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

From: Mathieu Desnoyers <[email-masked]>
Date: Wed, 12 Jan 2022 10:57:55 -0500 (EST)

----- On Jan 8, 2022, at 11:43 AM, Andy Lutomirski [email-masked] wrote:

> 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.

My disagreement with "membarrier: Make the post-switch-mm barrier explicit"
may affect this patch significantly, or even make it irrelevant.

Thanks,

Mathieu

> 
> 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 | 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com