[PATCH 3/4] lockding/lockdep: Avoid to find wrong lock dep path in check_irq_usage()

From: Boqun Feng
Date: Fri Jun 18 2021 - 13:02:01 EST


In the step #3 of check_irq_usage(), we seach backwards to find a lock
whose usage conflicts the usage of @target_entry1 on safe/unsafe.
However, we should only keep the irq-unsafe usage of @target_entry1 into
consideration, because it could be a case where a lock is hardirq-unsafe
but soft-safe, and in check_irq_usage() we find it because its
hardirq-unsafe could result into a hardirq-safe-unsafe deadlock, but
currently since we don't filter out the other usage bits, so we may find
a lock dependency path softirq-unsafe -> softirq-safe, which in fact
doesn't cause a deadlock. And this may cause misleading lockdep splats.

Fix this by only keeping LOCKF_ENABLED_IRQ_ALL bits when we try the
backwards search.

Reported-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
kernel/locking/lockdep.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 74d084a398be..6ff1e8405a83 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2768,8 +2768,18 @@ static int check_irq_usage(struct task_struct *curr, struct held_lock *prev,
* Step 3: we found a bad match! Now retrieve a lock from the backward
* list whose usage mask matches the exclusive usage mask from the
* lock found on the forward list.
+ *
+ * Note, we should only keep the LOCKF_ENABLED_IRQ_ALL bits, considering
+ * the follow case:
+ *
+ * When trying to add A -> B to the graph, we find that there is a
+ * hardirq-safe L, that L -> ... -> A, and another hardirq-unsafe M,
+ * that B -> ... -> M. However M is **softirq-safe**, if we use exact
+ * invert bits of M's usage_mask, we will find another lock N that is
+ * **softirq-unsafe** and N -> ... -> A, however N -> .. -> M will not
+ * cause a inversion deadlock.
*/
- backward_mask = original_mask(target_entry1->class->usage_mask);
+ backward_mask = original_mask(target_entry1->class->usage_mask & LOCKF_ENABLED_IRQ_ALL);

ret = find_usage_backwards(&this, backward_mask, &target_entry);
if (bfs_error(ret)) {
--
2.30.2