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Subject[PATCH] move xchg/cmpxchg to atomic.h

On parisc-linux mailing list, Grant Grundler wrote:
> After surveying all the arches that define __HAVE_ARCH_CMPXCHG:
>
> ./include/asm-alpha/system.h:#define __HAVE_ARCH_CMPXCHG 1
> ./include/asm-i386/system.h:#define __HAVE_ARCH_CMPXCHG 1
> ./include/asm-ia64/system.h:#define __HAVE_ARCH_CMPXCHG 1
> ./include/asm-ppc/system.h:#define __HAVE_ARCH_CMPXCHG 1
> ./include/asm-sparc64/system.h:#define __HAVE_ARCH_CMPXCHG 1
>
> I've come to the conclusion xchg/cmpxchg definitions do NOT
> belong in system.h. AFAICT, all the above use Load Linked semantics
> (or in the i386 case, operation is atomic). In other words, xchg/cmpxchg
> are atomic operations. Shouldn't xchg/cmpxchg definitions live
> with other atomic operations - asm/atomic.h?

On Sat, 30 Dec 2000 16:46:57 +0000 (GMT), Alan Cox replied:
| Seems a reasonable thing to try and move to atomic.h yes

Fundemental problem is parisc only supports one atomic operation
(LDCW/LDCD) and uses spinlocks for all atomic operations including
xchg/cmpxchg. Issue is dependencies between system.h, atomic.h
and spinlock.h are *really* ugly and prevented parisc port from
inlining xchg/cmpxchg definitions. This is a first step in fixing
that problem.

I've already made this change to the parisc-linux source tree for parisc
and parisc64 builds. Below is the i386 patch for linux-2.4.0-prerelease.
This is a simple cut/paste.

thanks,
grant

diff -ruNp linux/include/asm-i386/atomic.h linux.patch/include/asm-i386/atomic.h
--- linux/include/asm-i386/atomic.h Sun Dec 31 11:10:16 2000
+++ linux.patch/include/asm-i386/atomic.h Mon Jan 1 23:28:08 2001
@@ -2,6 +2,7 @@
#define __ARCH_I386_ATOMIC__

#include <linux/config.h>
+#include <linux/bitops.h> /* for LOCK_PREFIX */

/*
* Atomic operations that C can't guarantee us. Useful for
@@ -111,4 +112,136 @@ __asm__ __volatile__(LOCK "andl %0,%1" \
__asm__ __volatile__(LOCK "orl %0,%1" \
: : "r" (mask),"m" (*addr) : "memory")

