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. Okay -- so do we need a change to the text here? >>> .\" 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(?). Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
