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

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

[Yandong.Zhao]

From: Yandong Zhao <yandong77520@gmail.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@arm.com>
Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
---
 arch/arm64/include/asm/simd.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

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.
@@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
 	 * false.
 	 */
 	return !in_irq() && !irqs_disabled() && !in_nmi() &&
-		!raw_cpu_read(kernel_neon_busy);
+		!this_cpu_read(kernel_neon_busy);
 }
 
 #else /* ! CONFIG_KERNEL_MODE_NEON */
-- 
1.9.1

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

[Will Deacon]

On Wed, Jul 11, 2018 at 07:06:28PM +0800, Yandong.Zhao wrote:
> From: Yandong Zhao <yandong77520@gmail.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@arm.com>
> Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
> ---
>  arch/arm64/include/asm/simd.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

Does this need to go to stable?

Will

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

[Dave Martin]

On Wed, Jul 11, 2018 at 01:05:19PM +0100, Will Deacon wrote:
> On Wed, Jul 11, 2018 at 07:06:28PM +0800, Yandong.Zhao wrote:
> > From: Yandong Zhao <yandong77520@gmail.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@arm.com>
> > Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
> > ---
> >  arch/arm64/include/asm/simd.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> Does this need to go to stable?

It should, yes, so we should probably add

Cc: stable@vger.kernel.org


Are you OK to pick that up?

Cheers
---Dave

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

[Dave Martin]

On Thu, Jul 12, 2018 at 11:29:38AM +0800, Yandong.Zhao wrote:
> From: Yandong Zhao <yandong77520@gmail.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")
> Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Reviewed-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Mark Rutland <mark.rutland@arm..com>
> Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
> ---
>  arch/arm64/include/asm/simd.h | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index fa8b3fe..6495cc5 100644
> --- a/arch/arm64/include/asm/simd.h
> +++ b/arch/arm64/include/asm/simd.h
> @@ -29,20 +29,15 @@
>  static __must_check inline bool may_use_simd(void)
>  {
>  	/*
> -	 * The raw_cpu_read() is racy if called with preemption enabled.
> -	 * 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.
> -	 * So, if we find it clear on some CPU then we're guaranteed to
> -	 * find it clear on any CPU we could migrate to.
> -	 *
> -	 * If we are in between kernel_neon_begin()...kernel_neon_end(),
> -	 * the flag will be set, but preemption is also disabled, so we
> -	 * can't migrate to another CPU and spuriously see it become
> -	 * false.
> +	 * 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.
>  	 */

This new explanation looks fine to me.

[...]

Cheers
---Dave

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

[Yandong.Zhao]

From: Yandong Zhao <yandong77520@gmail.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")
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm..com>
Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
---
 arch/arm64/include/asm/simd.h | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe..6495cc5 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -29,20 +29,15 @@
 static __must_check inline bool may_use_simd(void)
 {
 	/*
-	 * The raw_cpu_read() is racy if called with preemption enabled.
-	 * 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.
-	 * So, if we find it clear on some CPU then we're guaranteed to
-	 * find it clear on any CPU we could migrate to.
-	 *
-	 * If we are in between kernel_neon_begin()...kernel_neon_end(),
-	 * the flag will be set, but preemption is also disabled, so we
-	 * can't migrate to another CPU and spuriously see it become
-	 * false.
+	 * 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.
 	 */
 	return !in_irq() && !irqs_disabled() && !in_nmi() &&
-		!raw_cpu_read(kernel_neon_busy);
+		!this_cpu_read(kernel_neon_busy);
 }
 
 #else /* ! CONFIG_KERNEL_MODE_NEON */
-- 
1.9.1

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

[Mark Rutland]

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.

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

[Will Deacon]

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..

Will

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

[Mark Rutland]

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:
> > From: Yandong Zhao <yandong77520@gmail.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.
> >
> > Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
> 
> I had a bit of trouble disentangling the per-cpu spaghetti to decide
> whether this may trigger warnings when CONFIG_DEBUG_PREEMPT=y, but I
> don't think so. So assuming this is *not* the case:

It shouldn't, since:

* this_cpu_*() are prempt-safe

* __this_cpu_*() are not preempt-safe (and warn when preemptible)

* raw_cpu_*() are not preempt safe (but don't warn when preemptible)

> Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> 
> 
> > ---
> >  arch/arm64/include/asm/simd.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > 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,
Mark.

> > @@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
> >          * false.
> >          */
> >         return !in_irq() && !irqs_disabled() && !in_nmi() &&
> > -               !raw_cpu_read(kernel_neon_busy);
> > +               !this_cpu_read(kernel_neon_busy);
> >  }
> >
> >  #else /* ! CONFIG_KERNEL_MODE_NEON */
> > --
> > 1.9.1
> >

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

[Dave Martin]

On Wed, Jul 11, 2018 at 09:09:59AM +0800, Yandong.Zhao wrote:
> From: Yandong Zhao <yandong77520@gmail.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.
> 
> Signed-off-by: Yandong Zhao <yandong77520@gmail.com>

Looks ok to me.  You can add the following tags:

Fixes: cb84d11e1625 ("arm64: neon: Remove support for nested or hardirq kernel-mode NEON")
Reviewed-by: Dave Martin <Dave.Martin@arm.com>


Cheers
---Dave


> ---
>  arch/arm64/include/asm/simd.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> 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.
> @@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
>  	 * false.
>  	 */
>  	return !in_irq() && !irqs_disabled() && !in_nmi() &&
> -		!raw_cpu_read(kernel_neon_busy);
> +		!this_cpu_read(kernel_neon_busy);
>  }
>  
>  #else /* ! CONFIG_KERNEL_MODE_NEON */
> -- 
> 1.9.1
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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

[Ard Biesheuvel]

On 11 July 2018 at 03:09, Yandong.Zhao <yandong77520@gmail.com> wrote:
> From: Yandong Zhao <yandong77520@gmail.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.
>
> Signed-off-by: Yandong Zhao <yandong77520@gmail.com>

I had a bit of trouble disentangling the per-cpu spaghetti to decide
whether this may trigger warnings when CONFIG_DEBUG_PREEMPT=y, but I
don't think so. So assuming this is *not* the case:

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>


> ---
>  arch/arm64/include/asm/simd.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> 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.
> @@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
>          * false.
>          */
>         return !in_irq() && !irqs_disabled() && !in_nmi() &&
> -               !raw_cpu_read(kernel_neon_busy);
> +               !this_cpu_read(kernel_neon_busy);
>  }
>
>  #else /* ! CONFIG_KERNEL_MODE_NEON */
> --
> 1.9.1
>

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

[Yandong.Zhao]

From: Yandong Zhao <yandong77520@gmail.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.

Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
---
 arch/arm64/include/asm/simd.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

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.
@@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
 	 * false.
 	 */
 	return !in_irq() && !irqs_disabled() && !in_nmi() &&
-		!raw_cpu_read(kernel_neon_busy);
+		!this_cpu_read(kernel_neon_busy);
 }
 
 #else /* ! CONFIG_KERNEL_MODE_NEON */
-- 
1.9.1

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

[Dave Martin]

On Tue, Jul 10, 2018 at 10:21:40AM +0800, Yandong.Zhao wrote:
> From: Yandong Zhao <yandong77520@gmail.com>
> 
> Operations for contexts where we do not want to do any checks for
> preemptions.  Unless strictly necessary, always use this_cpu_read()
> instead.  Because of the kernel_neon_busy here we have to make sure
> that it is the current cpu.

I find this wording a bit confusing.

Does the following make look OK to you?

--8<--

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.

-->8--

> 
> Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
> ---
>  arch/arm64/include/asm/simd.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index fa8b3fe..8b97f8b 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 my subsequently migrate to another CPU.

"my" -> "may"

(apologies if I was responsible for that typo)


[...]

Cheers
---Dave

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

[Yandong.Zhao]

From: Yandong Zhao <yandong77520@gmail.com>

Operations for contexts where we do not want to do any checks for
preemptions.  Unless strictly necessary, always use this_cpu_read()
instead.  Because of the kernel_neon_busy here we have to make sure
that it is the current cpu.

Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
---
 arch/arm64/include/asm/simd.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe..8b97f8b 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 my 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.
@@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
 	 * false.
 	 */
 	return !in_irq() && !irqs_disabled() && !in_nmi() &&
-		!raw_cpu_read(kernel_neon_busy);
+		!this_cpu_read(kernel_neon_busy);
 }
 
 #else /* ! CONFIG_KERNEL_MODE_NEON */
-- 
1.9.1