[PATCH v2 1/2] lockdep: Fix OOO unlock when hlocks need merging

[Imre Deak]

The sequence

	static DEFINE_WW_CLASS(test_ww_class);

	struct ww_acquire_ctx ww_ctx;
	struct ww_mutex ww_lock_a;
	struct ww_mutex ww_lock_b;
	struct mutex lock_c;
	struct mutex lock_d;

	ww_acquire_init(&ww_ctx, &test_ww_class);

	ww_mutex_init(&ww_lock_a, &test_ww_class);
	ww_mutex_init(&ww_lock_b, &test_ww_class);

	mutex_init(&lock_c);

	ww_mutex_lock(&ww_lock_a, &ww_ctx);

	mutex_lock(&lock_c);

	ww_mutex_lock(&ww_lock_b, &ww_ctx);

	mutex_unlock(&lock_c);		(*)

	ww_mutex_unlock(&ww_lock_b);
	ww_mutex_unlock(&ww_lock_a);

	ww_acquire_fini(&ww_ctx);

triggers the following WARN in __lock_release() when doing the unlock at *:

	DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - 1);

The problem is that the WARN check doesn't take into account the merging
of ww_lock_a and ww_lock_b which results in decreasing curr->lockdep_depth
by 2 not only 1.

Note that the following sequence doesn't trigger the WARN, since there
won't be any hlock merging.

	ww_acquire_init(&ww_ctx, &test_ww_class);

	ww_mutex_init(&ww_lock_a, &test_ww_class);
	ww_mutex_init(&ww_lock_b, &test_ww_class);

	mutex_init(&lock_c);
	mutex_init(&lock_d);

	ww_mutex_lock(&ww_lock_a, &ww_ctx);

	mutex_lock(&lock_c);
	mutex_lock(&lock_d);

	ww_mutex_lock(&ww_lock_b, &ww_ctx);

	mutex_unlock(&lock_d);

	ww_mutex_unlock(&ww_lock_b);
	ww_mutex_unlock(&ww_lock_a);

	mutex_unlock(&lock_c);

	ww_acquire_fini(&ww_ctx);

In general both of the above two sequences are valid and shouldn't
trigger any lockdep warning.

Fix this by taking the decrement due to the hlock merging into account
during lock release and hlock class re-setting. Merging can't happen
during lock downgrading since there won't be a new possibility to merge
hlocks in that case, so add a WARN if merging still happens then.

v2:
- Clarify the commit log and use mutex_lock() instead of mutex_trylock()
  in the example sequences for simplicity.

Cc: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
---
 kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c40fba54e324..967352d32af1 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3714,7 +3714,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 				hlock->references = 2;
 			}
 
-			return 1;
+			return 2;
 		}
 	}
 
@@ -3920,22 +3920,33 @@ static struct held_lock *find_held_lock(struct task_struct *curr,
 }
 
 static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
-			      int idx)
+				int idx, bool *first_merged)
 {
 	struct held_lock *hlock;
+	int first_idx = idx;
 
 	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
 		return 0;
 
 	for (hlock = curr->held_locks + idx; idx < depth; idx++, hlock++) {
-		if (!__lock_acquire(hlock->instance,
+		switch (__lock_acquire(hlock->instance,
 				    hlock_class(hlock)->subclass,
 				    hlock->trylock,
 				    hlock->read, hlock->check,
 				    hlock->hardirqs_off,
 				    hlock->nest_lock, hlock->acquire_ip,
-				    hlock->references, hlock->pin_count))
+				    hlock->references, hlock->pin_count)) {
+		case 0:
 			return 1;
+		case 1:
+			break;
+		case 2:
+			*first_merged = idx == first_idx;
+			break;
+		default:
+			WARN_ON(1);
+			return 0;
+		}
 	}
 	return 0;
 }
@@ -3948,6 +3959,7 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
 	struct task_struct *curr = current;
 	struct held_lock *hlock;
 	struct lock_class *class;
+	bool first_merged = false;
 	unsigned int depth;
 	int i;
 
@@ -3973,14 +3985,14 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
 	curr->lockdep_depth = i;
 	curr->curr_chain_key = hlock->prev_chain_key;
 
-	if (reacquire_held_locks(curr, depth, i))
+	if (reacquire_held_locks(curr, depth, i, &first_merged))
 		return 0;
 
 	/*
 	 * I took it apart and put it back together again, except now I have
 	 * these 'spare' parts.. where shall I put them.
 	 */
-	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth))
+	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - first_merged))
 		return 0;
 	return 1;
 }
@@ -3989,6 +4001,7 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 {
 	struct task_struct *curr = current;
 	struct held_lock *hlock;
+	bool first_merged = false;
 	unsigned int depth;
 	int i;
 
@@ -4014,7 +4027,7 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 	hlock->read = 1;
 	hlock->acquire_ip = ip;
 
-	if (reacquire_held_locks(curr, depth, i))
+	if (reacquire_held_locks(curr, depth, i, &first_merged))
 		return 0;
 
 	/*
@@ -4023,6 +4036,11 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 	 */
 	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth))
 		return 0;
+
+	/* Merging can't happen with unchanged classes.. */
+	if (DEBUG_LOCKS_WARN_ON(first_merged))
+		return 0;
+
 	return 1;
 }
 
@@ -4038,6 +4056,7 @@ __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
 {
 	struct task_struct *curr = current;
 	struct held_lock *hlock;
+	bool first_merged = false;
 	unsigned int depth;
 	int i;
 
@@ -4093,14 +4112,15 @@ __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
 	if (i == depth-1)
 		return 1;
 
-	if (reacquire_held_locks(curr, depth, i + 1))
+	if (reacquire_held_locks(curr, depth, i + 1, &first_merged))
 		return 0;
 
 	/*
 	 * We had N bottles of beer on the wall, we drank one, but now
 	 * there's not N-1 bottles of beer left on the wall...
+	 * Pouring two of the bottles together is acceptable.
 	 */
-	DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth-1);
+	DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - 1 - first_merged);
 
 	/*
 	 * Since reacquire_held_locks() would have called check_chain_key()
-- 
2.17.1