Re: [PATCH] sched_ext: Fix NULL pointer deref and warnings during scx teardown

[Tejun Heo <tj@kernel.org>]

On Mon, Feb 02, 2026 at 07:54:50PM +0100, Andrea Righi wrote:
> I'm able to reproduce the NULL pointer dereference in set_cpu_allowed_scx()
> quite easily running `stress-ng --race-sched 0` with an scx scheduler that
> is intentionally starving tasks, triggering a stall => disable.
> 
> I think this is what's happening:
> 
>  CPU0                                      CPU1
>  ----                                      ----
>  __sched_setscheduler()
>    task_rq_lock(p)
> 
>    next_class = __setscheduler_class()
>      // next_class is ext_sched_class
>                                            scx_disable_workfn()
>                                              scx_set_enable_state(SCX_DISABLING)
> 
>                                              scx_task_iter_start()
>                                              while ((p = next())) {
> 					       ...
>                                                p->sched_class = fair_sched_class
> 					       ...
>                                              }
>                                              scx_task_iter_stop()
> 
>                                              synchronize_rcu()
>                                              RCU_INIT_POINTER(scx_root, NULL)
> 
>    scoped_guard(sched_change, ...) {
>      p->sched_class = next_class;
>        // next_class is still ext_sched_class,
>        // overwriting fair_sched_class!
>    }
>    // Guard ends, calls sched_change_end()
>    //   switching_to_scx() called
>    //   scx_root == NULL => returns early
> 
>    task_rq_unlock(p)
> 
>    sched_setaffinity(p)
>      set_cpus_allowed_scx()
>        sch = scx_root; // scx_root == NULL => BUG!

Does the following patch fix the issue?

Thanks.

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 136b01950a62..1fc2b358a175 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4234,7 +4234,13 @@ static void scx_disable_workfn(struct kthread_work *work)
 	 * Here, every runnable task is guaranteed to make forward progress and
 	 * we can safely use blocking synchronization constructs. Actually
 	 * disable ops.
+	 *
+	 * Wait for all CPUs to observe %SCX_DISABLING. Otherwise,
+	 * task_should_scx() can see %SCX_ENABLED and __sched_setscheduler() put
+	 * a task into sched_ext while we're migrating tasks out below.
 	 */
+	synchronize_rcu();
+
 	mutex_lock(&scx_enable_mutex);
 
 	static_branch_disable(&__scx_switched_all);