Re: Next round: revised futex(2) man page for review

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 5 Aug 2015, Darren Hart wrote:
> On Mon, Jul 27, 2015 at 02:07:15PM +0200, Michael Kerrisk (man-pages) wrote:
> > .\" FIXME XXX ===== Start of adapted Hart/Guniguntala text =====
> > .\"       The following text is drawn from the Hart/Guniguntala paper
> > .\"       (listed in SEE ALSO), but I have reworded some pieces
> > .\"       significantly. Please check it.
> > 
> >        The PI futex operations described below  differ  from  the  other
> >        futex  operations  in  that  they impose policy on the use of the
> >        value of the futex word:
> > 
> >        *  If the lock is not acquired, the futex word's value  shall  be
> >           0.
> > 
> >        *  If  the  lock is acquired, the futex word's value shall be the
> >           thread ID (TID; see gettid(2)) of the owning thread.
> > 
> >        *  If the lock is owned and there are threads contending for  the
> >           lock,  then  the  FUTEX_WAITERS  bit shall be set in the futex
> >           word's value; in other words, this value is:
> > 
> >               FUTEX_WAITERS | TID
> > 
> > 
> >        Note that a PI futex word never just has the value FUTEX_WAITERS,
> >        which is a permissible state for non-PI futexes.
> 
> The second clause is inappropriate. I don't know if that was yours or
> mine, but non-PI futexes do not have a kernel defined value policy, so
> ==FUTEX_WAITERS cannot be a "permissible state" as any value is
> permissible for non-PI futexes, and none have a kernel defined state.

Depends. If the regular futex is configured as robust, then we have a
kernel defined value policy as well.

> > .\" FIXME I'm not quite clear on the meaning of the following sentence.
> > .\"       Is this trying to say that while blocked in a
> > .\"       FUTEX_WAIT_REQUEUE_PI, it could happen that another
> > .\"       task does a FUTEX_WAKE on uaddr that simply causes
> > .\"       a normal wake, with the result that the FUTEX_WAIT_REQUEUE_PI
> > .\"       does not complete? What happens then to the FUTEX_WAIT_REQUEUE_PI
> > .\"       opertion? Does it remain blocked, or does it unblock
> > .\"       In which case, what does user space see?
> > 
> >               The
> >               waiter   can  be  removed  from  the  wait  on  uaddr  via
> >               FUTEX_WAKE without requeueing on uaddr2.
> 
> Userspace should see the task wake and continue executing. This would
> effectively be a cancelation operation - which I didn't think was
> supported. Thomas?

We probably never intended to support it, but looking at the code it
works (did not try it though). It returns to user space with
-EWOULDBLOCK. So it basically behaves like any other spurious wakeup.
 
Thanks,

	tglx