+
+/* xchg/cmpxchg moved from asm/system.h */
+#define xchg(ptr,v) ((__typeof__(*(ptr)))__xchg((unsigned long)(v),(ptr),sizeof(*(ptr))))
+
+#define tas(ptr) (xchg((ptr),1))
+
+struct __xchg_dummy { unsigned long a[100]; };
+#define __xg(x) ((struct __xchg_dummy *)(x))
+
+
+/*
+ * The semantics of XCHGCMP8B are a bit strange, this is why
+ * there is a loop and the loading of %%eax and %%edx has to
+ * be inside. This inlines well in most cases, the cached
+ * cost is around ~38 cycles. (in the future we might want
+ * to do an SIMD/3DNOW!/MMX/FPU 64-bit store here, but that
+ * might have an implicit FPU-save as a cost, so it's not
+ * clear which path to go.)
+ */
+extern inline void __set_64bit (unsigned long long * ptr,
+ unsigned int low, unsigned int high)
+{
+__asm__ __volatile__ (
+ "1: movl (%0), %%eax;
+ movl 4(%0), %%edx;
+ cmpxchg8b (%0);
+ jnz 1b"
+ :: "D"(ptr),
+ "b"(low),
+ "c"(high)
+ :
+ "ax","dx","memory");
+}
+
+extern void inline __set_64bit_constant (unsigned long long *ptr,
+ unsigned long long value)
+{
+ __set_64bit(ptr,(unsigned int)(value), (unsigned int)((value)>>32ULL));
+}
+#define ll_low(x) *(((unsigned int*)&(x))+0)
+#define ll_high(x) *(((unsigned int*)&(x))+1)
+
+extern void inline __set_64bit_var (unsigned long long *ptr,
+ unsigned long long value)
+{
+ __set_64bit(ptr,ll_low(value), ll_high(value));
+}
+
+#define set_64bit(ptr,value) \
+(__builtin_constant_p(value) ? \
+ __set_64bit_constant(ptr, value) : \
+ __set_64bit_var(ptr, value) )
+
+#define _set_64bit(ptr,value) \
+(__builtin_constant_p(value) ? \
+ __set_64bit(ptr, (unsigned int)(value), (unsigned int)((value)>>32ULL) ) : \
+ __set_64bit(ptr, ll_low(value), ll_high(value)) )
+
+/*
+ * Note: no "lock" prefix even on SMP: xchg always implies lock anyway
+ * Note 2: xchg has side effect, so that attribute volatile is necessary,
+ * but generally the primitive is invalid, *ptr is output argument. --ANK
+ */
+static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int size)
+{
+ switch (size) {
+ case 1:
+ __asm__ __volatile__("xchgb %b0,%1"
+ :"=q" (x)
+ :"m" (*__xg(ptr)), "0" (x)
+ :"memory");
+ break;
+ case 2:
+ __asm__ __volatile__("xchgw %w0,%1"
+ :"=r" (x)
+ :"m" (*__xg(ptr)), "0" (x)
+ :"memory");
+ break;
+ case 4:
+ __asm__ __volatile__("xchgl %0,%1"
+ :"=r" (x)
+ :"m" (*__xg(ptr)), "0" (x)
+ :"memory");
+ break;
+ }
+ return x;
+}
+
+/*
+ * Atomic compare and exchange. Compare OLD with MEM, if identical,
+ * store NEW in MEM. Return the initial value in MEM. Success is
+ * indicated by comparing RETURN with OLD.
+ */
+
+#ifdef CONFIG_X86_CMPXCHG
+#define __HAVE_ARCH_CMPXCHG 1
+
+static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
+ unsigned long new, int size)
+{
+ unsigned long prev;
+ switch (size) {
+ case 1:
+ __asm__ __volatile__(LOCK_PREFIX "cmpxchgb %b1,%2"
+ : "=a"(prev)
+ : "q"(new), "m"(*__xg(ptr)), "0"(old)
+ : "memory");
+ return prev;
+ case 2:
+ __asm__ __volatile__(LOCK_PREFIX "cmpxchgw %w1,%2"
+ : "=a"(prev)
+ : "q"(new), "m"(*__xg(ptr)), "0"(old)
+ : "memory");
+ return prev;
+ case 4:
+ __asm__ __volatile__(LOCK_PREFIX "cmpxchgl %1,%2"
+ : "=a"(prev)
+ : "q"(new), "m"(*__xg(ptr)), "0"(old)
+ : "memory");
+ return prev;
+ }
+ return old;
+}
+
+#define cmpxchg(ptr,o,n)\
+ ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\
+ (unsigned long)(n),sizeof(*(ptr))))
+
+#else
+/* Compiling for a 386 proper. Is it worth implementing via cli/sti? */
#endif
+
+#endif /* __ARCH_I386_ATOMIC__ */
diff -ruNp linux/include/asm-i386/system.h linux.patch/include/asm-i386/system.h
--- linux/include/asm-i386/system.h Sun Dec 31 11:10:16 2000
+++ linux.patch/include/asm-i386/system.h Mon Jan 1 23:27:58 2001
@@ -4,7 +4,6 @@
#include <linux/config.h>
#include <linux/kernel.h>
#include <asm/segment.h>
-#include <linux/bitops.h> /* for LOCK_PREFIX */

