[PATCH 1/1] hung_task: fix warnings caused by unaligned lock pointers

[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, the runtime checks are adjusted. The first WARN_ON_ONCE in
hung_task_set_blocker() is changed to a simple 'if' that returns silently
for unaligned pointers. The second, now-invalid WARN_ON_ONCE in
hung_task_clear_blocker() is then removed.

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/hung_task.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
index 34e615c76ca5..69640f266a69 100644
--- a/include/linux/hung_task.h
+++ b/include/linux/hung_task.h
@@ -20,6 +20,10 @@
  * always zero. So we can use these bits to encode the specific blocking
  * type.
  *
+ * Note that on architectures like m68k with only 2-byte alignment, the
+ * blocker tracking mechanism gracefully does nothing for any lock that is
+ * not 4-byte aligned.
+ *
  * Type encoding:
  * 00 - Blocked on mutex			(BLOCKER_TYPE_MUTEX)
  * 01 - Blocked on semaphore			(BLOCKER_TYPE_SEM)
@@ -45,7 +49,7 @@ static inline void hung_task_set_blocker(void *lock, unsigned long type)
 	 * If the lock pointer matches the BLOCKER_TYPE_MASK, return
 	 * without writing anything.
 	 */
-	if (WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK))
+	if (lock_ptr & BLOCKER_TYPE_MASK)
 		return;
 
 	WRITE_ONCE(current->blocker, lock_ptr | type);
@@ -53,8 +57,6 @@ static inline void hung_task_set_blocker(void *lock, unsigned long type)
 
 static inline void hung_task_clear_blocker(void)
 {
-	WARN_ON_ONCE(!READ_ONCE(current->blocker));
-
 	WRITE_ONCE(current->blocker, 0UL);
 }
 
-- 
2.49.0