Re: handle_exit_race && PF_EXITING

Thomas Gleixner
2019-11-06T11:07:43Z

On Wed, 6 Nov 2019, Oleg Nesterov wrote:
> On 11/06, Thomas Gleixner wrote:
> > > + if (unlikely(p->flags & PF_EXITPIDONE)) {
> > > + /* exit_pi_state_list() was already called */
> > > raw_spin_unlock_irq(&p->pi_lock);
> > > put_task_struct(p);
> > > - return ret;
> > > + return -ESRCH;
> >
> > But, this is incorrect because we'd return -ESRCH to user space while the
> > futex value still has the TID of the exiting task set which will
> > subsequently cleanout the futex and set the owner died bit.
>
> Heh. Of course this is not correct. As I said, this patch should be adapted
> to the current code. See below.
>
> > See da791a667536 ("futex: Cure exit race") for example.
>
> Thomas, I simply can't resist ;)
>
> I reported this race when I sent this patch in 2015,
>
> https://lore.kernel.org/lkml/20150205181014.GA20244@redhat.com/
>
> but somehow that discussion died with no result.

Yes. I was not paying attention for some reason. Don't ask me what happened
in Feb. 2015 :)

But even if we adapt that patch to the current code it won't solve the
-ESRCH issue I described above.

> > Guess why that code has more corner case handling than actual
> > functionality. :)
>
> I know why. To confuse me!

Of course. As Rusty said: "Futexes are also cursed"

Thanks,

tglx