FAILED: patch "[PATCH] mm/kmemleak: report RCU-tasks quiescent states during the" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 3541a2b06ecd78ba333188df04368dcf97273d6a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090346-skyrocket-early-2da9@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3541a2b06ecd78ba333188df04368dcf97273d6a Mon Sep 17 00:00:00 2001
From: Breno Leitao <leitao@debian.org>
Date: Mon, 20 Jul 2026 06:23:45 -0700
Subject: [PATCH] mm/kmemleak: report RCU-tasks quiescent states during the
 scan

kmemleak_scan() can run for ages on large debug kernels. It was
causing some soft-lockups which I got fixed with commit
3175fcfec8b16baeb ("mm/kmemleak: avoid soft lockup when scanning task
stacks") with our beloved cond_resched().

I've got the fix above deployed in the Meta fleet, and now I am seeing:

  INFO: rcu_tasks detected stalls on tasks:
    task:kmemleak state:R ... nvcsw: 274/274 holdout: 1 idle_cpu: -1/3
     scan_block
     scan_gray_list
     kmemleak_scan

and, worse, blocks the callers waiting on that grace period.  Here a BPF
struct_ops map free, which waits via synchronize_rcu_mult(call_rcu,
call_rcu_tasks), is stuck long enough to also trip the hung task check:

  INFO: task kworker/...:bpf_map_free_deferred blocked for 122 seconds
     __wait_rcu_gp
     bpf_struct_ops_map_free

Then I've learned that cond_resched() is not an RCU-tasks quiescent
state, so, we need to use stronger primitives.

Use cond_resched_tasks_rcu_qs() at the scan reschedule points so the scan
reports an RCU-tasks quiescent state as it proceeds.

Inspired by commit b96285e10aad ("tracing: Have osnoise_main() add a
quiescent state for task rcu").

Link: https://lore.kernel.org/20260720-kmemleak_rcu_task-v1-1-5b460ade777d@debian.org
Fixes: c4b28963fd79 ("mm/kmemleak: rely on rcu for task stack scanning")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: SJ Park <sj@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Puranjay Mohan <puranjay@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index e96e9efd19b0..0a6045c857d6 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1571,7 +1571,7 @@ static int scan_large_block(void *start, void *end)
 		if (scan_block(start, next, NULL))
 			return 1;
 		start = next;
-		cond_resched();
+		cond_resched_tasks_rcu_qs();
 	}
 
 	return 0;
@@ -1608,7 +1608,7 @@ static void scan_object(struct kmemleak_object *object)
 			scan_block(start, end, object);
 
 			raw_spin_unlock_irqrestore(&object->lock, flags);
-			cond_resched();
+			cond_resched_tasks_rcu_qs();
 			raw_spin_lock_irqsave(&object->lock, flags);
 			if (!(object->flags & OBJECT_ALLOCATED))
 				break;
@@ -1630,7 +1630,7 @@ static void scan_object(struct kmemleak_object *object)
 				break;
 
 			raw_spin_unlock_irqrestore(&object->lock, flags);
-			cond_resched();
+			cond_resched_tasks_rcu_qs();
 			raw_spin_lock_irqsave(&object->lock, flags);
 		} while (object->flags & OBJECT_ALLOCATED);
 	} else {
@@ -1658,7 +1658,7 @@ static void scan_gray_list(void)
 	 */
 	object = list_entry(gray_list.next, typeof(*object), gray_list);
 	while (&object->gray_list != &gray_list) {
-		cond_resched();
+		cond_resched_tasks_rcu_qs();
 
 		/* may add new objects to the list */
 		if (!scan_should_stop())
@@ -1693,7 +1693,7 @@ static void kmemleak_cond_resched(struct kmemleak_object *object)
 	raw_spin_unlock_irq(&kmemleak_lock);
 
 	rcu_read_unlock();
-	cond_resched();
+	cond_resched_tasks_rcu_qs();
 	rcu_read_lock();
 
 	raw_spin_lock_irq(&kmemleak_lock);
@@ -1738,7 +1738,7 @@ static void kmemleak_scan_task_stacks(void)
 			}
 			put_task_struct(p);
 		}
-		cond_resched();
+		cond_resched_tasks_rcu_qs();
 	} while (pid && !stop);
 }
 
@@ -1915,7 +1915,7 @@ static void kmemleak_scan(void)
 			struct page *page = pfn_to_online_page(pfn);
 
 			if (!(pfn & 63))
-				cond_resched();
+				cond_resched_tasks_rcu_qs();
 
 			if (!page)
 				continue;