locking/lockdep: Fix stack trace caching logic

check_prev_add() caches saved stack trace in static trace variable
to avoid duplicate save_trace() calls in dependencies involving trylocks.
But that caching logic contains a bug. We may not save trace on first
iteration due to early return from check_prev_add(). Then on the
second iteration when we actually need the trace we don't save it
because we think that we've already saved it.

Let check_prev_add() itself control when stack is saved.

There is another bug. Trace variable is protected by graph lock.
But we can temporary release graph lock during printing.

Fix this by invalidating cached stack trace when we release graph lock.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: glider@google.com
Cc: kcc@google.com
Cc: peter@hurleysoftware.com
Cc: sasha.levin@oracle.com
Link: http://lkml.kernel.org/r/1454593240-121647-1-git-send-email-dvyukov@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -1822,7 +1822,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next,
  */
 static int
 check_prev_add(struct task_struct *curr, struct held_lock *prev,
-	       struct held_lock *next, int distance, int trylock_loop)
+	       struct held_lock *next, int distance, int *stack_saved)
 {
 	struct lock_list *entry;
 	int ret;
@@ -1883,8 +1883,11 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 		}
 	}
 
-	if (!trylock_loop && !save_trace(&trace))
+	if (!*stack_saved) {
+		if (!save_trace(&trace))
 			return 0;
+		*stack_saved = 1;
+	}
 
 	/*
 	 * Ok, all validations passed, add the new lock
@@ -1907,6 +1910,8 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 	 * Debugging printouts:
 	 */
 	if (verbose(hlock_class(prev)) || verbose(hlock_class(next))) {
+		/* We drop graph lock, so another thread can overwrite trace. */
+		*stack_saved = 0;
 		graph_unlock();
 		printk("\n new dependency: ");
 		print_lock_name(hlock_class(prev));
@@ -1929,7 +1934,7 @@ static int
 check_prevs_add(struct task_struct *curr, struct held_lock *next)
 {
 	int depth = curr->lockdep_depth;
-	int trylock_loop = 0;
+	int stack_saved = 0;
 	struct held_lock *hlock;
 
 	/*
@@ -1956,7 +1961,7 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
 		 */
 		if (hlock->read != 2 && hlock->check) {
 			if (!check_prev_add(curr, hlock, next,
-						distance, trylock_loop))
+						distance, &stack_saved))
 				return 0;
 			/*
 			 * Stop after the first non-trylock entry,
@@ -1979,7 +1984,6 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
 		if (curr->held_locks[depth].irq_context !=
 				curr->held_locks[depth-1].irq_context)
 			break;
-		trylock_loop = 1;
 	}
 	return 1;
 out_bug: