Re: ARM board lockups/hangs triggered by locks and mutexes

[Rafał Miłecki]

On 2.08.2023 00:10, Rafał Miłecki wrote:
> Unfortunately enabling *any* of following options:
> CONFIG_DEBUG_RT_MUTEXES=y
> CONFIG_DEBUG_SPINLOCK=y
> CONFIG_DEBUG_MUTEXES=y
> seems to make locksup/hangs go away. I tried for few hours.

I decided to find out why enabling CONFIG_DEBUG_MUTEXES "fixes" kernel /
device stability for me. I tried enabling manually code that normally
hides behind the #ifdev CONFIG_DEBUG_MUTEXES.

Attached to this e-mail is a small patch that is enough to make my
kernel stable (mutex-fix-bcm53573.diff).

#####

It's not what's the most interesting thought. What really doesn't make
sense anymore is that below diff (on top of attached one) brings back
hangs/lockups.

I triple checked that. Dropping a single unused function breaks kernel /
device stability on BCM53573!

AFAIK the only thing below diff actually affects is location of symbols
(I actually verified that by comparing System.map before and after -
over 22'000 of relocated symbols).

Can some unfortunate location of symbols cause those hangs/lockups?


diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 4fe40910f..c440222a4 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -34,6 +34,8 @@ void debug_mutex_lock_common(struct mutex *lock, struct mutex_waiter *waiter)
 	INIT_LIST_HEAD(&waiter->list);
 }

+/* Dropping below function brings back hangs/lockups & reboots */
+#if 0
 void debug_mutex_wake_waiter(struct mutex *lock, struct mutex_waiter *waiter)
 {
 	lockdep_assert_held(&lock->wait_lock);
@@ -41,6 +43,7 @@ void debug_mutex_wake_waiter(struct mutex *lock, struct mutex_waiter *waiter)
 	DEBUG_LOCKS_WARN_ON(waiter->magic != waiter);
 	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 }
+#endif

 void debug_mutex_free_waiter(struct mutex_waiter *waiter)
 {
diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 479bc96c3..15bd4691b 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -57,9 +57,7 @@ struct mutex {
 	struct optimistic_spin_queue osq; /* Spinner MCS lock */
 #endif
 	struct list_head	wait_list;
-#ifdef CONFIG_DEBUG_MUTEXES
 	void			*magic;
-#endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;
 #endif
@@ -73,12 +71,10 @@ struct mutex_waiter {
 	struct list_head	list;
 	struct task_struct	*task;
 	struct ww_acquire_ctx	*ww_ctx;
-#ifdef CONFIG_DEBUG_MUTEXES
 	void			*magic;
-#endif
 };
 
-#ifdef CONFIG_DEBUG_MUTEXES
+#if 1 //def CONFIG_DEBUG_MUTEXES
 
 #define __DEBUG_MUTEX_INITIALIZER(lockname)				\
 	, .magic = &lockname
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d0e639497..8fef4485e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -958,10 +958,8 @@ struct task_struct {
 	struct rt_mutex_waiter		*pi_blocked_on;
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
 	/* Mutex deadlock detection: */
 	struct mutex_waiter		*blocked_on;
-#endif
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 	int				non_block_count;
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index 45452facf..b22e6ecd8 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -12,7 +12,7 @@ CFLAGS_REMOVE_mutex-debug.o = $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_rtmutex-debug.o = $(CC_FLAGS_FTRACE)
 endif
 
-obj-$(CONFIG_DEBUG_MUTEXES) += mutex-debug.o
+obj-y += mutex-debug.o
 obj-$(CONFIG_LOCKDEP) += lockdep.o
 ifeq ($(CONFIG_PROC_FS),y)
 obj-$(CONFIG_LOCKDEP) += lockdep_proc.o
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index b02fff282..6dc3f80a3 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -946,9 +946,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
 	might_sleep();
 
-#ifdef CONFIG_DEBUG_MUTEXES
 	DEBUG_LOCKS_WARN_ON(lock->magic != lock);
-#endif
 
 	ww = container_of(lock, struct ww_mutex, base);
 	if (ww_ctx) {
@@ -1417,9 +1415,7 @@ int __sched mutex_trylock(struct mutex *lock)
 {
 	bool locked;
 
-#ifdef CONFIG_DEBUG_MUTEXES
 	DEBUG_LOCKS_WARN_ON(lock->magic != lock);
-#endif
 
 	locked = __mutex_trylock(lock);
 	if (locked)