Re: [PATCH] sched: allow preempt notifiers to self-unregister.

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-12-16 at 17:15 +0100, Pierre Habouzit wrote:

>   As a background, this need is because I have some kind of module code
>   that uses this facility to evaluate how many of a group of threads are
>   concurrently running (to regulate a pool of threads).

Typically such stuff is only merged along with whomever uses it.

>   Hence I install those callbacks for the thread registering themselves
>   and want to keep them until the thread dies. Sadly I have no way to
>   unregister those callbacks right now, but for horrible hacks (involving
>   private delayed queues processed regularly walked to kfree() the
>   structures referencing pids that are dead, urgh).

kfree_rcu() is the 'normal' way to cheat your way out of this.

> diff --git a/kernel/sched.c b/kernel/sched.c
> index d6b149c..2653169 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -3106,7 +3106,7 @@ static void fire_sched_in_preempt_notifiers(struct task_struct *curr)
>  	struct preempt_notifier *notifier;
>  	struct hlist_node *node;
>  
> -	hlist_for_each_entry(notifier, node, &curr->preempt_notifiers, link)
> +	hlist_for_each_entry_safe(notifier, node, &curr->preempt_notifiers, link)
>  		notifier->ops->sched_in(notifier, raw_smp_processor_id());
>  }
>  
> @@ -3117,7 +3117,7 @@ fire_sched_out_preempt_notifiers(struct task_struct *curr,
>  	struct preempt_notifier *notifier;
>  	struct hlist_node *node;
>  
> -	hlist_for_each_entry(notifier, node, &curr->preempt_notifiers, link)
> +	hlist_for_each_entry_safe(notifier, node, &curr->preempt_notifiers, link)
>  		notifier->ops->sched_out(notifier, next);
>  }

This adds to scheduler hot paths, it needs very good justification.