Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Alan Stern
2023-01-25T15:34:49Z

On Wed, Jan 25, 2023 at 07:05:20AM -0800, Paul E. McKenney wrote:
> On Wed, Jan 25, 2023 at 02:10:08PM +0100, Jonas Oberhauser wrote:
> >
> >
> > On 1/25/2023 3:20 AM, Paul E. McKenney wrote:
> > > On Tue, Jan 24, 2023 at 08:54:56PM -0500, Alan Stern wrote:
> > > > On Tue, Jan 24, 2023 at 02:54:49PM -0800, Paul E. McKenney wrote:
> > > > >
> > > > > Within the Linux kernel, the rule for a given RCU "domain" is that if
> > > > > an event follows a grace period in pretty much any sense of the word,
> > > > > then that event sees the effects of all events in all read-side critical
> > > > > sections that began prior to the start of that grace period.
> > > > >
> > > > > Here the senses of the word "follow" include combinations of rf, fr,
> > > > > and co, combined with the various acyclic and irreflexive relations
> > > > > defined in LKMM.
> > > > The LKMM says pretty much the same thing. In fact, it says the event
> > > > sees the effects of all events po-before the unlock of (not just inside)
> > > > any read-side critical section that began prior to the start of the
> > > > grace period.
> > > >
> > > > > > And are these anything the memory model needs to worry about?
> > > > > Given that several people, yourself included, are starting to use LKMM
> > > > > to analyze the Linux-kernel RCU implementations, maybe it does.
> > > > >
> > > > > Me, I am happy either way.
> > > > Judging from your description, I don't think we have anything to worry
> > > > about.
> > > Sounds good, and let's proceed on that assumption then. We can always
> > > revisit later if need be.
> > >
> > > Thanx, Paul
> >
> > FWIW, I currently don't see a need for either RCU nor "base" LKMM to have
> > this kind of guarantee.
>
> In the RCU case, it is because it is far easier to provide this guarantee,
> even though it is based on hardware and compilers rather than LKMM,
> than it would be to explain to some random person why the access that
> is intuitively clearly after the grace period can somehow come before it.
>
> > But I'm curious for why it doesn't exist in LKMM -- is it because of Alpha
> > or some other issues that make it hard to guarantee (like a compiler merging
> > two threads and optimizing or something?), or is it simply that it seemed
> > like a complicated guarantee with no discernible upside, or something else?
>
> Because to the best of my knowledge, no one has ever come up with a
> use for 2+2W and friends that isn't better handled by some much more
> straightforward pattern of accesses. So we did not guarantee it in LKMM.
>
> Yes, you could argue that my "ease of explanation" paragraph above is
> a valid use case, but I am not sure that this is all that compelling of
> an argument. ;-)

Are we all talking about the same thing? There were two different
guarantees mentioned above:

The RCU guarantee about writes in a read-side critical section
becoming visible to all CPUs before a later grace period ends;

The guarantee about the 2+2W pattern and friends being
forbidden.

The LKMM includes the first of these but not the second (for the reason
Paul stated).

Alan