From 824975ef190e7dcb77718d1cc2cb53769b16d918 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Fri, 3 Jul 2009 12:39:07 +0200 Subject: [PATCH] x86: atomic64: Improve atomic64_add_return() Linus noted (based on Eric Dumazet's numbers) that we would probably be better off not trying an atomic_read() in atomic64_add_return() but intead intentionally let the first cmpxchg8b fail - to get a cache-friendly 'give me ownership of this cacheline' transaction. That can then be followed by the real cmpxchg8b which sets the value local to the CPU. 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 | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/arch/x86/lib/atomic64_32.c b/arch/x86/lib/atomic64_32.c index 5fc1e2caa544..61959627e1e1 100644 --- a/arch/x86/lib/atomic64_32.c +++ b/arch/x86/lib/atomic64_32.c @@ -76,13 +76,22 @@ u64 atomic64_read(atomic64_t *ptr) */ u64 atomic64_add_return(u64 delta, atomic64_t *ptr) { - u64 old_val, new_val; + /* + * Try first with a (probably 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; do { - old_val = atomic_read(ptr); + old_val = real_val; new_val = old_val + delta; - } while (atomic64_cmpxchg(ptr, old_val, new_val) != old_val); + real_val = atomic64_cmpxchg(ptr, old_val, new_val); + + } while (real_val != old_val); return new_val; }