[PATCH] lockdep: make print_lock_name() robust against non-existing lock_class

[Sebastian Andrzej Siewior]

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@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/locking/lockdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ba77ab5f64dd..260155a2cb89 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -530,6 +530,10 @@ static void print_lock_name(struct lock_class *class)
 {
 	char usage[LOCK_USAGE_CHARS];
 
+	if (!class) {
+		printk(" (<NONE>)");
+		return;
+	}
 	get_usage_chars(class, usage);
 
 	printk(" (");
-- 
2.1.4

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