[PATCH v2] mm: Fix possible deadlock in console_trylock_spinning

[Gu Bowen <gubowen5@huawei.com>]

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;
-- 
2.25.1