[PATCH 1/2] sched_ext: Include exiting tasks in cgroup iter

Tejun Heo
2026-04-28T00:16:38Z

a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup") made
css_task_iter_advance() skip exiting tasks so cgroup.procs stays consistent
with waitpid() visibility. Unfortunately, this broke scx_task_iter.

scx_task_iter walks either scx_tasks (global) or a cgroup subtree via
css_task_iter() and the two modes are expected to cover the same set of
tasks. After the above change the cgroup-scoped mode silently skips tasks
past exit_signals() that are still on scx_tasks.

scx_sub_enable_workfn()'s abort path is one of the symptoms: an exiting
SCX_TASK_SUB_INIT task can race past the cgroup iter leaking
__scx_init_task() state. Other iterations share the same gap.

Add CSS_TASK_ITER_WITH_DEAD to opt out of the skip and use it from
scx_task_iter().

Fixes: b0e4c2f8a0f0 ("sched_ext: Implement cgroup subtree iteration for scx_task_iter")
Reported-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/cgroup.h | 1 +
kernel/cgroup/cgroup.c | 8 +++++---
kernel/sched/ext.c | 6 ++++--
3 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e52160e85af4..f6d037a30fd8 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -53,6 +53,7 @@ struct kernel_clone_args;
enum css_task_iter_flags {
CSS_TASK_ITER_PROCS = (1U << 0), /* walk only threadgroup leaders */
CSS_TASK_ITER_THREADED = (1U << 1), /* walk all threaded css_sets in the domain */
+ CSS_TASK_ITER_WITH_DEAD = (1U << 2), /* include exiting tasks */
CSS_TASK_ITER_SKIPPED = (1U << 16), /* internal flags */
};

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 1f084ee71443..e51ce4cd3739 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5059,10 +5059,12 @@ static void css_task_iter_advance(struct css_task_iter *it)

task = list_entry(it->task_pos, struct task_struct, cg_list);
/*
- * Hide tasks that are exiting but not yet removed. Keep zombie
- * leaders with live threads visible.
+ * Hide tasks that are exiting but not yet removed by default. Keep
+ * zombie leaders with live threads visible. Usages that need to walk
+ * every existing task can opt out via CSS_TASK_ITER_WITH_DEAD.
*/
- if ((task->flags & PF_EXITING) && !atomic_read(&task->signal->live))
+ if (!(it->flags & CSS_TASK_ITER_WITH_DEAD) &&
+ (task->flags & PF_EXITING) && !atomic_read(&task->signal->live))
goto repeat;

if (it->flags & CSS_TASK_ITER_PROCS) {
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9eda20e5fdb8..cf43be8ac1aa 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -766,7 +766,8 @@ static void scx_task_iter_start(struct scx_task_iter *iter, struct cgroup *cgrp)
lockdep_assert_held(&cgroup_mutex);
iter->cgrp = cgrp;
iter->css_pos = css_next_descendant_pre(NULL, &iter->cgrp->self);
- css_task_iter_start(iter->css_pos, 0, &iter->css_iter);
+ css_task_iter_start(iter->css_pos, CSS_TASK_ITER_WITH_DEAD,
+ &iter->css_iter);
return;
}
#endif
@@ -866,7 +867,8 @@ static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter)
iter->css_pos = css_next_descendant_pre(iter->css_pos,
&iter->cgrp->self);
if (iter->css_pos)
- css_task_iter_start(iter->css_pos, 0, &iter->css_iter);
+ css_task_iter_start(iter->css_pos, CSS_TASK_ITER_WITH_DEAD,
+ &iter->css_iter);
}
return NULL;
}
--
2.53.0