linux-kernel - [PATCH v6 4/4] atomic: Add option for weaker alignment check

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Message-ID: <0c8ef95ea5e9a949a28412a3c791b2167592ae80.1767169542.git.fthain@linux-m68k.org>
Date: Wed, 31 Dec 2025 19:25:42 +1100
From: Finn Thain <fthain@...ux-m68k.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
    Peter Zijlstra <peterz@...radead.org>,
    Will Deacon <will@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>,
    Boqun Feng <boqun.feng@...il.com>,
    Gary Guo <gary@...yguo.net>,
    Mark Rutland <mark.rutland@....com>,
    linux-arch@...r.kernel.org,
    linux-kernel@...r.kernel.org,
    linux-m68k@...ts.linux-m68k.org
Subject: [PATCH v6 4/4] 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.

Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
---
Changed since v5:
 - Rebased.

Changed since v3:
 - Dropped #include <linux/cache.h> to avoid a build failure on arm64.
 - Rewrote Kconfig help text to better describe preferred alignment.
 - Refactored to avoid line-wrapping and duplication.
---
 include/linux/instrumented.h | 25 ++++++++++++++++---------
 lib/Kconfig.debug            |  8 ++++++++
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index bcd1113b55a1..e8983eb8a888 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -56,6 +56,19 @@ static __always_inline void instrument_read_write(const volatile void *v, size_t
 	kcsan_check_read_write(v, size);
 }
 
+static __always_inline void instrument_atomic_check_alignment(const volatile void *v, size_t size)
+{
+#ifndef __DISABLE_BUG_TABLE
+	if (IS_ENABLED(CONFIG_DEBUG_ATOMIC)) {
+		unsigned int mask = size - 1;
+
+		if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_LARGEST_ALIGN))
+			mask &= sizeof(struct { long x; } __aligned_largest) - 1;
+		WARN_ON_ONCE((unsigned long)v & mask);
+	}
+#endif
+}
+
 /**
  * instrument_atomic_read - instrument atomic read access
  * @v: address of access
@@ -68,9 +81,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
-#ifndef __DISABLE_BUG_TABLE
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
-#endif
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
@@ -85,9 +96,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
-#ifndef __DISABLE_BUG_TABLE
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
-#endif
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
@@ -102,9 +111,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
-#ifndef __DISABLE_BUG_TABLE
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
-#endif
+	instrument_atomic_check_alignment(v, size);
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 4b4d1445ef9c..977a2c9163a2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1369,6 +1369,14 @@ config DEBUG_ATOMIC
 
 	  This option has potentially significant overhead.
 
+config DEBUG_ATOMIC_LARGEST_ALIGN
+	bool "Check alignment only up to __aligned_largest"
+	depends on DEBUG_ATOMIC
+	help
+	  If you say Y here then the check for natural alignment of
+	  atomic accesses will be constrained to the compiler's largest
+	  alignment for scalar types.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config LOCK_DEBUGGING_SUPPORT
-- 
2.49.1