[PATCH 2/2] lockdep: Fix merging of hlocks with non-zero references

[Imre Deak <imre.deak@intel.com>]

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 ww_mutex ww_lock_c;
	struct mutex lock_c;

	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);
	ww_mutex_init(&ww_lock_c, &test_ww_class);

	mutex_init(&lock_c);

	WARN_ON(ww_mutex_lock(&ww_lock_a, &ww_ctx));

	WARN_ON(!mutex_trylock(&lock_c));

	WARN_ON(ww_mutex_lock(&ww_lock_b, &ww_ctx));
	WARN_ON(ww_mutex_lock(&ww_lock_c, &ww_ctx));

	mutex_unlock(&lock_c);	(*)

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

	ww_acquire_fini(&ww_ctx); (**)

will trigger the following error in __lock_release() when calling
mutex_release() at **:

	DEBUG_LOCKS_WARN_ON(depth <= 0)

The problem is that the hlock merging happening at * updates the
references for test_ww_class incorrectly to 3 whereas it should've
updated it to 4 (representing all the instances for ww_ctx and
ww_lock_[abc]).

Fix this by updating the references during merging correctly taking into
account that we can have non-zero references (both for the hlock that we
merge into another hlock or for the hlock we are mering into).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 kernel/locking/lockdep.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 967352d32af1..9e2a4ab6c731 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3637,6 +3637,11 @@ print_lock_nested_lock_not_held(struct task_struct *curr,
 
 static int __lock_is_held(const struct lockdep_map *lock, int read);
 
+static int hlock_reference(int reference)
+{
+	return reference ? : 1;
+}
+
 /*
  * This gets called for every mutex_lock*()/spin_lock*() operation.
  * We maintain the dependency maps and validate the locking attempt:
@@ -3702,17 +3707,15 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	if (depth) {
 		hlock = curr->held_locks + depth - 1;
 		if (hlock->class_idx == class_idx && nest_lock) {
-			if (hlock->references) {
-				/*
-				 * Check: unsigned int references overflow.
-				 */
-				if (DEBUG_LOCKS_WARN_ON(hlock->references == UINT_MAX))
-					return 0;
+			/*
+			 * Check: unsigned int references overflow.
+			 */
+			if (DEBUG_LOCKS_WARN_ON(hlock_reference(hlock->references) >
+						UINT_MAX - hlock_reference(references)))
+				return 0;
 
-				hlock->references++;
-			} else {
-				hlock->references = 2;
-			}
+			hlock->references = hlock_reference(hlock->references) +
+					    hlock_reference(references);
 
 			return 2;
 		}
-- 
2.17.1