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author | Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> | 2008-01-30 13:30:47 +0100 |
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committer | Ingo Molnar <mingo@elte.hu> | 2008-01-30 13:30:47 +0100 |
commit | 2c0b8a7578f7653e1e5312a5232e8ead563cf477 (patch) | |
tree | bc220bff551e39f9faff17e807d828094f420607 /include/asm-x86/cmpxchg_32.h | |
parent | 5f627f8e122a163ce53908d55e088247db31f1d7 (diff) | |
download | linux-2c0b8a7578f7653e1e5312a5232e8ead563cf477.tar.bz2 |
x86: fall back on interrupt disable in cmpxchg8b on 80386 and 80486
Actually, on 386, cmpxchg and cmpxchg_local fall back on
cmpxchg_386_u8/16/32: it disables interruptions around non atomic
updates to mimic the cmpxchg behavior.
The comment:
/* Poor man's cmpxchg for 386. Unsuitable for SMP */
already present in cmpxchg_386_u32 tells much about how this cmpxchg
implementation should not be used in a SMP context. However, the cmpxchg_local
can perfectly use this fallback, since it only needs to be atomic wrt the local
cpu.
This patch adds a cmpxchg_486_u64 and uses it as a fallback for cmpxchg64
and cmpxchg64_local on 80386 and 80486.
Q:
but why is it called cmpxchg_486 when the other functions are called
A:
Because the standard cmpxchg is missing only on 386, but cmpxchg8b is
missing both on 386 and 486.
Citing Intel's Instruction set reference:
cmpxchg:
This instruction is not supported on Intel processors earlier than the
Intel486 processors.
cmpxchg8b:
This instruction encoding is not supported on Intel processors earlier
than the Pentium processors.
Q:
What's the reason to have cmpxchg64_local on 32 bit architectures?
Without that need all this would just be a few simple defines.
A:
cmpxchg64_local on 32 bits architectures takes unsigned long long
parameters, but cmpxchg_local only takes longs. Since we have cmpxchg8b
to execute a 8 byte cmpxchg atomically on pentium and +, it makes sense
to provide a flavor of cmpxchg and cmpxchg_local using this instruction.
Also, for 32 bits architectures lacking the 64 bits atomic cmpxchg, it
makes sense _not_ to define cmpxchg64 while cmpxchg could still be
available.
Moreover, the fallback for cmpxchg8b on i386 for 386 and 486 is a
However, cmpxchg64_local will be emulated by disabling interrupts on all
architectures where it is not supported atomically.
Therefore, we *could* turn cmpxchg64_local into a cmpxchg_local, but it
would make the 386/486 fallbacks ugly, make its design different from
cmpxchg/cmpxchg64 (which really depends on atomic operations and cannot
be emulated) and require the __cmpxchg_local to be expressed as a macro
rather than an inline function so the parameters would not be fixed to
unsigned long long in every case.
So I think cmpxchg64_local makes sense there, but I am open to
suggestions.
Q:
Are there any callers?
A:
I am actually using it in LTTng in my timestamping code. I use it to
work around CPUs with asynchronous TSCs. I need to update 64 bits
values atomically on this 32 bits architecture.
Changelog:
- Ran though checkpatch.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'include/asm-x86/cmpxchg_32.h')
-rw-r--r-- | include/asm-x86/cmpxchg_32.h | 122 |
1 files changed, 79 insertions, 43 deletions
diff --git a/include/asm-x86/cmpxchg_32.h b/include/asm-x86/cmpxchg_32.h index f86ede28f6dc..cea1dae288a7 100644 --- a/include/asm-x86/cmpxchg_32.h +++ b/include/asm-x86/cmpxchg_32.h @@ -105,15 +105,24 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz #ifdef CONFIG_X86_CMPXCHG #define __HAVE_ARCH_CMPXCHG 1 -#define cmpxchg(ptr,o,n)\ - ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\ - (unsigned long)(n),sizeof(*(ptr)))) -#define sync_cmpxchg(ptr,o,n)\ - ((__typeof__(*(ptr)))__sync_cmpxchg((ptr),(unsigned long)(o),\ - (unsigned long)(n),sizeof(*(ptr)))) -#define cmpxchg_local(ptr,o,n)\ - ((__typeof__(*(ptr)))__cmpxchg_local((ptr),(unsigned long)(o),\ - (unsigned long)(n),sizeof(*(ptr)))) +#define cmpxchg(ptr, o, n) \ + ((__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o), \ + (unsigned long)(n), sizeof(*(ptr)))) +#define sync_cmpxchg(ptr, o, n) \ + ((__typeof__(*(ptr)))__sync_cmpxchg((ptr), (unsigned long)(o), \ + (unsigned long)(n), sizeof(*(ptr)))) +#define cmpxchg_local(ptr, o, n) \ + ((__typeof__(*(ptr)))__cmpxchg_local((ptr), (unsigned long)(o), \ + (unsigned long)(n), sizeof(*(ptr)))) +#endif + +#ifdef CONFIG_X86_CMPXCHG64 +#define cmpxchg64(ptr, o, n) \ + ((__typeof__(*(ptr)))__cmpxchg64((ptr), (unsigned long long)(o), \ + (unsigned long long)(n))) +#define cmpxchg64_local(ptr, o, n) \ + ((__typeof__(*(ptr)))__cmpxchg64_local((ptr), (unsigned long long)(o),\ + (unsigned long long)(n))) #endif static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old, @@ -203,6 +212,34 @@ static inline unsigned long __cmpxchg_local(volatile void *ptr, return old; } +static inline unsigned long long __cmpxchg64(volatile void *ptr, + unsigned long long old, unsigned long long new) +{ + unsigned long long prev; + __asm__ __volatile__(LOCK_PREFIX "cmpxchg8b %3" + : "=A"(prev) + : "b"((unsigned long)new), + "c"((unsigned long)(new >> 32)), + "m"(*__xg(ptr)), + "0"(old) + : "memory"); + return prev; +} + +static inline unsigned long long __cmpxchg64_local(volatile void *ptr, + unsigned long long old, unsigned long long new) +{ + unsigned long long prev; + __asm__ __volatile__("cmpxchg8b %3" + : "=A"(prev) + : "b"((unsigned long)new), + "c"((unsigned long)(new >> 32)), + "m"(*__xg(ptr)), + "0"(old) + : "memory"); + return prev; +} + #ifndef CONFIG_X86_CMPXCHG /* * Building a kernel capable running on 80386. It may be necessary to @@ -228,7 +265,7 @@ static inline unsigned long cmpxchg_386(volatile void *ptr, unsigned long old, return old; } -#define cmpxchg(ptr,o,n) \ +#define cmpxchg(ptr, o, n) \ ({ \ __typeof__(*(ptr)) __ret; \ if (likely(boot_cpu_data.x86 > 3)) \ @@ -239,7 +276,7 @@ static inline unsigned long cmpxchg_386(volatile void *ptr, unsigned long old, (unsigned long)(n), sizeof(*(ptr))); \ __ret; \ }) -#define cmpxchg_local(ptr,o,n) \ +#define cmpxchg_local(ptr, o, n) \ ({ \ __typeof__(*(ptr)) __ret; \ if (likely(boot_cpu_data.x86 > 3)) \ @@ -252,38 +289,37 @@ static inline unsigned long cmpxchg_386(volatile void *ptr, unsigned long old, }) #endif -static inline unsigned long long __cmpxchg64(volatile void *ptr, unsigned long long old, - unsigned long long new) -{ - unsigned long long prev; - __asm__ __volatile__(LOCK_PREFIX "cmpxchg8b %3" - : "=A"(prev) - : "b"((unsigned long)new), - "c"((unsigned long)(new >> 32)), - "m"(*__xg(ptr)), - "0"(old) - : "memory"); - return prev; -} +#ifndef CONFIG_X86_CMPXCHG64 +/* + * Building a kernel capable running on 80386 and 80486. It may be necessary + * to simulate the cmpxchg8b on the 80386 and 80486 CPU. + */ -static inline unsigned long long __cmpxchg64_local(volatile void *ptr, - unsigned long long old, unsigned long long new) -{ - unsigned long long prev; - __asm__ __volatile__("cmpxchg8b %3" - : "=A"(prev) - : "b"((unsigned long)new), - "c"((unsigned long)(new >> 32)), - "m"(*__xg(ptr)), - "0"(old) - : "memory"); - return prev; -} +extern unsigned long long cmpxchg_486_u64(volatile void *, u64, u64); + +#define cmpxchg64(ptr, o, n) \ +({ \ + __typeof__(*(ptr)) __ret; \ + if (likely(boot_cpu_data.x86 > 4)) \ + __ret = __cmpxchg64((ptr), (unsigned long long)(o), \ + (unsigned long long)(n)); \ + else \ + __ret = cmpxchg_486_u64((ptr), (unsigned long long)(o), \ + (unsigned long long)(n)); \ + __ret; \ +}) +#define cmpxchg64_local(ptr, o, n) \ +({ \ + __typeof__(*(ptr)) __ret; \ + if (likely(boot_cpu_data.x86 > 4)) \ + __ret = __cmpxchg64_local((ptr), (unsigned long long)(o), \ + (unsigned long long)(n)); \ + else \ + __ret = cmpxchg_486_u64((ptr), (unsigned long long)(o), \ + (unsigned long long)(n)); \ + __ret; \ +}) + +#endif -#define cmpxchg64(ptr,o,n)\ - ((__typeof__(*(ptr)))__cmpxchg64((ptr),(unsigned long long)(o),\ - (unsigned long long)(n))) -#define cmpxchg64_local(ptr,o,n)\ - ((__typeof__(*(ptr)))__cmpxchg64_local((ptr),(unsigned long long)(o),\ - (unsigned long long)(n))) #endif |