Re: [PATCH] sched_ext: don't rehome a dead task in scx_cgroup_task_migrated

[Tao Cui]

Hi,

在 2026/8/15 03:04, Tejun Heo 写道:
> Hello,
> 
> On Tue, Aug 11, 2026 at 06:31:22PM +0800, Tao Cui wrote:
>> A task can exit between cgroup migration commit and the MIGRATED callback:
>> sched_ext_dead() marks it SCX_TASK_DEAD before cgroup_task_dead() removes it
>> from the migration list, so scx_cgroup_task_migrated() can pick up a dead
>> task and call scx_rehome_task(), which re-enables it and leaks the BPF
>> scheduler's per-task resources. The other scx_rehome_task() callers already
>> check for this; do the same here.
> 
> I don't think this window exists. SCX_TASK_DEAD is set only by
> sched_ext_dead() from finish_task_switch(), which a task reaches only
> after exit_signals(), and exit_signals() sets PF_EXITING inside
> cgroup_threadgroup_change_begin(). The MIGRATED notifiers run inside
> cgroup_migrate_execute() with the same rwsem write-held through
> cgroup_attach_lock(), so no task in the set can enter the exit path
> until the migration is done. Tasks which were already exiting are
> filtered out by the PF_EXITING test in cgroup_migrate_add_task().
> 
> The DEAD tests you referenced are in scx_task_iter walks which run
> without the threadgroup rwsem, where dying tasks can actually show up.
> 
> Did you try to reproduce the leak? When code review turns up a
> suspected bug, it's a good idea to reproduce it first to verify the
> assumptions before writing a fix.
> 
You're right, I missed that. 
I walked it again, a task past exit_signals() is filtered out by the PF_EXITING test in cgroup_migrate_add_task(), and a task mid-migration can't reach exit_signals() until the rwsem is dropped, so scx_cgroup_task_migrated() always sees a live task. The DEAD checks I pointed at are on task iteration paths without the rwsem, so they don't apply here.

I didn't try to reproduce the leak first. I will next time.

Please disregard this patch.Sorry for the noise.

Thanks,
Tao> Thanks.
> 
> --
> tejun