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 different case than cmpxchg (which is only required for 386). Using different code makes this easier. 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 CC: clameter@sgi.com CC: mingo@redhat.com --- arch/i386/kernel/cpu/intel.c | 17 +++++ include/asm-i386/cmpxchg.h | 122 +++++++++++++++++++++++++++---------------- 2 files changed, 96 insertions(+), 43 deletions(-) Index: linux-2.6-lttng/arch/i386/kernel/cpu/intel.c =================================================================== --- linux-2.6-lttng.orig/arch/i386/kernel/cpu/intel.c 2007-10-31 17:41:12.000000000 -0400 +++ linux-2.6-lttng/arch/i386/kernel/cpu/intel.c 2007-10-31 17:41:34.000000000 -0400 @@ -350,5 +350,22 @@ unsigned long cmpxchg_386_u32(volatile v EXPORT_SYMBOL(cmpxchg_386_u32); #endif +#ifndef CONFIG_X86_CMPXCHG64 +unsigned long long cmpxchg_486_u64(volatile void *ptr, u64 old, u64 new) +{ + u64 prev; + unsigned long flags; + + /* Poor man's cmpxchg8b for 386 and 486. Unsuitable for SMP */ + local_irq_save(flags); + prev = *(u64 *)ptr; + if (prev == old) + *(u64 *)ptr = new; + local_irq_restore(flags); + return prev; +} +EXPORT_SYMBOL(cmpxchg_486_u64); +#endif + // arch_initcall(intel_cpu_init); Index: linux-2.6-lttng/include/asm-i386/cmpxchg.h =================================================================== --- linux-2.6-lttng.orig/include/asm-i386/cmpxchg.h 2007-10-31 17:41:12.000000000 -0400 +++ linux-2.6-lttng/include/asm-i386/cmpxchg.h 2007-10-31 17:49:54.000000000 -0400 @@ -105,15 +105,24 @@ static inline unsigned long __xchg(unsig #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_lo 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( 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( (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( }) #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 -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/