On Wed, Oct 07, 2015 at 10:34:19AM +0100, Michael Kerrisk (man-pages) wrote: > On 08/19/2015 03:40 PM, Thomas Gleixner wrote: > > 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. > Right. > Okay -- so do we need a change to the text here? Hrm. We probably need a way to indicate that kernel-defined futex word value policy only applies to PI and or ROBUST futexes. > > >>> .\" 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. > > Again, I assume no changes are required to the man page(?). I'd rather not document this as supported or intended behavior. FUTEX_WAIT_REQUEUE_PI is documented as being paired with and only with FUTEX_CMP_REQUEUE_PI. Anything else is undefined behavior. If we want to support a cancelation, it should be deliberate - and we should probably test it ;-) -- Darren Hart Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
