x86-64: support native xadd rwsem implementation

This one is much faster than the spinlock based fallback rwsem code,
with certain artifical benchmarks having shown 300%+ improvement on
threaded page faults etc.

Again, note the 32767-thread limit here. So this really does need that
whole "make rwsem_count_t be 64-bit and fix the BIAS values to match"
extension on top of it, but that is conceptually a totally independent
issue.

NOT TESTED! The original patch that this all was based on were tested by
KAMEZAWA Hiroyuki, but maybe I screwed up something when I created the
cleaned-up series, so caveat emptor..

Also note that it _may_ be a good idea to mark some more registers
clobbered on x86-64 in the inline asms instead of saving/restoring them.
They are inline functions, but they are only used in places where there
are not a lot of live registers _anyway_, so doing for example the
clobbers of %r8-%r11 in the asm wouldn't make the fast-path code any
worse, and would make the slow-path code smaller.

(Not that the slow-path really matters to that degree. Saving a few
unnecessary registers is the _least_ of our problems when we hit the slow
path. The instruction/cycle counting really only matters in the fast
path).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <alpine.LFD.2.00.1001121810410.17145@localhost.localdomain>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index cffd754f3039..c80245131fdc 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -39,4 +39,5 @@ else
         lib-y += thunk_64.o clear_page_64.o copy_page_64.o
         lib-y += memmove_64.o memset_64.o
         lib-y += copy_user_64.o rwlock_64.o copy_user_nocache_64.o
+	lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem_64.o
 endif