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authorPeter Zijlstra <peterz@infradead.org>2015-04-15 17:11:57 +0200
committerIngo Molnar <mingo@kernel.org>2015-04-17 09:42:14 +0200
commitd7bc3197b41e0a1af6677e83f8736e93a1575ce0 (patch)
tree8b9dbb106e7c287182b3812c0469a1fea8e46c95
parent6a16dda86ebbcfe690c753c3fb469b4f9ad5a5ef (diff)
downloadlinux-d7bc3197b41e0a1af6677e83f8736e93a1575ce0.tar.bz2
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>
-rw-r--r--kernel/locking/lockdep.c16
1 files changed, 15 insertions, 1 deletions
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ba77ab5f64dd..a0831e1b99f4 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);
}