[Xen-devel] Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction

[Ingo Molnar <mingo@xxxxxxx>]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxx>
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@xxxxxxxx>
    Signed-off-by: Ingo Molnar <mingo@xxxxxxx>

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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