[PATCH] mm/kmemleak: avoid deadlock by moving pr_warn() outside kmemleak_lock

[Breno Leitao <leitao@debian.org>]

When netpoll is enabled, calling pr_warn_once() while holding
kmemleak_lock in mem_pool_alloc() can cause a deadlock due to lock
inversion with the netconsole subsystem. This occurs because
pr_warn_once() may trigger netpoll, which eventually leads to
__alloc_skb() and back into kmemleak code, attempting to reacquire
kmemleak_lock.

This is the path for the deadlock.

mem_pool_alloc()
  -> raw_spin_lock_irqsave(&kmemleak_lock, flags);
      -> pr_warn_once()
          -> netconsole subsystem
	     -> netpoll
	         -> __alloc_skb
		   -> __create_object
		     -> raw_spin_lock_irqsave(&kmemleak_lock, flags);

Fix this by setting a flag and issuing the pr_warn_once() after
kmemleak_lock is released.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Fixes: c5665868183fec ("mm: kmemleak: use the memory pool for early allocations")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/kmemleak.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 8d588e6853110..e0333455c7384 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -470,6 +470,7 @@ static struct kmemleak_object *mem_pool_alloc(gfp_t gfp)
 {
 	unsigned long flags;
 	struct kmemleak_object *object;
+	bool warn = false;
 
 	/* try the slab allocator first */
 	if (object_cache) {
@@ -488,8 +489,10 @@ static struct kmemleak_object *mem_pool_alloc(gfp_t gfp)
 	else if (mem_pool_free_count)
 		object = &mem_pool[--mem_pool_free_count];
 	else
-		pr_warn_once("Memory pool empty, consider increasing CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE\n");
+		warn = true;
 	raw_spin_unlock_irqrestore(&kmemleak_lock, flags);
+	if (warn)
+		pr_warn_once("Memory pool empty, consider increasing CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE\n");
 
 	return object;
 }

---
base-commit: 260f6f4fda93c8485c8037865c941b42b9cba5d2
change-id: 20250731-kmemleak_lock-4e04743ea79e

Best regards,
--  
Breno Leitao <leitao@debian.org>