Our syztester report the lockdep WARNING [1]. kmemleak_scan_thread() invokes scan_block() which may invoke a nomal printk() to print warning message. This can cause a deadlock in the scenario reported below:
CPU0 CPU1 ---- ---- lock(kmemleak_lock); lock(&port->lock); lock(kmemleak_lock); lock(console_owner);
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() if we want to deferred the printing [2].
This patch also fix some similar cases that need to use the printk deferring [3].
[1] https://lore.kernel.org/all/20250730094914.566582-1-gubowen5@huawei.com/ [2] https://lore.kernel.org/all/5ca375cd-4a20-4807-b897-68b289626550@redhat.com/ [3] https://lore.kernel.org/all/aJCir5Wh362XzLSx@arm.com/ ====================
Cc: stable@vger.kernel.org # 5.10 Signed-off-by: Gu Bowen gubowen5@huawei.com --- mm/kmemleak.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-)
diff --git a/mm/kmemleak.c b/mm/kmemleak.c index 4801751cb6b6..381145dde54f 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -390,9 +390,15 @@ static struct kmemleak_object *lookup_object(unsigned long ptr, int alias) else if (object->pointer == 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; } } @@ -433,8 +439,15 @@ static struct kmemleak_object *mem_pool_alloc(gfp_t gfp) list_del(&object->object_list); else if (mem_pool_free_count) object = &mem_pool[--mem_pool_free_count]; - else + else { + /* + * Printk deferring due to the kmemleak_lock held. + * This is done to avoid deadlock. + */ + printk_deferred_enter(); pr_warn_once("Memory pool empty, consider increasing CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE\n"); + printk_deferred_exit(); + } raw_spin_unlock_irqrestore(&kmemleak_lock, flags);
return object; @@ -632,6 +645,11 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size, else if (parent->pointer + parent->size <= 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); /* @@ -639,6 +657,7 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size, * be freed while the kmemleak_lock is held. */ dump_object_info(parent); + printk_deferred_exit(); kmem_cache_free(object_cache, object); object = NULL; goto out;