linux-kernel - Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading from scalar types

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Date:   Wed, 22 Apr 2020 15:54:13 +0100
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-arch@...r.kernel.org, kernel-team@...roid.com,
        Mark Rutland <mark.rutland@....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>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux@...musvillemoes.dk
Subject: Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading
 from scalar types

On Tue, Apr 21, 2020 at 04:15:34PM +0100, Will Deacon wrote:
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index e970f97a7fcb..233066c92f6f 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -210,6 +210,27 @@ struct ftrace_likely_data {
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
>  
> +/*
> + * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
> + *			       non-scalar types unchanged.
> + *
> + * We build this out of a couple of helper macros in a vain attempt to
> + * help you keep your lunch down while reading it.
> + */
> +#define __pick_scalar_type(x, type, otherwise)					\
> +	__builtin_choose_expr(__same_type(x, type), (type)0, otherwise)
> +
> +#define __pick_integer_type(x, type, otherwise)					\
> +	__pick_scalar_type(x, unsigned type,					\
> +		__pick_scalar_type(x, signed type, otherwise))
> +
> +#define __unqual_scalar_typeof(x) typeof(					\
> +	__pick_integer_type(x, char,						\

Rasmus pointed out to me that 'char' is not __builtin_types_compatible_p()
with either 'signed char' or 'unsigned char', so I'll roll in the diff below
to handle this case.

Will

--->8

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 233066c92f6f..6ed0612bc143 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -220,9 +220,14 @@ struct ftrace_likely_data {
 #define __pick_scalar_type(x, type, otherwise)					\
 	__builtin_choose_expr(__same_type(x, type), (type)0, otherwise)
 
+/*
+ * 'char' is not type-compatible with either 'signed char' or 'unsigned char',
+ * so we include the naked type here as well as the signed/unsigned variants.
+ */
 #define __pick_integer_type(x, type, otherwise)					\
-	__pick_scalar_type(x, unsigned type,					\
-		__pick_scalar_type(x, signed type, otherwise))
+	__pick_scalar_type(x, type,						\
+		__pick_scalar_type(x, unsigned type,				\
+			__pick_scalar_type(x, signed type, otherwise)))
 
 #define __unqual_scalar_typeof(x) typeof(					\
 	__pick_integer_type(x, char,						\