -58,7 +58,7 @@ static inline void set_bit(unsigned int nr, volatile unsigned long *addr)
> if (IS_IMMEDIATE(nr)) {
> asm volatile(LOCK_PREFIX "orb %1,%0"
> : CONST_MASK_ADDR(nr, addr)
> - : "i" (CONST_MASK(nr))
> + : "ir" (CONST_MASK(nr))
> : "memory");
> } else {
> asm volatile(LOCK_PREFIX "bts %1,%0"
> @@ -95,7 +95,7 @@ static inline void clear_bit(int nr, volatile unsigned long *addr)
> if (IS_IMMEDIATE(nr)) {
> asm volatile(LOCK_PREFIX "andb %1,%0"
> : CONST_MASK_ADDR(nr, addr)
> - : "i" (~CONST_MASK(nr)));
> + : "ir" (~CONST_MASK(nr)));
> } else {
> asm volatile(LOCK_PREFIX "btr %1,%0"
> : BITOP_ADDR(addr)
>



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