linux-kernel - Re: [PATCH 1/2] sched_ext: Fix ops.dequeue() semantics

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Message-ID: <aYMTOg8PoMKxK432@gpd4>
Date: Wed, 4 Feb 2026 10:36:58 +0100
From: Andrea Righi <arighi@...dia.com>
To: Kuba Piecuch <jpiecuch@...gle.com>
Cc: Christian Loehle <christian.loehle@....com>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>,
	Emil Tsalapatis <emil@...alapatis.com>,
	Daniel Hodges <hodgesd@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched_ext: Fix ops.dequeue() semantics

Hi Kuba,

sorry for the late response.

On Mon, Feb 02, 2026 at 01:59:24PM +0000, Kuba Piecuch wrote:
> Hi Andrea,
> 
> On Mon Feb 2, 2026 at 7:45 AM UTC, Andrea Righi wrote:
> > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> > index 6d6f1253039d8..d8fed4a49195d 100644
> > --- a/kernel/sched/ext.c
> > +++ b/kernel/sched/ext.c
> > @@ -2248,7 +2248,7 @@ static void finish_dispatch(struct scx_sched *sch, struct rq *rq,
> >  			p->scx.flags |= SCX_TASK_OPS_ENQUEUED;
> >  		} else {
> >  			if (p->scx.flags & SCX_TASK_OPS_ENQUEUED)
> > -				SCX_CALL_OP_TASK(sch, SCX_KF_REST, dequeue, task_rq(p), p, 0);
> > +				SCX_CALL_OP_TASK(sch, SCX_KF_REST, dequeue, rq, p, 0);
> >  
> >  			p->scx.flags &= ~SCX_TASK_OPS_ENQUEUED;
> >  		}
> 
> This looks risky from a locking perspective. Are we relying on
> SCX_OPSS_DISPATCHING to protect against racing dequeues? If so, it might
> be worth calling out in a comment.

You're right, we're relying on SCX_OPSS_DISPATCHING to protect against
racing dequeues and this definitely deserves a comment. How about something
like the following?

Thanks,
-Andrea

---
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 292adf10fee1b..b189339e74101 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2260,6 +2260,15 @@ static void finish_dispatch(struct scx_sched *sch, struct rq *rq,
 		if (!is_terminal_dsq(dsq_id)) {
 			p->scx.flags |= SCX_TASK_OPS_ENQUEUED;
 		} else {
+			/*
+			 * Locking: we're holding the @rq lock (the
+			 * dispatch CPU's rq), but not necessarily
+			 * task_rq(p), since @p may be from a remote CPU.
+			 *
+			 * This is safe because SCX_OPSS_DISPATCHING state
+			 * prevents racing dequeues, any concurrent
+			 * ops_dequeue() will wait for this state to clear.
+			 */
 			if (p->scx.flags & SCX_TASK_OPS_ENQUEUED)
 				SCX_CALL_OP_TASK(sch, SCX_KF_REST, dequeue, rq, p, 0);