[PATCH 2/2] sched_ext: Skip past-sched_ext_dead() tasks in scx_task_iter_next_locked()

Tejun Heo
2026-04-28T00:16:39Z

scx_task_iter's cgroup-scoped mode can return tasks whose
sched_ext_dead() has already completed: cgroup_task_dead() removes
from cset->tasks after sched_ext_dead() in finish_task_switch() and is
irq-work deferred on PREEMPT_RT. The global mode is fine -
sched_ext_dead() removes from scx_tasks via list_del_init() first.

Callers (sub-sched enable prep/abort/apply, scx_sub_disable(),
scx_fail_parent()) assume returned tasks are still on @sch and trip
WARN_ON_ONCE() or operate on torn-down state otherwise.

Set %SCX_TASK_OFF_TASKS in sched_ext_dead() under @p's rq lock and
have scx_task_iter_next_locked() skip flagged tasks under the same
lock. Setter and reader serialize on the per-task rq lock - no race.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/sched/ext.h | 1 +
kernel/sched/ext.c | 33 +++++++++++++++++++++++++--------
2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 1a3af2ea2a79..adb9a4de068a 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -101,6 +101,7 @@ enum scx_ent_flags {
SCX_TASK_DEQD_FOR_SLEEP = 1 << 3, /* last dequeue was for SLEEP */
SCX_TASK_SUB_INIT = 1 << 4, /* task being initialized for a sub sched */
SCX_TASK_IMMED = 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
+ SCX_TASK_OFF_TASKS = 1 << 6, /* removed from scx_tasks by sched_ext_dead() */

/*
* Bits 8 and 9 are used to carry task state:
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index cf43be8ac1aa..6c3c40499404 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -928,16 +928,27 @@ static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
*
* Test for idle_sched_class as only init_tasks are on it.
*/
- if (p->sched_class != &idle_sched_class)
- break;
- }
- if (!p)
- return NULL;
+ if (p->sched_class == &idle_sched_class)
+ continue;

- iter->rq = task_rq_lock(p, &iter->rf);
- iter->locked_task = p;
+ iter->rq = task_rq_lock(p, &iter->rf);
+ iter->locked_task = p;

- return p;
+ /*
+ * cgroup_task_dead() removes the dead tasks from cset->tasks
+ * after sched_ext_dead() and cgroup iteration may see tasks
+ * which already finished sched_ext_dead(). %SCX_TASK_OFF_TASKS
+ * is set by sched_ext_dead() under @p's rq lock. Test it to
+ * avoid visiting tasks which are already dead from SCX POV.
+ */
+ if (p->scx.flags & SCX_TASK_OFF_TASKS) {
+ __scx_task_iter_rq_unlock(iter);
+ continue;
+ }
+
+ return p;
+ }
+ return NULL;
}

/**
@@ -3816,6 +3827,11 @@ void sched_ext_dead(struct task_struct *p)
/*
* @p is off scx_tasks and wholly ours. scx_root_enable()'s READY ->
* ENABLED transitions can't race us. Disable ops for @p.
+ *
+ * %SCX_TASK_OFF_TASKS synchronizes against cgroup task iteration - see
+ * scx_task_iter_next_locked(). NONE tasks need no marking: cgroup
+ * iteration is only used from sub-sched paths, which require root
+ * enabled. Root enable transitions every live task to at least READY.
*/
if (scx_get_task_state(p) != SCX_TASK_NONE) {
struct rq_flags rf;
@@ -3823,6 +3839,7 @@ void sched_ext_dead(struct task_struct *p)

rq = task_rq_lock(p, &rf);
scx_disable_and_exit_task(scx_task_sched(p), p);
+ p->scx.flags |= SCX_TASK_OFF_TASKS;
task_rq_unlock(rq, p, &rf);
}
}
--
2.53.0