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authorMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>2008-01-30 13:30:47 +0100
committerIngo Molnar <mingo@elte.hu>2008-01-30 13:30:47 +0100
commit2c0b8a7578f7653e1e5312a5232e8ead563cf477 (patch)
treebc220bff551e39f9faff17e807d828094f420607 /arch/x86/kernel/cpu
parent5f627f8e122a163ce53908d55e088247db31f1d7 (diff)
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 'arch/x86/kernel/cpu')
-rw-r--r--arch/x86/kernel/cpu/intel.c17
1 files changed, 17 insertions, 0 deletions
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index cc8c501b9f3..867ff94579b 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -342,5 +342,22 @@ unsigned long cmpxchg_386_u32(volatile void *ptr, u32 old, u32 new)
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);