x86: Add memory modify constraints to xchg() and cmpxchg()

commit 113fc5a6e8c2288619ff7e8187a6f556b7e0d372 upstream.

[ Backport to .32 by Tomáš Janoušek <tomi@nomi.cz> ]

xchg() and cmpxchg() modify their memory operands, not merely read
them.  For some versions of gcc the "memory" clobber has apparently
dealt with the situation, but not for all.

Originally-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Glauber Costa <glommer@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Peter Palfrader <peter@palfrader.org>
Cc: Greg KH <gregkh@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Zachary Amsden <zamsden@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
LKML-Reference: <4C4F7277.8050306@zytor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed