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 <torvalds@linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
LKML-Reference: <alpine.LFD.2.01.0907021653030.3210@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

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;
 }