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

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Message-ID: <20180711160738.okzpxenezq52etol@lakrids.cambridge.arm.com>
Date:   Wed, 11 Jul 2018 17:07:38 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        "Yandong.Zhao" <yandong77520@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Catalin Marinas <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 05:03:15PM +0100, Will Deacon wrote:
> On Wed, Jul 11, 2018 at 04:47:58PM +0100, Mark Rutland wrote:
> > On Wed, Jul 11, 2018 at 09:20:03AM +0200, Ard Biesheuvel wrote:
> > > On 11 July 2018 at 03:09, Yandong.Zhao <yandong77520@...il.com> wrote:
> > > > diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> > > > index fa8b3fe..784a8c2 100644
> > > > --- a/arch/arm64/include/asm/simd.h
> > > > +++ b/arch/arm64/include/asm/simd.h
> > > > @@ -29,7 +29,8 @@
> > > >  static __must_check inline bool may_use_simd(void)
> > > >  {
> > > >         /*
> > > > -        * The raw_cpu_read() is racy if called with preemption enabled.
> > > > +        * The this_cpu_read() is racy if called with preemption enabled,
> > > > +        * since the task may subsequently migrate to another CPU.
> > > >          * This is not a bug: kernel_neon_busy is only set when
> > > >          * preemption is disabled, so we cannot migrate to another CPU
> > > >          * while it is set, nor can we migrate to a CPU where it is set.
> > 
> > It would be nice if we could clarify the "is racy" part here.
> > 
> > How about:
> > 
> > 	/*
> > 	 * kernel_neon_busy is only set while preemption is disabled,
> > 	 * and is clear whenever preemption is enabled. Since
> > 	 * this_cpu_read() is atomic w.r.t. preemption, kernel_neon_busy
> > 	 * cannot change under our feet -- if it's set we cannot be
> > 	 * migrated, and if it's clear we cannot be migrated to a CPU
> > 	 * where it is set.
> > 	 */
> > 
> > With that:
> > 
> > Reviewed-by: Mark Rutland <mark.rutland@.....com>
> 
> Thanks. Applied with the updated comment and your tag..

Cheer!

Mar.