linux-kernel - [RFC v3 5/5] atomic: Add option for weaker alignment check

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Message-ID: <d4ab82f5646f901b7c556c4cb8f8adf4096f0c50.1759875560.git.fthain@linux-m68k.org>
Date: Wed, 08 Oct 2025 09:19:20 +1100
From: Finn Thain <fthain@...ux-m68k.org>
To: Peter Zijlstra <peterz@...radead.org>,
    Will Deacon <will@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
    Boqun Feng <boqun.feng@...il.com>,
    Jonathan Corbet <corbet@....net>,
    Mark Rutland <mark.rutland@....com>,
    Arnd Bergmann <arnd@...db.de>,
    linux-kernel@...r.kernel.org,
    linux-arch@...r.kernel.org,
    Geert Uytterhoeven <geert@...ux-m68k.org>,
    linux-m68k@...r.kernel.org
Subject: [RFC v3 5/5] atomic: Add option for weaker alignment check

Add a new Kconfig symbol to make CONFIG_DEBUG_ATOMIC more useful on
those architectures which do not align dynamic allocations to
8-byte boundaries. Without this, CONFIG_DEBUG_ATOMIC produces excessive
WARN splats.
---
 include/linux/instrumented.h | 14 +++++++++++---
 lib/Kconfig.debug            |  9 +++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index 402a999a0d6b..3c9a570dbfa7 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -8,11 +8,13 @@
 #define _LINUX_INSTRUMENTED_H
 
 #include <linux/bug.h>
+#include <linux/cache.h>
 #include <linux/compiler.h>
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
 #include <linux/kmsan-checks.h>
 #include <linux/types.h>
+#include <vdso/align.h>
 
 /**
  * instrument_read - instrument regular read access
@@ -68,7 +70,9 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	if (IS_ENABLED(CONFIG_DEBUG_ATOMIC))
+		WARN_ON_ONCE(v != PTR_ALIGN(v, IS_ENABLED(CONFIG_DEBUG_ATOMIC_LARGEST_ALIGN) ?
+					       __LARGEST_ALIGN : size));
 }
 
 /**
@@ -83,7 +87,9 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	if (IS_ENABLED(CONFIG_DEBUG_ATOMIC))
+		WARN_ON_ONCE(v != PTR_ALIGN(v, IS_ENABLED(CONFIG_DEBUG_ATOMIC_LARGEST_ALIGN) ?
+					       __LARGEST_ALIGN : size));
 }
 
 /**
@@ -98,7 +104,9 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	if (IS_ENABLED(CONFIG_DEBUG_ATOMIC))
+		WARN_ON_ONCE(v != PTR_ALIGN(v, IS_ENABLED(CONFIG_DEBUG_ATOMIC_LARGEST_ALIGN) ?
+					       __LARGEST_ALIGN : size));
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d82626b7d6be..f81b8a9b9216 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1373,6 +1373,15 @@ config DEBUG_ATOMIC
 
 	  This option has potentially significant overhead.
 
+config DEBUG_ATOMIC_LARGEST_ALIGN
+	bool "Check alignment only up to __LARGEST_ALIGN"
+	depends on DEBUG_ATOMIC
+	help
+	  Say Y here if you require natural alignment of atomic accesses
+	  only up to long word boundaries. That is, char, short, int and long
+	  will be checked for natural alignment but anything larger checked
+	  only for alignment with a long word boundary.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config LOCK_DEBUGGING_SUPPORT
-- 
2.49.1