[PATCH] i386: generic cmpxchg

- Make cmpxchg generally available on the i386 platform.

- Provide emulation of cmpxchg suitable for uniprocessor if built and run on
  386.

From: Christoph Lameter <clameter@sgi.com>

- Cut down patch and small style changes.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 87 insertions, 3 deletions

diff --git a/arch/i386/kernel/cpu/intel.c b/arch/i386/kernel/cpu/intel.c
index 43601de0f633..c28d26fb5f24 100644
--- a/arch/i386/kernel/cpu/intel.c
+++ b/arch/i386/kernel/cpu/intel.c
@@ -6,6 +6,7 @@
 #include <linux/bitops.h>
 #include <linux/smp.h>
 #include <linux/thread_info.h>
+#include <linux/module.h>
 
 #include <asm/processor.h>
 #include <asm/msr.h>
@@ -264,5 +265,52 @@ __init int intel_cpu_init(void)
         return 0;
 }
 
+#ifndef CONFIG_X86_CMPXCHG
+unsigned long cmpxchg_386_u8(volatile void *ptr, u8 old, u8 new)
+{
+        u8 prev;
+        unsigned long flags;
+
+        /* Poor man's cmpxchg for 386. Unsuitable for SMP */
+        local_irq_save(flags);
+        prev = *(u8 *)ptr;
+        if (prev == old)
+                *(u8 *)ptr = new;
+        local_irq_restore(flags);
+        return prev;
+}
+EXPORT_SYMBOL(cmpxchg_386_u8);
+
+unsigned long cmpxchg_386_u16(volatile void *ptr, u16 old, u16 new)
+{
+        u16 prev;
+        unsigned long flags;
+
+        /* Poor man's cmpxchg for 386. Unsuitable for SMP */
+        local_irq_save(flags);
+        prev = *(u16 *)ptr;
+        if (prev == old)
+                *(u16 *)ptr = new;
+        local_irq_restore(flags);
+        return prev;
+}
+EXPORT_SYMBOL(cmpxchg_386_u16);
+
+unsigned long cmpxchg_386_u32(volatile void *ptr, u32 old, u32 new)
+{
+        u32 prev;
+        unsigned long flags;
+
+        /* Poor man's cmpxchg for 386. Unsuitable for SMP */
+        local_irq_save(flags);
+        prev = *(u32 *)ptr;
+        if (prev == old)
+                *(u32 *)ptr = new;
+        local_irq_restore(flags);
+        return prev;
+}
+EXPORT_SYMBOL(cmpxchg_386_u32);
+#endif
+
 // arch_initcall(intel_cpu_init);
 
diff --git a/include/asm-i386/system.h b/include/asm-i386/system.h
index 97d52ac49e46..772f85da1206 100644
--- a/include/asm-i386/system.h
+++ b/include/asm-i386/system.h
@@ -263,6 +263,10 @@ 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))))
+#endif
 
 static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
                                       unsigned long new, int size)
@@ -291,10 +295,42 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
         return old;
 }
 
-#define cmpxchg(ptr,o,n)\
-        ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\
-                                        (unsigned long)(n),sizeof(*(ptr))))
+#ifndef CONFIG_X86_CMPXCHG
+/*
+ * Building a kernel capable running on 80386. It may be necessary to
+ * simulate the cmpxchg on the 80386 CPU. For that purpose we define
+ * a function for each of the sizes we support.
+ */
 
+extern unsigned long cmpxchg_386_u8(volatile void *, u8, u8);
+extern unsigned long cmpxchg_386_u16(volatile void *, u16, u16);
+extern unsigned long cmpxchg_386_u32(volatile void *, u32, u32);
+
+static inline unsigned long cmpxchg_386(volatile void *ptr, unsigned long old,
+                                      unsigned long new, int size)
+{
+        switch (size) {
+        case 1:
+                return cmpxchg_386_u8(ptr, old, new);
+        case 2:
+                return cmpxchg_386_u16(ptr, old, new);
+        case 4:
+                return cmpxchg_386_u32(ptr, old, new);
+        }
+        return old;
+}
+
+#define cmpxchg(ptr,o,n)                                                \
+({                                                                      \
+        __typeof__(*(ptr)) __ret;                                       \
+        if (likely(boot_cpu_data.x86 > 3))                              \
+                __ret = __cmpxchg((ptr), (unsigned long)(o),            \
+                                        (unsigned long)(n), sizeof(*(ptr))); \
+        else                                                            \
+                __ret = cmpxchg_386((ptr), (unsigned long)(o),          \
+                                        (unsigned long)(n), sizeof(*(ptr))); \
+        __ret;                                                          \
+})
 #endif
 
 #ifdef CONFIG_X86_CMPXCHG64