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

[Christophe Leroy]

Le 08/01/2022 à 17:43, Andy Lutomirski a écrit :
> 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.

I can't find this function membarrier_finish_switch_mm(), neither in 
Linus tree, nor in linux-next tree.

> 
> 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);

Are you sure that's what you want ?

It now means you have:

	if (!new_on_cpu)
	switch_mmu_context(prev, next, tsk);


>   
>   	/*
>   	 * The actual HW switching method differs between the various