Messages in this thread | | | Date | Tue, 10 Dec 2013 21:33:30 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: process 'stuck' at exit. |
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On Tue, 10 Dec 2013, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 11:18 AM, Thomas Gleixner <tglx@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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