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>

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index cc8c501b9f396d2fb0d63804e7a867875141a2be..867ff94579be6b692b14864db4623d1223aab641 100644 (file)
--- 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);
 

diff --git a/include/asm-x86/cmpxchg_32.h b/include/asm-x86/cmpxchg_32.h
index f86ede28f6dc84750ab60250cc7d122534d9c74a..cea1dae288a74b72e567d45ef963f3d5a271eb78 100644 (file)
--- 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