Re: [RFC][PATCH] cgroup: Fix cgroup_subsys::exit callback

[Li Zefan <lizf@cn.fujitsu.com>]

(just came back from vacation)

>> How about if the exit callback was moved before the preceding
>> task_unlock()? Since I think the scheduler is still the only user of
>> the exit callback, redefining the locking semantics should be fine.
> 
> Like the below? Both the perf and sched exit callback are fine with
> being called under task_lock afaict, but I haven't actually ran with
> lockdep enabled to see if I missed something.
> 

This change should be fine.

...

> +
> +	if (run_callbacks && need_forkexit_callback) {
> +		/*
> +		 * modular subsystems can't use callbacks, so no need to lock
> +		 * the subsys array
> +		 */
> +		for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
> +			struct cgroup_subsys *ss = subsys[i];
> +			if (ss->exit) {
> +				struct cgroup *old_cgrp =
> +					rcu_dereference_raw(cg->subsys[i])->cgroup;
> +				struct cgroup *cgrp = task_cgroup(tsk, i);
> +				ss->exit(ss, cgrp, old_cgrp, tsk);

Since both sched and perf won't use the 2 args @cgrp and @old_cgrp,
don't bother to change the ->exit interface?

> +			}
> +		}
> +	}
>  	task_unlock(tsk);
> +
>  	if (cg)
>  		put_css_set_taskexit(cg);
>  }