lockdep: Make print_lock() robust against concurrent release

During sysrq's show-held-locks command it is possible that
hlock_class() returns NULL for a given lock. The result is then (after
the warning):

	|BUG: unable to handle kernel NULL pointer dereference at 0000001c
	|IP: [<c1088145>] get_usage_chars+0x5/0x100
	|Call Trace:
	| [<c1088263>] print_lock_name+0x23/0x60
	| [<c1576b57>] print_lock+0x5d/0x7e
	| [<c1088314>] lockdep_print_held_locks+0x74/0xe0
	| [<c1088652>] debug_show_all_locks+0x132/0x1b0
	| [<c1315c48>] sysrq_handle_showlocks+0x8/0x10

This *might* happen because the thread on the other CPU drops the lock
after we are looking ->lockdep_depth and ->held_locks points no longer
to a lock that is held.

The fix here is to simply ignore it and continue.
Reported-by: Andreas Messerschmid <andreas@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -551,7 +551,21 @@ static void print_lockdep_cache(struct lockdep_map *lock)
 
 static void print_lock(struct held_lock *hlock)
 {
-	print_lock_name(hlock_class(hlock));
+	/*
+	 * We can be called locklessly through debug_show_all_locks() so be
+	 * extra careful, the hlock might have been released and cleared.
+	 */
+	unsigned int class_idx = hlock->class_idx;
+
+	/* Don't re-read hlock->class_idx, can't use READ_ONCE() on bitfields: */
+	barrier();
+
+	if (!class_idx || (class_idx - 1) >= MAX_LOCKDEP_KEYS) {
+		printk("<RELEASED>\n");
+		return;
+	}
+
+	print_lock_name(lock_classes + class_idx - 1);
 	printk(", at: ");
 	print_ip_sym(hlock->acquire_ip);
 }