linux-kernel - Re: [RFC][PATCH] sched/ext: Avoid null ptr traversal when ->put_prev

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Message-ID: <DESR4ZKLE36H.3MT2T8V3VLV7U@google.com>
Date: Mon, 08 Dec 2025 10:10:07 +0000
From: Kuba Piecuch <jpiecuch@...gle.com>
To: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>, Qais Yousef <qyousef@...alina.io>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Valentin Schneider <vschneid@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya <Metin.Kaya@....com>, 
	Xuewen Yan <xuewen.yan94@...il.com>, K Prateek Nayak <kprateek.nayak@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Suleiman Souhlal <suleiman@...gle.com>, kuyo chang <kuyo.chang@...iatek.com>, 
	hupu <hupu.gm@...il.com>, Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>, 
	Andrea Righi <arighi@...dia.com>, Changwoo Min <changwoo@...lia.com>, <sched-ext@...ts.linux.dev>, 
	<kernel-team@...roid.com>
Subject: Re: [RFC][PATCH] sched/ext: Avoid null ptr traversal when
 ->put_prev_task() is called with NULL next

Hello John,

On Sat Dec 6, 2025 at 2:22 AM UTC, John Stultz wrote:
> I can't say I understand why this doesn't trip usually without
> proxy-exec. And in newer kernels there are way fewer
> put_prev_task(), and I can't easily reproduce the issue now
> even with proxy-exec.

That's probably because put_prev_task_scx() with next == NULL is always
preceded by a dequeue, clearing SCX_TASK_QUEUED from p->scx.flags, so we don't
reach the problematic sched_class_above() check because it only happens when
the flag is set.

> But we still have one put_prev_task() call left in core.c that
> seems like it could trip this, so I wanted to send this out for
> consideration.

I'm assuming you're referring to the one in sched_change_begin().
It looks like it's impossible for an outside observer holding a CPU's rq lock
to observe a task that is running on that CPU and isn't queued, i.e.
'running' implies 'queued' (I'm new to the scheduler so I may be wrong here).
That would explain why dequeue_task() is always called before put_prev_task().
Does proxy execution break that assumption?

Best,
Kuba