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Date:   Wed, 16 Jun 2021 14:36:37 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm
 switch
To:     Andy Lutomirski <luto@kernel.org>, x86@kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        linuxppc-dev@lists.ozlabs.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>,
        Peter Zijlstra <peterz@infradead.org>
References: <cover.1623813516.git.luto@kernel.org>
        <d9034f951329830b766feb08099c15b714e08e8f.1623813516.git.luto@kernel.org>
In-Reply-To: <d9034f951329830b766feb08099c15b714e08e8f.1623813516.git.luto@kernel.org>
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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():
>=20
>        /*
>         * 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;
>=20
> 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 =3D=3D NULL and would, in fact, OOPS if prev =3D=3D NULL.

Yeah that's strange, code definitely doesn't match comment. Good catch.

>=20
> 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=20
digging into this too.

Thanks,
Nick

>=20
> 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
>=20
> 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,
> =20
>  	if (new_on_cpu)
>  		radix_kvm_prefetch_workaround(next);
> -	else
> -		membarrier_arch_switch_mm(prev, next, tsk);
> =20
>  	/*
>  	 * The actual HW switching method differs between the various
> --=20
> 2.31.1
>=20
>=20