On Tue, 10 Dec 2013, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 11:18 AM, Thomas Gleixner <t...@linutronix.de> wrote:
> >
> > So this is pretty unlikely. The retry requires:
> >
> > get_futex_value_locked() == EFAULT;
> >
> > Now we drop the hash bucket locks and do:
> >
> > get_user();
> >
> > And if that get_user() faults again, we bail out.
>
> I think you need to look closer.
>
> We have at least also that "futex_proxy_trylock_atomic() returns
> -EAGAIN" case. Which triggers at some exit condition. Another thread
> in the same group, perhaps never completing the exit because it's
> waiting for this one? I dunno, I didn't look any closer (but this does
> make me think "Hey, we should add Oleg to the Cc too", since
> PF_EXITING is involved).. So maybe there is some situation where that
> EAGAIN will keep happening, forever..
>
> Now, I'm *not* saying that that is it. It's quite possible/likely some
> other loop, but I do have to say that it sure isn't _obvious_. And
> that whole EAGAIN return case is quite deep and special, so ...
The -EAGAIN is when the user value changed, simplified:
oldval = *uval;
sys_futex(....., oldval)
do_futex(...) {
if (oldval != *uval)
return -EAGAIN;
}
sys_exit();
And user space loops on that.
Thanks,
tglx
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