[PATCH 06/23] powerpc/membarrier: Remove special barrier on mm switch

[Andy Lutomirski]

powerpc did the following on some, but not all, paths through
switch_mm_irqs_off():

       /*
        * Only need the full barrier when switching between processes.
        * Barrier when switching from kernel to userspace is not
        * required here, given that it is implied by mmdrop(). Barrier
        * when switching from userspace to kernel is not needed after
        * store to rq->curr.
        */
       if (likely(!(atomic_read(&next->membarrier_state) &
                    (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
                     MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
               return;

This is puzzling: if !prev, then one might expect that we are switching
from kernel to user, not user to kernel, which is inconsistent with the
comment.  But this is all nonsense, because the one and only caller would
never have prev == NULL and would, in fact, OOPS if prev == NULL.

In any event, this code is unnecessary, since the new generic
membarrier_finish_switch_mm() provides the same barrier without arch help.

arch/powerpc/include/asm/membarrier.h remains as an empty header,
because a later patch in this series will add code to it.

Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Cc: Paul Mackerras <paulus at samba.org>
Cc: linuxppc-dev at lists.ozlabs.org
Cc: Nicholas Piggin <npiggin at gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Signed-off-by: Andy Lutomirski <luto at kernel.org>
---
 arch/powerpc/include/asm/membarrier.h | 24 ------------------------
 arch/powerpc/mm/mmu_context.c         |  1 -
 2 files changed, 25 deletions(-)

diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
index de7f79157918..b90766e95bd1 100644
--- a/arch/powerpc/include/asm/membarrier.h
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -1,28 +1,4 @@
 #ifndef _ASM_POWERPC_MEMBARRIER_H
 #define _ASM_POWERPC_MEMBARRIER_H
 
-static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
-					     struct mm_struct *next,
-					     struct task_struct *tsk)
-{
-	/*
-	 * Only need the full barrier when switching between processes.
-	 * Barrier when switching from kernel to userspace is not
-	 * required here, given that it is implied by mmdrop(). Barrier
-	 * when switching from userspace to kernel is not needed after
-	 * store to rq->curr.
-	 */
-	if (IS_ENABLED(CONFIG_SMP) &&
-	    likely(!(atomic_read(&next->membarrier_state) &
-		     (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
-		      MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
-		return;
-
-	/*
-	 * The membarrier system call requires a full memory barrier
-	 * after storing to rq->curr, before going back to user-space.
-	 */
-	smp_mb();
-}
-
 #endif /* _ASM_POWERPC_MEMBARRIER_H */
diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.c
index 74246536b832..5f2daa6b0497 100644
--- a/arch/powerpc/mm/mmu_context.c
+++ b/arch/powerpc/mm/mmu_context.c
@@ -84,7 +84,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		asm volatile ("dssall");
 
 	if (!new_on_cpu)
-		membarrier_arch_switch_mm(prev, next, tsk);
 
 	/*
 	 * The actual HW switching method differs between the various
-- 
2.33.1