linux-kernel - Re: [tip:sched/core] [sched/fair] 79104becf4: BUG:kernel_NULL_pointer

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Message-ID: <20251105084335.121098-1-sieberf@amazon.com>
Date: Wed, 5 Nov 2025 10:43:33 +0200
From: Fernand Sieber <sieberf@...zon.com>
To: kernel test robot <oliver.sang@...el.com>
CC: <oe-lkp@...ts.linux.dev>, <lkp@...el.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	<aubrey.li@...ux.intel.com>, <yu.c.chen@...el.com>
Subject: Re: [tip:sched/core] [sched/fair] 79104becf4: BUG:kernel_NULL_pointer_dereference,address

Hi Peter,

I have done more code analysis and I think the following code is a better 
approach to handle proxy tasks:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 00f9d6c05d4cf..65eb98a779f91 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8983,9 +8983,10 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct t
  */
 static void yield_task_fair(struct rq *rq)
 {
-	struct task_struct *curr = rq->curr;
-	struct cfs_rq *cfs_rq = task_cfs_rq(curr);
-	struct sched_entity *se = &curr->se;
+	struct task_struct *rq_curr = rq->curr;
+	struct cfs_rq *cfs_rq = task_cfs_rq(rq_curr);
+	struct sched_entity *se = &rq_curr->se;
+	bool curr = cfs_rq->curr == se;
 
 	/*
 	 * Are we the only task in the tree?
@@ -9016,8 +9017,12 @@ static void yield_task_fair(struct rq *rq)
 	 * ends up quickly running away.
 	 */
 	if (entity_eligible(cfs_rq, se)) {
+		if (!curr)
+			__dequeue_entity(cfs_rq, se);
 		se->vruntime = se->deadline;
 		se->deadline += calc_delta_fair(se->slice, se);
+		if (!curr)
+			__enqueue_entity(cfs_rq, se);
 		update_min_vruntime(cfs_rq);
 	}
 }

I'll prepare rev4 of the patch accordingly.

--Fernand



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