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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH 4.19 056/191] powerpc: select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
In-Reply-To: <20201103203239.940977599@linuxfoundation.org>
References: <20201103203232.656475008@linuxfoundation.org> <20201103203239.940977599@linuxfoundation.org>
Date:   Wed, 04 Nov 2020 11:20:17 +1100
Message-ID: <87361qug5a.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
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Precedence: bulk

Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
> From: Nicholas Piggin <npiggin@gmail.com>
>
> [ Upstream commit 66acd46080bd9e5ad2be4b0eb1d498d5145d058e ]
>
> powerpc uses IPIs in some situations to switch a kernel thread away
> from a lazy tlb mm, which is subject to the TLB flushing race
> described in the changelog introducing ARCH_WANT_IRQS_OFF_ACTIVATE_MM.
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> Link: https://lore.kernel.org/r/20200914045219.3736466-3-npiggin@gmail.com
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  arch/powerpc/Kconfig                   | 1 +
>  arch/powerpc/include/asm/mmu_context.h | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index f38d153d25861..0bc53f0e37c0f 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -152,6 +152,7 @@ config PPC
>  	select ARCH_USE_BUILTIN_BSWAP
>  	select ARCH_USE_CMPXCHG_LOCKREF		if PPC64
>  	select ARCH_WANT_IPC_PARSE_VERSION
> +	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM

This depends on upstream commit:

  d53c3dfb23c4 ("mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race")


Which I don't see in 4.19 stable, or in the email thread here.

So this shouldn't be backported to 4.19 unless that commit is also
backported.

cheers