[PATCH] arm64: xchg: prevent warning if return value is unused

[will.deacon@arm.com (Will Deacon)]

Some users of xchg() don't bother using the return value, which results
in a compiler warning like the following (from kgdb):

In file included from linux/arch/arm64/include/asm/atomic.h:27:0,
                 from include/linux/atomic.h:4,
                 from include/linux/spinlock.h:402,
                 from include/linux/seqlock.h:35,
                 from include/linux/time.h:5,
                 from include/uapi/linux/timex.h:56,
                 from include/linux/timex.h:56,
                 from include/linux/sched.h:19,
                 from include/linux/pid_namespace.h:4,
                 from kernel/debug/debug_core.c:30:
kernel/debug/debug_core.c: In function ?kgdb_cpu_enter?:
linux/arch/arm64/include/asm/cmpxchg.h:75:3: warning: value computed is not used [-Wunused-value]
  ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))
   ^
linux/arch/arm64/include/asm/atomic.h:132:30: note: in expansion of macro ?xchg?
 #define atomic_xchg(v, new) (xchg(&((v)->counter), new))

kernel/debug/debug_core.c:504:4: note: in expansion of macro ?atomic_xchg?
    atomic_xchg(&kgdb_active, cpu);
    ^

This patch makes use of the same trick as we do for cmpxchg, by assigning
the return value to a dummy variable in the xchg() macro itself.

Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 arch/arm64/include/asm/cmpxchg.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
index 57c0fa7bf711..ddb9d7830558 100644
--- a/arch/arm64/include/asm/cmpxchg.h
+++ b/arch/arm64/include/asm/cmpxchg.h
@@ -72,7 +72,12 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
 }
 
 #define xchg(ptr,x) \
-	((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))
+({ \
+	__typeof__(*(ptr)) __ret; \
+	__ret = (__typeof__(*(ptr))) \
+		__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))); \
+	__ret; \
+})
 
 static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 				      unsigned long new, int size)
-- 
1.9.2