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

From: Nicholas Piggin <[email-masked]>
Date: Wed, 16 Jun 2021 14:36:37 +1000

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 <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: [email-masked]
> Cc: Nicholas Piggin <[email-masked]>
> Cc: Mathieu Desnoyers <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Signed-off-by: Andy Lutomirski <[email-masked]>
> ---
>  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
> 
>