Re: [PATCH v2 2/3] futex: Create set_robust_list2

[André Almeida]

Hi Peter,

Em 04/11/2024 08:22, Peter Zijlstra escreveu:
> On Fri, Nov 01, 2024 at 01:21:46PM -0300, André Almeida wrote:
> > @@ -1046,24 +1095,44 @@ static inline void exit_pi_state_list(struct task_struct *curr) { }
> >   
> >   static void futex_cleanup(struct task_struct *tsk)
> >   {
> > +	struct robust_list2_entry *curr, *n;
> > +	struct list_head *list2 = &tsk->robust_list2;
> > +
> >   #ifdef CONFIG_64BIT
> >   	if (unlikely(tsk->robust_list)) {
> > -		exit_robust_list64(tsk);
> > +		exit_robust_list64(tsk, tsk->robust_list);
> >   		tsk->robust_list = NULL;
> >   	}
> >   #else
> >   	if (unlikely(tsk->robust_list)) {
> > -		exit_robust_list32(tsk);
> > +		exit_robust_list32(tsk, (struct robust_list_head32 *) tsk->robust_list);
> >   		tsk->robust_list = NULL;
> >   	}
> >   #endif
> >   
> >   #ifdef CONFIG_COMPAT
> >   	if (unlikely(tsk->compat_robust_list)) {
> > -		exit_robust_list32(tsk);
> > +		exit_robust_list32(tsk, tsk->compat_robust_list);
> >   		tsk->compat_robust_list = NULL;
> >   	}
> >   #endif
> > +	/*
> > +	 * Walk through the linked list, parsing robust lists and freeing the
> > +	 * allocated lists
> > +	 */
> > +	if (unlikely(!list_empty(list2))) {
> > +		list_for_each_entry_safe(curr, n, list2, list) {
> > +			if (curr->head != NULL) {
> > +				if (curr->list_type == ROBUST_LIST_64BIT)
> > +					exit_robust_list64(tsk, curr->head);
> > +				else if (curr->list_type == ROBUST_LIST_32BIT)
> > +					exit_robust_list32(tsk, curr->head);
> > +				curr->head = NULL;
> > +			}
> > +			list_del_init(&curr->list);
> > +			kfree(curr);
> > +		}
> > +	}
> >   
> >   	if (unlikely(!list_empty(&tsk->pi_state_list)))
> >   		exit_pi_state_list(tsk);
>
> I'm still digesting this, but the above seems particularly silly.
>
> Should not the legacy lists also be on the list of lists? I mean, it
> makes no sense to have two completely separate means of tracking lists.

You are asking if, whenever someone calls set_robust_list() or 
compat_set_robust_list() to be inserted into &current->robust_list2 
instead of using tsk->robust_list and tsk->compat_robust_list?

I was thinking of doing that, but my current implementation has a 
kmalloc() call for every insertion, and I wasn't sure if I could add 
this new latency to the old set_robust_list() syscall. Assuming it is 
usually called just once during the thread initialization perhaps it 
shouldn't cause much harm I guess.