From 3a8d1788b37435baf6c296f4ea8beb4fa4955f44 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Fri, 3 Jul 2009 19:56:36 +0200 Subject: x86: atomic64: Improve atomic64_xchg() 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 Cc: Eric Dumazet Cc: Peter Zijlstra Cc: Mike Galbraith Cc: Paul Mackerras Cc: Arnaldo Carvalho de Melo Cc: Frederic Weisbecker Cc: David Howells Cc: Andrew Morton Cc: Arnd Bergmann LKML-Reference: Signed-off-by: Ingo Molnar --- arch/x86/lib/atomic64_32.c | 21 +++++++++++++++------ 1 file changed, 15 insertions(+), 6 deletions(-) (limited to 'arch/x86/lib') diff --git a/arch/x86/lib/atomic64_32.c b/arch/x86/lib/atomic64_32.c index 6722a09..a804f96 100644 --- a/arch/x86/lib/atomic64_32.c +++ b/arch/x86/lib/atomic64_32.c @@ -33,14 +33,23 @@ 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; } @@ -91,13 +100,13 @@ 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; -- cgit v1.1