arm64: xchg: Implement cmpxchg_double

The arm64 architecture has the ability to exclusively load and store
a pair of registers from an address (ldxp/stxp). Also the SLUB can take
advantage of a cmpxchg_double implementation to avoid taking some
locks.

This patch provides an implementation of cmpxchg_double for 64-bit
pairs, and activates the logic required for the SLUB to use these
functions (HAVE_ALIGNED_STRUCT_PAGE and HAVE_CMPXCHG_DOUBLE).

Also definitions of this_cpu_cmpxchg_8 and this_cpu_cmpxchg_double_8
are wired up to cmpxchg_local and cmpxchg_double_local (rather than the
stock implementations that perform non-atomic operations with
interrupts disabled) as they are used by the SLUB.

On a Juno platform running on only the A57s I get quite a noticeable
performance improvement with 5 runs of hackbench on v3.17:

         Baseline | With Patch
 -----------------+-----------
 Mean    119.2312 | 106.1782
 StdDev    0.4919 |   0.4494

(times taken to complete `./hackbench 100 process 1000', in seconds)

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 2c3c2ca6f8bc..1e0e4671dd25 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -34,6 +34,7 @@ config ARM64
         select GENERIC_TIME_VSYSCALL
         select HANDLE_DOMAIN_IRQ
         select HARDIRQS_SW_RESEND
+        select HAVE_ALIGNED_STRUCT_PAGE if SLUB
         select HAVE_ARCH_AUDITSYSCALL
         select HAVE_ARCH_JUMP_LABEL
         select HAVE_ARCH_KGDB
@@ -41,6 +42,7 @@ config ARM64
         select HAVE_BPF_JIT
         select HAVE_C_RECORDMCOUNT
         select HAVE_CC_STACKPROTECTOR
+        select HAVE_CMPXCHG_DOUBLE
         select HAVE_DEBUG_BUGVERBOSE
         select HAVE_DEBUG_KMEMLEAK
         select HAVE_DMA_API_DEBUG