Messages in this thread | | | Date | Thu, 5 Feb 2015 17:27:25 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 0/1] futex: check PF_KTHREAD rather than !p->mm to filter out kthreads |
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On Wed, Feb 04, 2015 at 09:25:09PM +0100, Oleg Nesterov wrote: > > I'm not entire sure why we need two PF flags for this; once PF_EXITING > > is set userspace is _dead_ and it doesn't make sense to keep adding > > (futex) PI-state to the task. > > This is what I _seem_ to understand: exit_robust_list(). Although I am > not sure this all is by design... > > And this is the reason why I still can't finish the patch. Perhaps I am > totally confused, but I think there is yet another problem here. > > Please forget about PF_EXIT.*. attach_to_pi_owner() returns -ESRCH if > futex_find_get_task() and even this looks wrong.
You'll have to help me out a little here; where do we unhash the PIDs? From what I can find we set PF_EXITING _before_ unhashing ourselves.
In fact, from what I can tell we only unhash after calling both exit_robust_list and exit_pi_state_list.
> Because handle_futex_death() > updates *uaddr lockless and does nothing if "pi". This means that the owner > of PI + robust mutex can go away (or just set PF_EXITPIDONE) and the caller > of futex_lock_pi() can miss unlock. > > Peter, could you confirm that this problem does exist, or I missed something?
So as long as we unhash _last_ I can't see this happening, we'll always find the task, the robust list walk doesn't care about PI state.
The exit_pi_state_list() will serialize against any concurrent attach that might be in progress -- and we nkow there won't be a new one since we've set PF_EXITING. And kill all the PI owners stuff.
But please, if you suspect, share a little more detail on how you see this happening, this is not code I've looked at in detail before.
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