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

[Lance Yang]

From: Lance Yang <lance.yang@xxxxxxxxx>

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@xxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@xxxxxxxxxxxxxx
Fixes: e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
---
 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