linux-kernel - Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation

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Date:   Thu, 2 Jul 2020 10:32:39 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Matt Turner <mattst88@...il.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Richard Henderson <rth@...ddle.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-alpha@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kernel-team@...roid.com
Subject: Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered
 implementation

On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
> Rather then relying on the core code to use smp_read_barrier_depends()
> as part of the READ_ONCE() definition, instead override __READ_ONCE()
> in the Alpha code so that it is treated the same way as
> smp_load_acquire().
> 
> Acked-by: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
>  arch/alpha/include/asm/barrier.h | 61 ++++----------------------------
>  arch/alpha/include/asm/rwonce.h  | 19 ++++++++++
>  2 files changed, 26 insertions(+), 54 deletions(-)
>  create mode 100644 arch/alpha/include/asm/rwonce.h
> 
> diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
> index 92ec486a4f9e..2ecd068d91d1 100644
> --- a/arch/alpha/include/asm/barrier.h
> +++ b/arch/alpha/include/asm/barrier.h
> @@ -2,64 +2,17 @@
>  #ifndef __BARRIER_H
>  #define __BARRIER_H
>  
> -#include <asm/compiler.h>
> -
>  #define mb()	__asm__ __volatile__("mb": : :"memory")
>  #define rmb()	__asm__ __volatile__("mb": : :"memory")
>  #define wmb()	__asm__ __volatile__("wmb": : :"memory")

> -#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
> +#define __smp_load_acquire(p)						\
> +({									\
> +	__unqual_scalar_typeof(*p) ___p1 =				\
> +		(*(volatile typeof(___p1) *)(p));			\
> +	compiletime_assert_atomic_type(*p);				\
> +	___p1;								\
> +})

Sorry if I'm being thick, but doesn't this need a barrier after the
volatile access to provide the acquire semantic?

IIUC prior to this commit alpha would have used the asm-generic
__smp_load_acquire, i.e.

| #ifndef __smp_load_acquire
| #define __smp_load_acquire(p)                                           \
| ({                                                                      \
|         __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);               \
|         compiletime_assert_atomic_type(*p);                             \
|         __smp_mb();                                                     \
|         (typeof(*p))___p1;                                              \
| })
| #endif

... where the __smp_mb() would be alpha's mb() from earlier in the patch
context, i.e.

| #define mb() __asm__ __volatile__("mb": : :"memory")

... so don't we need similar before returning ___p1 above in
__smp_load_acquire() (and also matching the old read_barrier_depends())?

[...]

> +#include <asm/barrier.h>
> +
> +/*
> + * Alpha is apparently daft enough to reorder address-dependent loads
> + * on some CPU implementations. Knock some common sense into it with
> + * a memory barrier in READ_ONCE().
> + */
> +#define __READ_ONCE(x)	__smp_load_acquire(&(x))

As above, I don't see a memory barrier implied here, so this doesn't
look quite right.

Thanks,
Mark.