From: Xie Yuanbin <qq570070308@gmail.com>
Date: Sun, 9 Nov 2025 08:37:15 +0000 (+0800)
Subject: include/linux/once_lite.h: fix judgment in WARN_ONCE with clang
X-Git-Tag: v6.19-rc1~70^2~78
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=242b872239f6a7deacbc20ab9406ea40cb738ec6;p=thirdparty%2Flinux.git

include/linux/once_lite.h: fix judgment in WARN_ONCE with clang

For c code:
```c
extern int xx;
void test(void)
{
	if (WARN_ONCE(xx, "x"))
		__asm__ volatile ("nop":::);
}
```

Clang will generate the following assembly code:
```assemble
test:
	movl	xx(%rip), %eax // Assume xx == 0 (likely case)
	testl	%eax, %eax // judge once
	je	.LBB0_3    // jump to .LBB0_3
	testb	$1, test.__already_done(%rip)
	je	.LBB0_2
.LBB0_3:
	testl	%eax, %eax // judge again
	je	.LBB0_5    // jump to .LBB0_5
.LBB0_4:
	nop
.LBB0_5:
	retq
	// omit
```

In the above code, `xx == 0` should be a likely case, but in this case,
xx has been judged twice.

Test info:
1. kernel source:
linux-next
commit 9c0826a5d9aa4d52206d ("Add linux-next specific files for 20251107")
2. compiler:
clang: Debian clang version 21.1.4 (8) with
Debian LLD 21.1.4 (compatible with GNU linkers)
3. config:
base on default x86_64_defconfig, and setting:
CONFIG_MITIGATION_RETHUNK=n
CONFIG_STACKPROTECTOR=n

Add unlikely to __ret_cond to help the compiler optimize correctly.

[akpm@linux-foundation.org: undo whitespace changes]
Link: https://lkml.kernel.org/r/20251109083715.24495-1-qq570070308@gmail.com
Signed-off-by: Xie Yuanbin <qq570070308@gmail.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Maninder Singh <maninder1.s@samsung.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/include/linux/once_lite.h b/include/linux/once_lite.h
index 27de7bc32a061..236592c4eeb1a 100644
--- a/include/linux/once_lite.h
+++ b/include/linux/once_lite.h
@@ -16,7 +16,7 @@
 		bool __ret_cond = !!(condition);			\
 		bool __ret_once = false;				\
 									\
-		if (unlikely(__ret_cond && !__already_done)) {		\
+		if (unlikely(__ret_cond) && unlikely(!__already_done)) {\
 			__already_done = true;				\
 			__ret_once = true;				\
 		}							\