[6/8] powerpc/membarrier: Remove special barrier on mm switch

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.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/powerpc/include/asm/membarrier.h | 27 ---------------------------
 arch/powerpc/mm/mmu_context.c         |  2 --
 2 files changed, 29 deletions(-)
 delete mode 100644 arch/powerpc/include/asm/membarrier.h

Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm:
> 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.

Yeah that's strange, code definitely doesn't match comment. Good catch.

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

If that's merged then I think this could be too. I'll do a bit more 
digging into this too.

Thanks,
Nick

> 
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/powerpc/include/asm/membarrier.h | 27 ---------------------------
>  arch/powerpc/mm/mmu_context.c         |  2 --
>  2 files changed, 29 deletions(-)
>  delete mode 100644 arch/powerpc/include/asm/membarrier.h
> 
> diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
> deleted file mode 100644
> index 6e20bb5c74ea..000000000000
> --- a/arch/powerpc/include/asm/membarrier.h
> +++ /dev/null
> @@ -1,27 +0,0 @@
> -#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 (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 a857af401738..8daa95b3162b 100644
> --- a/arch/powerpc/mm/mmu_context.c
> +++ b/arch/powerpc/mm/mmu_context.c
> @@ -85,8 +85,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>  
>  	if (new_on_cpu)
>  		radix_kvm_prefetch_workaround(next);
> -	else
> -		membarrier_arch_switch_mm(prev, next, tsk);
>  
>  	/*
>  	 * The actual HW switching method differs between the various
> -- 
> 2.31.1
> 
>