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authorIngo Molnar <mingo@elte.hu>2009-07-03 12:39:07 +0200
committerIngo Molnar <mingo@elte.hu>2009-07-03 13:26:42 +0200
commit824975ef190e7dcb77718d1cc2cb53769b16d918 (patch)
treef5f9f180bef531795e992b34ef417e9822cab3e9 /arch
parent69237f94e65d3d7f539f1adb98ef68685c595004 (diff)
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>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/lib/atomic64_32.c15
1 files changed, 12 insertions, 3 deletions
diff --git a/arch/x86/lib/atomic64_32.c b/arch/x86/lib/atomic64_32.c
index 5fc1e2caa54..61959627e1e 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;
}