From: Nicholas Piggin <npiggin@gmail•com>
To: Andy Lutomirski <luto@kernel•org>, x86@kernel•org
Cc: Dave Hansen <dave.hansen@intel•com>,
Peter Zijlstra <peterz@infradead•org>,
LKML <linux-kernel@vger•kernel.org>,
linux-mm@kvack•org,
Mathieu Desnoyers <mathieu.desnoyers@efficios•com>,
Paul Mackerras <paulus@samba•org>,
Andrew Morton <akpm@linux-foundation•org>,
linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch
Date: Wed, 16 Jun 2021 14:36:37 +1000 [thread overview]
Message-ID: <1623817894.lsnlb539ub.astroid@bobo.none> (raw)
In-Reply-To: <d9034f951329830b766feb08099c15b714e08e8f.1623813516.git.luto@kernel.org>
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
>
>
next prev parent reply other threads:[~2021-06-16 4:37 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1623813516.git.luto@kernel.org>
2021-06-16 3:21 ` [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2021-06-16 4:36 ` Nicholas Piggin [this message]
2021-06-16 3:21 ` [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2021-06-16 4:45 ` Nicholas Piggin
2021-06-16 18:52 ` Andy Lutomirski
2021-06-16 23:48 ` Andy Lutomirski
2021-06-18 15:27 ` Christophe Leroy
2021-06-16 10:20 ` Will Deacon
2021-06-16 23:58 ` Andy Lutomirski
2021-06-17 14:47 ` Mathieu Desnoyers
2021-06-18 0:12 ` Andy Lutomirski
2021-06-18 16:31 ` Mathieu Desnoyers
2021-06-18 19:58 ` Andy Lutomirski
2021-06-18 20:09 ` Mathieu Desnoyers
2021-06-19 6:02 ` Nicholas Piggin
2021-06-19 15:50 ` Andy Lutomirski
2021-06-20 2:10 ` Nicholas Piggin
2021-06-17 15:16 ` Mathieu Desnoyers
2021-06-18 0:13 ` Andy Lutomirski
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