Our syztester report the lockdep WARNING [1], which was identified in stable kernel version 5.10. However, this deadlock path no longer exists due to the refactoring of console_lock in v6.2-rc1 [2]. Coincidentally, there are two types of deadlocks that we have found here. One is the ABBA deadlock, as mentioned above [1], and the other is the AA deadlock was reported by Breno [3]. The latter's deadlock issue persists.

To solve this problem, switch to printk_safe mode before printing warning message, this will redirect all printk()-s to a special per-CPU buffer, which will be flushed later from a safe context (irq work), and this deadlock problem can be avoided. The proper API to use should be printk_deferred_enter()/printk_deferred_exit() [4].

[1] https://lore.kernel.org/all/20250730094914.566582-1-gubowen5@huawei.com/ [2] https://lore.kernel.org/all/20221116162152.193147-1-john.ogness@linutronix.d... [3] https://lore.kernel.org/all/20250731-kmemleak_lock-v1-1-728fd470198f@debian.... [4] https://lore.kernel.org/all/5ca375cd-4a20-4807-b897-68b289626550@redhat.com/ ====================

Signed-off-by: Gu Bowen gubowen5@huawei.com --- mm/kmemleak.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c index 84265983f239..26113b89d09b 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -437,9 +437,15 @@ static struct kmemleak_object *__lookup_object(unsigned long ptr, int alias, else if (untagged_objp == untagged_ptr || alias) return object; else { + /* + * Printk deferring due to the kmemleak_lock held. + * This is done to avoid deadlock. + */ + printk_deferred_enter(); kmemleak_warn("Found object by alias at 0x%08lx\n", ptr); dump_object_info(object); + printk_deferred_exit(); break; } } @@ -736,6 +742,11 @@ static int __link_object(struct kmemleak_object *object, unsigned long ptr, else if (untagged_objp + parent->size <= untagged_ptr) link = &parent->rb_node.rb_right; else { + /* + * Printk deferring due to the kmemleak_lock held. + * This is done to avoid deadlock. + */ + printk_deferred_enter(); kmemleak_stop("Cannot insert 0x%lx into the object search tree (overlaps existing)\n", ptr); /* @@ -743,6 +754,7 @@ static int __link_object(struct kmemleak_object *object, unsigned long ptr, * be freed while the kmemleak_lock is held. */ dump_object_info(parent); + printk_deferred_exit(); return -EEXIST; } } @@ -858,8 +870,14 @@ static void delete_object_part(unsigned long ptr, size_t size, object = __find_and_remove_object(ptr, 1, objflags); if (!object) { #ifdef DEBUG + /* + * Printk deferring due to the kmemleak_lock held. + * This is done to avoid deadlock. + */ + printk_deferred_enter(); kmemleak_warn("Partially freeing unknown object at 0x%08lx (size %zu)\n", ptr, size); + printk_deferred_exit(); #endif goto unlock; }