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SubjectRe: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction

* 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)

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

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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