lockdep: Don't create the wrong dependency on hlock->check == 0

Test-case:

	DEFINE_MUTEX(m1);
	DEFINE_MUTEX(m2);
	DEFINE_MUTEX(mx);

	void lockdep_should_complain(void)
	{
		lockdep_set_novalidate_class(&mx);

		// m1 -> mx -> m2
		mutex_lock(&m1);
		mutex_lock(&mx);
		mutex_lock(&m2);
		mutex_unlock(&m2);
		mutex_unlock(&mx);
		mutex_unlock(&m1);

		// m2 -> m1 ; should trigger the warning
		mutex_lock(&m2);
		mutex_lock(&m1);
		mutex_unlock(&m1);
		mutex_unlock(&m2);
	}

this doesn't trigger any warning, lockdep can't detect the trivial
deadlock.

This is because lock(&mx) correctly avoids m1 -> mx dependency, it
skips validate_chain() due to mx->check == 0. But lock(&m2) wrongly
adds mx -> m2 and thus m1 -> m2 is not created.

rcu_lock_acquire()->lock_acquire(check => 0) is fine due to read == 2,
so currently only __lockdep_no_validate__ can trigger this problem.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140120182010.GA26498@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -1936,12 +1936,12 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
 
 	for (;;) {
 		int distance = curr->lockdep_depth - depth + 1;
-		hlock = curr->held_locks + depth-1;
+		hlock = curr->held_locks + depth - 1;
 		/*
 		 * Only non-recursive-read entries get new dependencies
 		 * added:
 		 */
-		if (hlock->read != 2) {
+		if (hlock->read != 2 && hlock->check) {
 			if (!check_prev_add(curr, hlock, next,
 						distance, trylock_loop))
 				return 0;