----- On Jan 8, 2022, at 11:43 AM, Andy Lutomirski luto@xxxxxxxxxx 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 <mpe@xxxxxxxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
> 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
