[PATCH] x86: atomic64: Improve atomic64_xchg()

From: Ingo Molnar
Date: Fri Jul 03 2009 - 13:56:36 EST


Remove the read-first logic from atomic64_xchg() and simplify
the loop.

This function was the last user of __atomic64_read() - remove it.

Also, change the 'real_val' assumption from the somewhat quirky
1ULL << 32 value to the (just as arbitrary, but simpler) value
of 0.

Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
LKML-Reference: <tip-05118ab8859492ac9ddda0154cf90e37b0a4a0b0@xxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
arch/x86/include/asm/atomic_32.h | 9 ---------
arch/x86/lib/atomic64_32.c | 21 +++++++++++++++------
2 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/atomic_32.h b/arch/x86/include/asm/atomic_32.h
index aa045de..d7c8849 100644
--- a/arch/x86/include/asm/atomic_32.h
+++ b/arch/x86/include/asm/atomic_32.h
@@ -268,15 +268,6 @@ typedef struct {

#define ATOMIC64_INIT(val) { (val) }

-/**
- * atomic64_read - read atomic64 variable
- * @ptr: pointer of type atomic64_t
- *
- * Atomically reads the value of @v.
- * Doesn't imply a read memory barrier.
- */
-#define __atomic64_read(ptr) ((ptr)->counter)
-
extern u64 atomic64_cmpxchg(atomic64_t *ptr, u64 old_val, u64 new_val);

/**
diff --git a/arch/x86/lib/atomic64_32.c b/arch/x86/lib/atomic64_32.c
index aab898c..0fac67b 100644
--- a/arch/x86/lib/atomic64_32.c
+++ b/arch/x86/lib/atomic64_32.c
@@ -31,14 +31,23 @@ EXPORT_SYMBOL(atomic64_cmpxchg);
* Atomically xchgs the value of @ptr to @new_val and returns
* the old value.
*/
-
u64 atomic64_xchg(atomic64_t *ptr, u64 new_val)
{
- u64 old_val;
+ /*
+ * Try first with a (possibly incorrect) assumption about
+ * what we have there. We'll do two loops most likely,
+ * but we'll get an ownership MESI transaction straight away
+ * instead of a read transaction followed by a
+ * flush-for-ownership transaction:
+ */
+ u64 old_val, real_val = 0;

do {
- old_val = __atomic64_read(ptr);
- } while (atomic64_cmpxchg(ptr, old_val, new_val) != old_val);
+ old_val = real_val;
+
+ real_val = atomic64_cmpxchg(ptr, old_val, new_val);
+
+ } while (real_val != old_val);

return old_val;
}
@@ -89,13 +98,13 @@ EXPORT_SYMBOL(atomic64_read);
noinline u64 atomic64_add_return(u64 delta, atomic64_t *ptr)
{
/*
- * Try first with a (probably incorrect) assumption about
+ * Try first with a (possibly incorrect) assumption about
* what we have there. We'll do two loops most likely,
* but we'll get an ownership MESI transaction straight away
* instead of a read transaction followed by a
* flush-for-ownership transaction:
*/
- u64 old_val, new_val, real_val = 1ULL << 32;
+ u64 old_val, new_val, real_val = 0;

do {
old_val = real_val;
--
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