From: Lance Yang lance.yang@linux.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, 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@linux-m68k.org Closes: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc... Fixes: e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker") Cc: stable@vger.kernel.org Signed-off-by: Lance Yang lance.yang@linux.dev --- 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); }