[RFC][PATCH v2 31/31] timers: Expand DEBUG_OBJECTS_TIMER to check if it ever was used

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

There's been too many bugs happening where a timer is removed, either by
del_timer() or even del_timer_sync() but get's re-armed again by a
workqueue or some other task. Then the timer is freed while it's still
queued to go off. When the timer eventually goes off, as its content no
longer exists, it causes a crash in the timer code.

This is very hard to debug because all evidence of who added the timer is
gone.

Currently, DEBUG_OBJECTS_TIMER will trigger if this happens, but as this
only happens rarely (but in the field, thousands of times) and may depend
on performing various tasks (USB unplug, CPU hotplug, suspend and resume),
not to mention that enabling DEBUG_OBJECTS_TIMER has too much overhead to
run in the field, it seldom catches these types of bugs.

Now that del_timer_shutdown() is to be called before freeing, move the
checks of DEBUG_OBJECTS_TIMER to if it ever gets armed to where
del_timer_shutdown() is called. If there's a case where a timer is armed,
and then freed without calling del_timer_shutdown() DEBUG_OBJECTS_TIMER
will now trigger on it.

This catches cases that are potential issues instead of just catching
when the race condition occurs.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: John Stultz <jstultz@xxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 include/linux/timer.h | 20 ++++++++++++++++++--
 kernel/time/timer.c   | 16 +++++++++++-----
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index daccfe33da42..d4d90149d015 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -18,6 +18,9 @@ struct timer_list {
 	void			(*function)(struct timer_list *);
 	u32			flags;
 
+#ifdef CONFIG_DEBUG_OBJECTS_TIMERS
+	u32			enabled;
+#endif
 #ifdef CONFIG_LOCKDEP
 	struct lockdep_map	lockdep_map;
 #endif
@@ -128,6 +131,19 @@ static inline void init_timer_on_stack_key(struct timer_list *timer,
 	init_timer_on_stack_key((_timer), (_fn), (_flags), NULL, NULL)
 #endif
 
+#ifdef CONFIG_DEBUG_OBJECTS_TIMERS
+#define __init_timer_debug(_timer, _fn, _flags)				\
+	do {								\
+		(_timer)->enabled = 0;					\
+		__init_timer((_timer), (_fn), (_flags));		\
+	} while (0)
+#else
+#define __init_timer_debug(_timer, _fn, _flags)				\
+	do {								\
+		__init_timer((_timer), (_fn), (_flags));		\
+	} while (0)
+#endif
+
 /**
  * timer_setup - prepare a timer for first use
  * @timer: the timer in question
@@ -139,7 +155,7 @@ static inline void init_timer_on_stack_key(struct timer_list *timer,
  * be used and must be balanced with a call to destroy_timer_on_stack().
  */
 #define timer_setup(timer, callback, flags)			\
-	__init_timer((timer), (callback), (flags))
+	__init_timer_debug((timer), (callback), (flags))
 
 #define timer_setup_on_stack(timer, callback, flags)		\
 	__init_timer_on_stack((timer), (callback), (flags))
@@ -207,7 +223,7 @@ static inline int del_timer_shutdown(struct timer_list *timer)
 	return __del_timer_sync(timer, true);
 }
 
-#define del_singleshot_timer_sync(t) del_timer_sync(t)
+#define del_singleshot_timer_sync(t) del_timer_shutdown(t)
 
 extern void init_timers(void);
 struct hrtimer;
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 111a3550b3f2..1d17552b3ede 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -774,17 +774,22 @@ static const struct debug_obj_descr timer_debug_descr = {
 
 static inline void debug_timer_init(struct timer_list *timer)
 {
-	debug_object_init(timer, &timer_debug_descr);
+	if (!timer->enabled)
+		debug_object_init(timer, &timer_debug_descr);
 }
 
 static inline void debug_timer_activate(struct timer_list *timer)
 {
-	debug_object_activate(timer, &timer_debug_descr);
+	if (!timer->enabled) {
+		timer->enabled = 1;
+		debug_object_activate(timer, &timer_debug_descr);
+	}
 }
 
 static inline void debug_timer_deactivate(struct timer_list *timer)
 {
-	debug_object_deactivate(timer, &timer_debug_descr);
+	if (timer->enabled)
+		debug_object_deactivate(timer, &timer_debug_descr);
 }
 
 static inline void debug_timer_assert_init(struct timer_list *timer)
@@ -828,7 +833,6 @@ static inline void debug_init(struct timer_list *timer)
 
 static inline void debug_deactivate(struct timer_list *timer)
 {
-	debug_timer_deactivate(timer);
 	trace_timer_cancel(timer);
 }
 
@@ -1281,8 +1285,10 @@ static int __try_to_del_timer_sync(struct timer_list *timer, bool free)
 
 	if (base->running_timer != timer)
 		ret = detach_if_pending(timer, base, true);
-	if (free)
+	if (free) {
 		timer->function = NULL;
+		debug_timer_deactivate(timer);
+	}
 
 	raw_spin_unlock_irqrestore(&base->lock, flags);
 
-- 
2.35.1