linux-kernel - [PATCH v2 1/2] x86/percpu: Fix clang warning when dealing with unsigned types

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Message-ID: <20241016182635.1156168-2-andriy.shevchenko@linux.intel.com>
Date: Wed, 16 Oct 2024 21:23:51 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Ingo Molnar <mingo@...nel.org>,
	Uros Bizjak <ubizjak@...il.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Dennis Zhou <dennis@...nel.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v2 1/2] x86/percpu: Fix clang warning when dealing with unsigned types

When percpu_add_op() is used with an unsigned argument, it prevents
kernel builds with clang, `make W=1` and CONFIG_WERROR=y:

net/ipv4/tcp_output.c:187:3: error: result of comparison of constant -1 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
  187 |                 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  188 |                               tp->compressed_ack);
      |                               ~~~~~~~~~~~~~~~~~~~
...
arch/x86/include/asm/percpu.h:238:31: note: expanded from macro 'percpu_add_op'
  238 |                               ((val) == 1 || (val) == -1)) ?            \
      |                                              ~~~~~ ^  ~~

Fix this by casting -1 to the type of the parameter and then compare.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 arch/x86/include/asm/percpu.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index c55a79d5feae..e525cd85f999 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -234,9 +234,10 @@ do {									\
  */
 #define percpu_add_op(size, qual, var, val)				\
 do {									\
-	const int pao_ID__ = (__builtin_constant_p(val) &&		\
-			      ((val) == 1 || (val) == -1)) ?		\
-				(int)(val) : 0;				\
+	const int pao_ID__ =						\
+		(__builtin_constant_p(val) &&				\
+			((val) == 1 ||					\
+			 (val) == (typeof(val))-1)) ? (int)(val) : 0;	\
 									\
 	if (0) {							\
 		typeof(var) pao_tmp__;					\
-- 
2.43.0.rc1.1336.g36b5255a03ac