Re: [PATCH sched_ext/for-7.2] sched_ext: add p->scx.tid and SCX_OPS_TID_TO_TASK lookup

[Tejun Heo]

Hello,

On Sun, Apr 19, 2026 at 07:02:47PM +0200, Andrea Righi wrote:
> I'm wondering if we should have a teardown function, called before disabling
> SCX_OPS_TID_TO_TASK and destroying scx_tid_hash, to explicitly remove all the
> scx_tid_hash entries via rhashtable_remove_fast().

rhashtable_remove_fast() doesn't clear tid_hash_node.next either - it
just unlinks from the bucket chain, leaving obj->next at whatever the
chain successor or nulls-marker put there. So explicit drain wouldn't
give us a cleaner post-state.

And the stale pointer is never read. After the static key is disabled,
scx_post_fork/sched_ext_dead/scx_bpf_tid_to_task all gate on the key and
skip the hash. On re-enable, rhashtable_init() creates fresh buckets and
task iteration re-inserts each live task via
rhashtable_lookup_insert_fast(), which unconditionally does

    RCU_INIT_POINTER(obj->next, head);

at include/linux/rhashtable.h:838. Lookups always traverse from
ht->tbl[hash], never from an embedded node.

Thanks.

-- 
tejun