Commit-ID: d7bc3197b41e0a1af6677e83f8736e93a1575ce0
Gitweb: http://git.kernel.org/tip/d7bc3197b41e0a1af6677e83f8736e93a1575ce0
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 15 Apr 2015 17:11:57 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 17 Apr 2015 09:42:14 +0200
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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/locking/lockdep.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ba77ab5..a0831e1 100644
--- 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);
}
--
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