----- On Sep 20, 2017, at 2:18 PM, Andy Lutomirski luto@xxxxxxxxxx wrote: > On Wed, Sep 20, 2017 at 11:13 AM, Mathieu Desnoyers > <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> >> ----- On Sep 20, 2017, at 12:02 PM, Andy Lutomirski luto@xxxxxxxxxx wrote: >> >> > On Sun, Sep 17, 2017 at 3:36 PM, Paul E. McKenney >> > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: >> >> Hello! >> >> >> >> Rough notes from our discussion last Thursday. Please reply to the >> >> group with any needed elaborations or corrections. >> >> >> >> Adding Andy and Michael on CC since this most closely affects their >> >> architectures. Also adding Dave Watson and Maged Michael because >> >> the preferred approach requires that processes wanting to use the >> >> lightweight sys_membarrier() do a registration step. >> > >> > Not to be too much of a curmudgeon, but I think that there should be a >> > real implementation of the isync membarrier before this get merged. >> > This series purports to solve two problems, ppc barriers and x86 >> > exit-without-isync, but it's very hard to evaluate whether it actually >> > solves the latter problem given the complete lack of x86 or isync code >> > in the current RFC. >> > >> > It still seems to me that you won't get any particular advantage for >> > using this registration mechanism on x86 even when you implement >> > isync. Unless I've misunderstood, the only real issue on x86 is that >> > you need a helper like arch_force_isync_before_usermode(), and that >> > helper doesn't presently exist. That means that this whole patchset >> > is standing on very dangerous ground: you'll end up with an efficient >> > implementation that works just fine without even requesting >> > registration on every architecture except ppc. That way lies >> > userspace bugs. >> >> My proposed RFC for private expedited membarrier enforces that all >> architectures perform the registration step. Using the "PRIVATE_EXPEDITED" >> command without prior process registration returns an error on all >> architectures. The goal here is to make all architectures behave in the >> same way, and it allows us to rely on process registration to deal >> with future arch-specific optimizations. > > Fair enough. > > That being said, on same architectures (which may well be all but > PPC), it might be nice if the registration call literally just sets a > flag in the mm saying that it happened so that the registration > enforcement can be done. My RFC patch does exactly that. :-) Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
