Re: [PATCH] arm64: neon: Fix function may_use_simd() return error status

From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Dave Martin <Dave.Martin@arm.com>,
	"Yandong.Zhao" <yandong77520@gmail.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	zhaoyd@thundersoft.com, zhaoxb@thundersoft.com,
	fanlc0801@thundersoft.com
Subject: Re: [PATCH] arm64: neon: Fix function may_use_simd() return error status
Date: Wed, 11 Jul 2018 17:07:38 +0100	[thread overview]
Message-ID: <20180711160738.okzpxenezq52etol@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20180711160315.GD18477@arm.com>

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@gmail.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@arm..com>
> 
> Thanks. Applied with the updated comment and your tag..

Cheer!

Mar.