Merge tag 'locking-urgent-2020-11-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking fixes from Thomas Gleixner:
"Two fixes for the locking subsystem:

- Prevent an unconditional interrupt enable in a futex helper
function which can be called from contexts which expect interrupts
to stay disabled across the call

- Don't modify lockdep chain keys in the validation process as that
causes chain inconsistency"

* tag 'locking-urgent-2020-11-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
lockdep: Avoid to modify chain keys in validate_chain()
futex: Don't enable IRQs unconditionally in put_pi_state()

kernel/futex.c

···
 	 */
 	if (pi_state->owner) {
 		struct task_struct *owner;
+		unsigned long flags;
 
-		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+		raw_spin_lock_irqsave(&pi_state->pi_mutex.wait_lock, flags);
 		owner = pi_state->owner;
 		if (owner) {
 			raw_spin_lock(&owner->pi_lock);
···
 			raw_spin_unlock(&owner->pi_lock);
 		}
 		rt_mutex_proxy_unlock(&pi_state->pi_mutex, owner);
-		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+		raw_spin_unlock_irqrestore(&pi_state->pi_mutex.wait_lock, flags);
 	}
 
 	if (current->pi_state_cache) {

kernel/locking/lockdep.c

···
  * (Note that this has to be done separately, because the graph cannot
  * detect such classes of deadlocks.)
  *
- * Returns: 0 on deadlock detected, 1 on OK, 2 on recursive read
+ * Returns: 0 on deadlock detected, 1 on OK, 2 if another lock with the same
+ * lock class is held but nest_lock is also held, i.e. we rely on the
+ * nest_lock to avoid the deadlock.
  */
 static int
 check_deadlock(struct task_struct *curr, struct held_lock *next)
···
 		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
 		 */
 		if ((next->read == 2) && prev->read)
-			return 2;
+			continue;
 
 		/*
 		 * We're holding the nest_lock, which serializes this lock's
···
 		if (!ret)
 			return 0;
 		/*
-		 * Mark recursive read, as we jump over it when
-		 * building dependencies (just like we jump over
-		 * trylock entries):
-		 */
-		if (ret == 2)
-			hlock->read = 2;
-		/*
 		 * Add dependency only if this lock is not the head
-		 * of the chain, and if it's not a secondary read-lock:
+		 * of the chain, and if the new lock introduces no more
+		 * lock dependency (because we already hold a lock with the
+		 * same lock class) nor deadlock (because the nest_lock
+		 * serializes nesting locks), see the comments for
+		 * check_deadlock().
 		 */
 		if (!chain_head && ret != 2) {
 			if (!check_prevs_add(curr, hlock))