Re: [PATCH sched_ext/for-7.3-fixes] sched_ext: Wait for SCX_OPSS_DISPATCHING before reenqueueing a task

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Tue, Sep 15, 2026 at 10:03:37AM -1000, Tejun Heo wrote:
> ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics") moved the final
> ops_state store in scx_dispatch_enqueue() after the DSQ unlock so that the
> custody update and ops.dequeue() precede it. A task can thus be found on a
> DSQ while still SCX_OPSS_DISPATCHING.
> 
> The dequeue and core-sched pick paths wait for the state to clear in
> ops_dequeue() but the reenqueue paths don't. A reenqueue in that window runs
> ops.enqueue() and sets SCX_OPSS_QUEUED before the dispatch has completed.
> The dispatcher's final store then overwrites it with SCX_OPSS_NONE and
> finish_dispatch() drops every later dispatch of the task.
> 
> Wait for SCX_OPSS_DISPATCHING to clear before dequeueing a task for
> reenqueue, the same way ops_dequeue() does.
> 
> Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> Cc: stable@vger.kernel.org # v7.1+
> Signed-off-by: Tejun Heo <tj@kernel.org>

Thanks for fixing this, it makes sense to me.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

-Andrea

> ---
>  kernel/sched/ext/ext.c      |   13 +++++++++++++
>  kernel/sched/ext/internal.h |    1 +
>  kernel/sched/ext/sub.c      |    1 +
>  3 files changed, 15 insertions(+)
> 
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -4404,6 +4404,17 @@ static bool local_task_should_reenq(stru
>  	return *reenq_flags & SCX_REENQ_ANY;
>  }
>  
> +/*
> + * The dispatcher stores the final ops_state after dropping the DSQ lock, so @p
> + * can be found on a DSQ while still %SCX_OPSS_DISPATCHING. Reenqueueing @p
> + * before that store lands would have it clobber the new %SCX_OPSS_QUEUED.
> + */
> +void scx_reenq_wait_dispatching(struct task_struct *p)
> +{
> +	if (unlikely(atomic_long_read_acquire(&p->scx.ops_state) == SCX_OPSS_DISPATCHING))
> +		wait_ops_state(p, SCX_OPSS_DISPATCHING);
> +}
> +
>  static u32 reenq_local(struct scx_sched *sch, struct rq *rq, u64 reenq_flags)
>  {
>  	LIST_HEAD(tasks);
> @@ -4447,6 +4458,7 @@ static u32 reenq_local(struct scx_sched
>  		if (!local_task_should_reenq(rq, p, &reenq_flags, &reason))
>  			continue;
>  
> +		scx_reenq_wait_dispatching(p);
>  		scx_dispatch_dequeue(rq, p);
>  
>  		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
> @@ -4570,6 +4582,7 @@ static void reenq_user(struct rq *rq, st
>  		}
>  
>  		/* @p is on @dsq, its rq and @dsq are locked */
> +		scx_reenq_wait_dispatching(p);
>  		dispatch_dequeue_locked(p, dsq);
>  		raw_spin_unlock(&dsq->lock);
>  
> --- a/kernel/sched/ext/internal.h
> +++ b/kernel/sched/ext/internal.h
> @@ -2078,6 +2078,7 @@ void scx_kick_cpu(struct scx_sched *sch,
>  u64 __scx_bpf_now(struct rq *rq);
>  void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
>  			u64 reenq_flags, struct rq *locked_rq);
> +void scx_reenq_wait_dispatching(struct task_struct *p);
>  int __scx_init_task(struct scx_sched *sch, struct task_struct *p,
>  		    struct cgroup *cgrp, bool fork);
>  void scx_enable_task(struct scx_sched *sch, struct task_struct *p);
> --- a/kernel/sched/ext/sub.c
> +++ b/kernel/sched/ext/sub.c
> @@ -801,6 +801,7 @@ void scx_reenq_reject(struct rq *rq)
>  		if (WARN_ON_ONCE(p->migration_pending))
>  			continue;
>  
> +		scx_reenq_wait_dispatching(p);
>  		scx_dispatch_dequeue(rq, p);
>  
>  		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))