rcu: Make arch select smp_mb__after_unlock_lock() strength

The definition of smp_mb__after_unlock_lock() is currently smp_mb()
for CONFIG_PPC and a no-op otherwise.  It would be better to instead
provide an architecture-selectable Kconfig option, and select the
strength of smp_mb__after_unlock_lock() based on that option.  This
commit therefore creates ARCH_WEAK_RELEASE_ACQUIRE, has PPC select it,
and bases the definition of smp_mb__after_unlock_lock() on this new
ARCH_WEAK_RELEASE_ACQUIRE Kconfig option.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Boqun Feng <boqun.feng@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: <linuxppc-dev@lists.ozlabs.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

arch/Kconfig

@@ -320,6 +320,9 @@ config HAVE_CMPXCHG_LOCAL
 config HAVE_CMPXCHG_DOUBLE
 	bool
 
+config ARCH_WEAK_RELEASE_ACQUIRE
+	bool
+
 config ARCH_WANT_IPC_PARSE_VERSION
 	bool
 

arch/powerpc/Kconfig

@@ -99,6 +99,7 @@ config PPC
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_CMPXCHG_LOCKREF		if PPC64
 	select ARCH_WANT_IPC_PARSE_VERSION
+	select ARCH_WEAK_RELEASE_ACQUIRE
 	select BINFMT_ELF
 	select BUILDTIME_EXTABLE_SORT
 	select CLONE_BACKWARDS

include/linux/rcupdate.h

@@ -1127,11 +1127,11 @@ do { \
  * if the UNLOCK and LOCK are executed by the same CPU or if the
  * UNLOCK and LOCK operate on the same lock variable.
  */
-#ifdef CONFIG_PPC
+#ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
 #define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
-#else /* #ifdef CONFIG_PPC */
+#else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
 #define smp_mb__after_unlock_lock()	do { } while (0)
-#endif /* #else #ifdef CONFIG_PPC */
+#endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
 
 
 #endif /* __LINUX_RCUPDATE_H */