locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE

locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE

MFENCE appears to be way slower than a locked instruction - let's use
LOCK ADD unconditionally, as we always did on old 32-bit.

Performance testing results:

  perf stat -r 10 -- ./virtio_ring_0_9 --sleep --host-affinity 0 --guest-affinity 0
  Before:
         0.922565990 seconds time elapsed                                          ( +-  1.15% )
  After:
         0.578667024 seconds time elapsed                                          ( +-  1.21% )

i.e. about ~60% faster.

Just poking at SP would be the most natural, but if we then read the
value from SP, we get a false dependency which will slow us down.

This was noted in this article:

  http://shipilev.net/blog/2014/on-the-fence-with-dependencies/

And is easy to reproduce by sticking a barrier in a small non-inline
function.

So let's use a negative offset - which avoids this problem since we
build with the red zone disabled.

For userspace, use an address just below the redzone.

The one difference between LOCK ADD and MFENCE is that LOCK ADD does
not affect CLFLUSH, previous patches converted all uses of CLFLUSH to
call mb(), such that changes to smp_mb() won't affect it.

Update mb/rmb/wmb() on 32-bit to use the negative offset, too, for
consistency.

As a follow-up, it might be worth considering switching users
of CLFLUSH to another API (e.g. clflush_mb()?) - we will
then be able to convert mb() to smp_mb() again.

Also arguably, GCC should switch to use LOCK ADD for __sync_synchronize().
This might be worth pursuing separately.

Suggested-by: Andy Lutomirski <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>