lockdep: Make print_lock() robust against concurrent release

Peter Zijlstra · Ingo Molnar · commit d7bc3197b41e · 2015-04-17T09:42:14.000+02:00
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: [&lt;c1088145&gt;] get_usage_chars+0x5/0x100
	|Call Trace:
	| [&lt;c1088263&gt;] print_lock_name+0x23/0x60
	| [&lt;c1576b57&gt;] print_lock+0x5d/0x7e
	| [&lt;c1088314&gt;] lockdep_print_held_locks+0x74/0xe0
	| [&lt;c1088652&gt;] debug_show_all_locks+0x132/0x1b0
	| [&lt;c1315c48&gt;] sysrq_handle_showlocks+0x8/0x10

This *might* happen because the thread on the other CPU drops the lock
after we are looking -&gt;lockdep_depth and -&gt;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 &lt;andreas@linutronix.de&gt;
Signed-off-by: Peter Zijlstra (Intel) &lt;peterz@infradead.org&gt;
Cc: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Cc: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Cc: Paul E. McKenney &lt;paulmck@linux.vnet.ibm.com&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Sebastian Andrzej Siewior &lt;bigeasy@linutronix.de&gt;
Cc: Thomas Gleixner &lt;tglx@linutronix.de&gt;
Signed-off-by: Ingo Molnar &lt;mingo@kernel.org&gt;
diff --git a/kernel/locking/lockdep.c b/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);
 }
