linux-kernel - Re: [RFC PATCH 8/8] arm64: barrier: Use '__unqual_scalar

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Message-ID: <20200110174506.GK33536@lakrids.cambridge.arm.com>
Date:   Fri, 10 Jan 2020 17:45:07 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...roid.com, Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH 8/8] arm64: barrier: Use '__unqual_scalar_typeof' for
 acquire/release macros

On Fri, Jan 10, 2020 at 04:56:36PM +0000, Will Deacon wrote:
> Passing volatile-qualified pointers to the arm64 implementations of the
> load-acquire/store-release macros results in a re-load from the stack
> and a bunch of associated stack-protector churn due to the temporary
> result variable inheriting the volatile semantics thanks to the use of
> 'typeof()'.
> 
> Define these temporary variables using 'unqual_scalar_typeof' to drop
> the volatile qualifier in the case that they are scalar types.
> 
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Will Deacon <will@...nel.org>

Based on my understanding of __unqual_scalar_typeof(), these changes
look sound to me:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  arch/arm64/include/asm/barrier.h | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 7d9cc5ec4971..fb4c27506ef4 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -76,8 +76,8 @@ static inline unsigned long array_index_mask_nospec(unsigned long idx,
>  #define __smp_store_release(p, v)					\
>  do {									\
>  	typeof(p) __p = (p);						\
> -	union { typeof(*p) __val; char __c[1]; } __u =			\
> -		{ .__val = (__force typeof(*p)) (v) };			\
> +	union { __unqual_scalar_typeof(*p) __val; char __c[1]; } __u =	\
> +		{ .__val = (__force __unqual_scalar_typeof(*p)) (v) };	\
>  	compiletime_assert_atomic_type(*p);				\
>  	kasan_check_write(__p, sizeof(*p));				\
>  	switch (sizeof(*p)) {						\
> @@ -110,7 +110,7 @@ do {									\
>  
>  #define __smp_load_acquire(p)						\
>  ({									\
> -	union { typeof(*p) __val; char __c[1]; } __u;			\
> +	union { __unqual_scalar_typeof(*p) __val; char __c[1]; } __u;	\
>  	typeof(p) __p = (p);						\
>  	compiletime_assert_atomic_type(*p);				\
>  	kasan_check_read(__p, sizeof(*p));				\
> @@ -136,33 +136,33 @@ do {									\
>  			: "Q" (*__p) : "memory");			\
>  		break;							\
>  	}								\
> -	__u.__val;							\
> +	(typeof(*p))__u.__val;						\
>  })
>  
>  #define smp_cond_load_relaxed(ptr, cond_expr)				\
>  ({									\
>  	typeof(ptr) __PTR = (ptr);					\
> -	typeof(*ptr) VAL;						\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
>  	for (;;) {							\
>  		VAL = READ_ONCE(*__PTR);				\
>  		if (cond_expr)						\
>  			break;						\
>  		__cmpwait_relaxed(__PTR, VAL);				\
>  	}								\
> -	VAL;								\
> +	(typeof(*ptr))VAL;						\
>  })
>  
>  #define smp_cond_load_acquire(ptr, cond_expr)				\
>  ({									\
>  	typeof(ptr) __PTR = (ptr);					\
> -	typeof(*ptr) VAL;						\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
>  	for (;;) {							\
>  		VAL = smp_load_acquire(__PTR);				\
>  		if (cond_expr)						\
>  			break;						\
>  		__cmpwait_relaxed(__PTR, VAL);				\
>  	}								\
> -	VAL;								\
> +	(typeof(*ptr))VAL;						\
>  })
>  
>  #include <asm-generic/barrier.h>
> -- 
> 2.25.0.rc1.283.g88dfdc4193-goog
>