Date: Mon, 7 Feb 2005 14:27:42 -0500 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-amd64@FreeBSD.org Cc: peter@FreeBSD.org Subject: Re: [PATCH] Change atomic operations to use fences for memory barriers Message-ID: <200502071427.42202.jhb@FreeBSD.org> In-Reply-To: <200502041558.28521.jhb@FreeBSD.org> References: <200502041558.28521.jhb@FreeBSD.org>
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On Friday 04 February 2005 03:58 pm, John Baldwin wrote:
> The patch below changes the atomic operations on amd64 to use the cheaper
> fence instructions for memory barriers. I'd like people to test it to see
> if 1) it breaks things or not, and 2) if it impacts performance either in a
> good way or a bad way. For this last I'm curious about both UP and SMP as
> my initial guess is that it will help on UP but might hurt on SMP.
Would help if I included it:
--- //depot/vendor/freebsd/src/sys/amd64/include/atomic.h 2003/11/21 03:05:42
+++ //depot/projects/smpng/sys/amd64/include/atomic.h 2004/11/19 20:16:10
@@ -70,7 +70,7 @@
int atomic_cmpset_int(volatile u_int *dst, u_int exp, u_int src);
int atomic_cmpset_long(volatile u_long *dst, u_long exp, u_long src);
-#define ATOMIC_STORE_LOAD(TYPE, LOP, SOP) \
+#define ATOMIC_STORE_LOAD(TYPE) \
u_##TYPE atomic_load_acq_##TYPE(volatile u_##TYPE *p); \
void atomic_store_rel_##TYPE(volatile u_##TYPE *p, u_##TYPE v)
@@ -162,30 +162,22 @@
#if defined(__GNUC__)
-#define ATOMIC_STORE_LOAD(TYPE, LOP, SOP) \
+#define ATOMIC_STORE_LOAD(TYPE) \
static __inline u_##TYPE \
atomic_load_acq_##TYPE(volatile u_##TYPE *p) \
{ \
- u_##TYPE res; \
+ u_##TYPE v; \
\
- __asm __volatile(__XSTRING(MPLOCKED) LOP \
- : "=a" (res), /* 0 (result) */\
- "+m" (*p) /* 1 */ \
- : : "memory"); \
- \
- return (res); \
+ v = *p; \
+ __asm __volatile("lfence" ::: "memory"); \
+ return (v); \
} \
\
-/* \
- * The XCHG instruction asserts LOCK automagically. \
- */ \
static __inline void \
atomic_store_rel_##TYPE(volatile u_##TYPE *p, u_##TYPE v)\
{ \
- __asm __volatile(SOP \
- : "+m" (*p), /* 0 */ \
- "+r" (v) /* 1 */ \
- : : "memory"); \
+ __asm __volatile("sfence" ::: "memory"); \
+ *p = v; \
} \
struct __hack
@@ -194,7 +186,7 @@
extern int atomic_cmpset_int(volatile u_int *, u_int, u_int);
extern int atomic_cmpset_long(volatile u_long *, u_long, u_long);
-#define ATOMIC_STORE_LOAD(TYPE, LOP, SOP) \
+#define ATOMIC_STORE_LOAD(TYPE) \
extern u_##TYPE atomic_load_acq_##TYPE(volatile u_##TYPE *p); \
extern void atomic_store_rel_##TYPE(volatile u_##TYPE *p, u_##TYPE v)
@@ -222,10 +214,10 @@
ATOMIC_ASM(add, long, "addq %1,%0", "ir", v);
ATOMIC_ASM(subtract, long, "subq %1,%0", "ir", v);
-ATOMIC_STORE_LOAD(char, "cmpxchgb %b0,%1", "xchgb %b1,%0");
-ATOMIC_STORE_LOAD(short,"cmpxchgw %w0,%1", "xchgw %w1,%0");
-ATOMIC_STORE_LOAD(int, "cmpxchgl %0,%1", "xchgl %1,%0");
-ATOMIC_STORE_LOAD(long, "cmpxchgq %0,%1", "xchgq %1,%0");
+ATOMIC_STORE_LOAD(char);
+ATOMIC_STORE_LOAD(short);
+ATOMIC_STORE_LOAD(int);
+ATOMIC_STORE_LOAD(long);
#undef ATOMIC_ASM
#undef ATOMIC_STORE_LOAD
--- //depot/vendor/freebsd/src/sys/amd64/include/bus_amd64.h 2005/01/05
20:20:40
+++ //depot/projects/smpng/sys/amd64/include/bus_amd64.h 2005/01/05 22:38:38
@@ -1215,9 +1215,9 @@
{
#ifdef __GNUC__
if (flags & BUS_SPACE_BARRIER_READ)
- __asm __volatile("lock; addl $0,0(%%rsp)" : : : "memory");
+ __asm __volatile("lfence" : : : "memory");
else
- __asm __volatile("" : : : "memory");
+ __asm __volatile("sfence" : : : "memory");
#endif
}
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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