Architectures without membarrier hooks don't need to emit the
empty membarrier_arch_switch_mm() static inline when
CONFIG_MEMBARRIER=y.

Adapt the CONFIG_MEMBARRIER=n counterpart to only emit the empty
membarrier_arch_switch_mm() for architectures with membarrier hooks.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Paul E. McKenney <redacted>
CC: Boqun Feng <redacted>
CC: Andrew Hunter <redacted>
CC: Maged Michael <redacted>
CC: gromer@google.com
CC: Avi Kivity <redacted>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <redacted>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Dave Watson <redacted>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: Will Deacon <redacted>
CC: Andy Lutomirski <luto@kernel.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-arch@vger.kernel.org
---
 include/linux/sched/mm.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index d5a9ab8f3836..b2767ecb21a8 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h

@@ -215,10 +215,6 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
 #ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
 #include <asm/membarrier.h>
 #else
-static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
-		struct mm_struct *next, struct task_struct *tsk)
-{
-}
 static inline void membarrier_arch_fork(struct task_struct *t,
 		unsigned long clone_flags)
 {

@@ -247,10 +243,12 @@ static inline void membarrier_execve(struct task_struct *t)
 	membarrier_arch_execve(t);
 }
 #else
+#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
 static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 		struct mm_struct *next, struct task_struct *tsk)
 {
 }
+#endif
 static inline void membarrier_fork(struct task_struct *t,
 		unsigned long clone_flags)
 {

-- 
2.11.0