Commit 7b1f8c61 authored by Boqun Feng's avatar Boqun Feng Committed by Peter Zijlstra
Browse files

lockding/lockdep: Avoid to find wrong lock dep path in check_irq_usage() 



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: default avatarJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: default avatarBoqun Feng <boqun.feng@gmail.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210618170110.3699115-4-boqun.feng@gmail.com
parent d4c157c7

kernel/locking/lockdep.c

+11 −1
Original line number Diff line number Diff line
@@ -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.

	 */

	backward_mask = original_mask(target_entry1->class->usage_mask);

	 *

	 * 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 & LOCKF_ENABLED_IRQ_ALL);



	ret = find_usage_backwards(&this, backward_mask, &target_entry);

	if (bfs_error(ret)) {