On Sat, 24 Jan 2015, Torvald Riegel wrote: > On Sat, 2015-01-24 at 11:05 +0100, Thomas Gleixner wrote: > > On Fri, 23 Jan 2015, Torvald Riegel wrote: > > > > > On Fri, 2015-01-16 at 16:46 -0800, Darren Hart wrote: > > > > On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)" > > > > <mtk.manpages@xxxxxxxxx> wrote: > > > > > > > > >Color me stupid, but I can't see this in futex_requeue(). Where is that > > > > >check that is "independent of the requeue type (normal/pi)"? > > > > > > > > > >When I look through futex_requeue(), all the likely looking sources > > > > >of EINVAL are governed by a check on the 'requeue_pi' argument. > > > > > > > > > > > > Right, in the non-PI case, I believe there are valid use cases: move to > > > > the back of the FIFO, for example (OK, maybe the only example?). > > > > > > But we never guarantee a futex is a FIFO, or do we? If we don't, then > > > such a requeue could be implemented as a no-op by the kernel, which > > > would sort of invalidate the use case. > > > > > > (And I guess we don't want to guarantee FIFO behavior for futexes.) > > > > The (current) behaviour is: > > > > real-time threads: FIFO per priority level > > sched-other threads: FIFO independent of nice level > > > > The wakeup is priority ordered. Highest priority level first. > > OK. > > But, just to be clear, do I correctly understand that you do not want to > guarantee FIFO behavior in the specified futex semantics? I think there > are cases where being able to *rely* on FIFO (now and on all future > kernels) would be helpful for users (e.g., on POSIX/C++11 condvars and I > assume in other ordered-wakeup cases too) -- but at the same time, this > would constrain future futex implementations. It would be a constraint, but I don't think it would be a horrible one. Though I have my doubts, that we can actually guarantee it under all circumstances. One thing comes to my mind right away: spurious wakeups. There is no way that we can guarantee FIFO ordering in the context of those. And we cannot prevent them either. Thanks, tglx -- 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
