[RFC PATCH for 4.14 1/2] membarrier: Remove unused code for architectures without membarrier hooks

[Mathieu Desnoyers]

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 at gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
CC: Peter Zijlstra <peterz at infradead.org>
CC: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng at gmail.com>
CC: Andrew Hunter <ahh at google.com>
CC: Maged Michael <maged.michael at gmail.com>
CC: gromer at google.com
CC: Avi Kivity <avi at scylladb.com>
CC: Benjamin Herrenschmidt <benh at kernel.crashing.org>
CC: Paul Mackerras <paulus at samba.org>
CC: Michael Ellerman <mpe at ellerman.id.au>
CC: Dave Watson <davejwatson at fb.com>
CC: Alan Stern <stern at rowland.harvard.edu>
CC: Will Deacon <will.deacon at arm.com>
CC: Andy Lutomirski <luto at kernel.org>
CC: Ingo Molnar <mingo at redhat.com>
CC: Alexander Viro <viro at zeniv.linux.org.uk>
CC: linuxppc-dev at lists.ozlabs.org
CC: linux-arch at 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