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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
To:     akpm@linux-foundation.org
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        linux-kernel@vger.kernel.org
Subject: [PATCH] linux/container_of.h: switch to static_assert
Date:   Fri, 15 Oct 2021 11:05:30 +0200
Message-Id: <20211015090530.2774079-1-linux@rasmusvillemoes.dk>
MIME-Version: 1.0
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Precedence: bulk

_Static_assert() is evaluated already in the compiler's frontend, and
gives a somehat more to-the-point error, compared to the BUILD_BUG_ON
macro, which only fires after the optimizer has had a chance to
eliminate calls to functions marked with
__attribute__((error)). In theory, this might make builds a tiny bit
faster.

There's also a little less gunk in the error message emitted:

lib/sort.c: In function ‘foo’:
./include/linux/build_bug.h:78:41: error: static assertion failed: "pointer type mismatch in container_of()"
   78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)

compared to

lib/sort.c: In function ‘foo’:
././include/linux/compiler_types.h:322:38: error: call to ‘__compiletime_assert_2’ declared with attribute error: pointer type mismatch in container_of()
  322 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)

While at it, fix the copy-pasto in container_of_safe().

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
akpm: This is obviously on top of Andy's kernel.h splitup series, so
should go along with those if acked.

 include/linux/container_of.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index dd56019838c6..2f4944b791b8 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -16,9 +16,9 @@
  */
 #define container_of(ptr, type, member) ({				\
 	void *__mptr = (void *)(ptr);					\
-	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
-			 !__same_type(*(ptr), void),			\
-			 "pointer type mismatch in container_of()");	\
+	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
+		      __same_type(*(ptr), void),			\
+		      "pointer type mismatch in container_of()");	\
 	((type *)(__mptr - offsetof(type, member))); })
 
 /**
@@ -31,9 +31,9 @@
  */
 #define container_of_safe(ptr, type, member) ({				\
 	void *__mptr = (void *)(ptr);					\
-	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
-			 !__same_type(*(ptr), void),			\
-			 "pointer type mismatch in container_of()");	\
+	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
+		      __same_type(*(ptr), void),			\
+		      "pointer type mismatch in container_of_safe()");	\
 	IS_ERR_OR_NULL(__mptr) ? ERR_CAST(__mptr) :			\
 		((type *)(__mptr - offsetof(type, member))); })
 
-- 
2.31.1