#ifdef __KERNEL__

@@ -123,136 +122,6 @@ static inline unsigned long get_limit(un
}

#define nop() __asm__ __volatile__ ("nop")
-
-#define xchg(ptr,v) ((__typeof__(*(ptr)))__xchg((unsigned long)(v),(ptr),sizeof(*(ptr))))
-
-#define tas(ptr) (xchg((ptr),1))
-
-struct __xchg_dummy { unsigned long a[100]; };
-#define __xg(x) ((struct __xchg_dummy *)(x))
-
-
-/*
- * The semantics of XCHGCMP8B are a bit strange, this is why
- * there is a loop and the loading of %%eax and %%edx has to
- * be inside. This inlines well in most cases, the cached
- * cost is around ~38 cycles. (in the future we might want
- * to do an SIMD/3DNOW!/MMX/FPU 64-bit store here, but that
- * might have an implicit FPU-save as a cost, so it's not
- * clear which path to go.)
- */
-extern inline void __set_64bit (unsigned long long * ptr,
- unsigned int low, unsigned int high)
-{
-__asm__ __volatile__ (
- "1: movl (%0), %%eax;
- movl 4(%0), %%edx;
- cmpxchg8b (%0);
- jnz 1b"
- :: "D"(ptr),
- "b"(low),
- "c"(high)
- :
- "ax","dx","memory");
-}
-
-extern void inline __set_64bit_constant (unsigned long long *ptr,
- unsigned long long value)
-{
- __set_64bit(ptr,(unsigned int)(value), (unsigned int)((value)>>32ULL));
-}
-#define ll_low(x) *(((unsigned int*)&(x))+0)
-#define ll_high(x) *(((unsigned int*)&(x))+1)
-
-extern void inline __set_64bit_var (unsigned long long *ptr,
- unsigned long long value)
-{
- __set_64bit(ptr,ll_low(value), ll_high(value));
-}
-
-#define set_64bit(ptr,value) \
-(__builtin_constant_p(value) ? \
- __set_64bit_constant(ptr, value) : \
- __set_64bit_var(ptr, value) )
-
-#define _set_64bit(ptr,value) \
-(__builtin_constant_p(value) ? \
- __set_64bit(ptr, (unsigned int)(value), (unsigned int)((value)>>32ULL) ) : \
- __set_64bit(ptr, ll_low(value), ll_high(value)) )
-
-/*
- * Note: no "lock" prefix even on SMP: xchg always implies lock anyway
- * Note 2: xchg has side effect, so that attribute volatile is necessary,
- * but generally the primitive is invalid, *ptr is output argument. --ANK
- */
-static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int size)
-{
- switch (size) {
- case 1:
- __asm__ __volatile__("xchgb %b0,%1"
- :"=q" (x)
- :"m" (*__xg(ptr)), "0" (x)
- :"memory");
- break;
- case 2:
- __asm__ __volatile__("xchgw %w0,%1"
- :"=r" (x)
- :"m" (*__xg(ptr)), "0" (x)
- :"memory");
- break;
- case 4:
- __asm__ __volatile__("xchgl %0,%1"
- :"=r" (x)
- :"m" (*__xg(ptr)), "0" (x)
- :"memory");
- break;
- }
- return x;
-}
-
-/*
- * Atomic compare and exchange. Compare OLD with MEM, if identical,
- * store NEW in MEM. Return the initial value in MEM. Success is
- * indicated by comparing RETURN with OLD.
- */
-
-#ifdef CONFIG_X86_CMPXCHG
-#define __HAVE_ARCH_CMPXCHG 1
-
-static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
- unsigned long new, int size)
-{
- unsigned long prev;
- switch (size) {
- case 1:
- __asm__ __volatile__(LOCK_PREFIX "cmpxchgb %b1,%2"
- : "=a"(prev)
- : "q"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
- return prev;
- case 2:
- __asm__ __volatile__(LOCK_PREFIX "cmpxchgw %w1,%2"
- : "=a"(prev)
- : "q"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
- return prev;
- case 4:
- __asm__ __volatile__(LOCK_PREFIX "cmpxchgl %1,%2"
- : "=a"(prev)
- : "q"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
- return prev;
- }
- return old;
-}
-
-#define cmpxchg(ptr,o,n)\
- ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\
- (unsigned long)(n),sizeof(*(ptr))))
-
-#else
-/* Compiling for a 386 proper. Is it worth implementing via cli/sti? */
-#endif

/*
* Force strict CPU ordering.
-
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