linux-kernel - Re: [PATCH] arm64: neon: Fix function may_use

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Message-ID: <20180711120519.GC15725@arm.com>
Date:   Wed, 11 Jul 2018 13:05:19 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Yandong.Zhao" <yandong77520@...il.com>
Cc:     Dave.Martin@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, ard.biesheuvel@...aro.org,
        catalin.marinas@....com, zhaoyd@...ndersoft.com,
        zhaoxb@...ndersoft.com, fanlc0801@...ndersoft.com
Subject: Re: [PATCH] arm64: neon: Fix function may_use_simd() return error
 status

On Wed, Jul 11, 2018 at 07:06:28PM +0800, Yandong.Zhao wrote:
> From: Yandong Zhao <yandong77520@...il.com>
> 
> It does not matter if the caller of may_use_simd() migrates to
> another cpu after the call, but it is still important that the
> kernel_neon_busy percpu instance that is read matches the cpu the
> task is running on at the time of the read.
> 
> This means that raw_cpu_read() is not sufficient.  kernel_neon_busy
> may appear true if the caller migrates during the execution of
> raw_cpu_read() and the next task to be scheduled in on the initial
> cpu calls kernel_neon_begin().
> 
> This patch replaces raw_cpu_read() with this_cpu_read() to protect
> against this race.
> 
> Fixes: cb84d11e1625 ("arm64: neon: Remove support for nested or hardirq kernel-mode NEON")
> Reviewed-by: Dave Martin <Dave.Martin@....com>
> Signed-off-by: Yandong Zhao <yandong77520@...il.com>
> ---
>  arch/arm64/include/asm/simd.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

Does this need to go to stable?

Will