Messages in this thread | | | Date | Mon, 20 Aug 2012 18:10:12 +0200 | From | Oleg Nesterov <> | Subject | Re: lockdep trace from posix timers |
| |
On 08/20, Peter Zijlstra wrote: > > On Mon, 2012-08-20 at 17:41 +0200, Oleg Nesterov wrote: > > > IMHO, this is just more natural. > > Depends on what you're used to I guess ;-)
I have to agree ;)
> > For example. keyctl_session_to_parent() does _cancel only to protect > > from exploits doing keyctl(KEYCTL_SESSION_TO_PARENT) in an endless > > loop. It could simply do task_work_add(), but in this case we need > > fifo for correctness. > > I'm not entirely sure I see, not doing the cancel would delay the free > until the executing of key_change_session_keyring()? doing that keyctl() > in an indefinite loop involves going back to userspace, so where's the > resource issue?
But the child does task_work_add(current->parent). IOW, there are 2 different tasks. Now suppose that ->parent sleeps.
> Also, I'm not seeing where the FIFO requirement comes from.
Again, suppose that ->parent sleeps. The last KEYCTL_SESSION_TO_PARENT request should "win". Although I agree, this is not that important.
> > > Iterating a single linked queue in fifo > > > seems more expensive than useful. > > > > Currently the list is fifo (we add to the last element), this is O(1). > > depends on what way you look at the list I guess, with a single linked > list there's only one end you can add to in O(1), so we're calling that > the tail?
Sorry, can't understand...
task->task_works points to the last node in the circular single-linked list, task_work_add() adds the new element after the last one and updates task->task_works. This is O(1).
> > But the list should be short, we can reverse it in _run() if we change > > task_work_add() to add to the head. > > Reversing a (single linked) list is O(n^2)..
Hmm. This is O(n). You can simply iterate over this list once, changing the ->next pointer to point back.
> which is indeed doable for > short lists, but why assume its short?
I agree, it is better to not do this.
Oleg.
| |