Re: Rough notes from sys_membarrier() lightning BoF
From: Mathieu Desnoyers
Date: Wed Sep 20 2017 - 15:56:20 EST
----- 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. :-)