linux-kernel - [PATCH 1/1] hung_task: fix warnings by enforcing alignment on lock structures

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Message-ID: <20250823074048.92498-1-lance.yang@linux.dev>
Date: Sat, 23 Aug 2025 15:40:48 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: akpm@...ux-foundation.org,
	fthain@...ux-m68k.org,
	geert@...ux-m68k.org,
	mhiramat@...nel.org,
	senozhatsky@...omium.org
Cc: lance.yang@...ux.dev,
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	joel.granados@...nel.org,
	jstultz@...gle.com,
	kent.overstreet@...ux.dev,
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	linux-kernel@...r.kernel.org,
	linux-m68k@...ts.linux-m68k.org,
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Subject: [PATCH 1/1] hung_task: fix warnings by enforcing alignment on lock structures

From: Lance Yang <lance.yang@...ux.dev>

The blocker tracking mechanism assumes that lock pointers are at least
4-byte aligned to use their lower bits for type encoding.

However, as reported by Geert Uytterhoeven, some architectures like m68k
only guarantee 2-byte alignment of 32-bit values. This breaks the
assumption and causes two related WARN_ON_ONCE checks to trigger.

To fix this, enforce a minimum of 4-byte alignment on the core lock
structures supported by the blocker tracking mechanism. This ensures the
algorithm's alignment assumption now holds true on all architectures.

This patch adds __aligned(4) to the definitions of "struct mutex",
"struct semaphore", and "struct rw_semaphore", resolving the warnings.

Thanks to Geert for bisecting!

Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Closes: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com
Fixes: e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker")
Cc: <stable@...r.kernel.org>
Signed-off-by: Lance Yang <lance.yang@...ux.dev>
---
 include/linux/mutex_types.h | 2 +-
 include/linux/rwsem.h       | 2 +-
 include/linux/semaphore.h   | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/mutex_types.h b/include/linux/mutex_types.h
index fdf7f515fde8..de798bfbc4c7 100644
--- a/include/linux/mutex_types.h
+++ b/include/linux/mutex_types.h
@@ -51,7 +51,7 @@ struct mutex {
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;
 #endif
-};
+} __aligned(4); /* For hung_task blocker tracking, which encodes type in LSBs */
 
 #else /* !CONFIG_PREEMPT_RT */
 /*
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index f1aaf676a874..f6ecf4a4710d 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -64,7 +64,7 @@ struct rw_semaphore {
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;
 #endif
-};
+} __aligned(4); /* For hung_task blocker tracking, which encodes type in LSBs */
 
 #define RWSEM_UNLOCKED_VALUE		0UL
 #define RWSEM_WRITER_LOCKED		(1UL << 0)
diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
index 89706157e622..ac9b9c87bfb7 100644
--- a/include/linux/semaphore.h
+++ b/include/linux/semaphore.h
@@ -20,7 +20,7 @@ struct semaphore {
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
 	unsigned long		last_holder;
 #endif
-};
+} __aligned(4); /* For hung_task blocker tracking, which encodes type in LSBs */
 
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
 #define __LAST_HOLDER_SEMAPHORE_INITIALIZER				\
-- 
2.49.0