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

Paul E. McKenney
2023-01-25T21:38:52Z

On Wed, Jan 25, 2023 at 03:46:14PM -0500, Alan Stern wrote:
> On Wed, Jan 25, 2023 at 11:46:51AM -0800, Paul E. McKenney wrote:
> > On Wed, Jan 25, 2023 at 02:08:59PM -0500, Alan Stern wrote:
> > > Why do you want the implementation to forbid it? The pattern of the
> > > litmus test resembles 3+3W, and you don't care whether the kernel allows
> > > that pattern. Do you?
> >
> > Jonas asked a similar question, so I am answering you both here.
> >
> > With (say) a release-WRITE_ONCE() chain implementing N+2W for some
> > N, it is reasonably well known that you don't get ordering, hardware
> > support otwithstanding. After all, none of the Linux kernel, C, and C++
> > memory models make that guarantee. In addition, the non-RCU barriers
> > and accesses that you can use to create N+2W have been in very wide use
> > for a very long time.
> >
> > Although RCU has been in use for almost as long as those non-RCU barriers,
> > it has not been in wide use for anywhere near that long. So I cannot
> > be so confident in ruling out some N+2W use case for RCU.
> >
> > Such a use case could play out as follows:
> >
> > 1. They try LKMM on it, see that LKMM allows it, and therefore find
> > something else that works just as well. This is fine.
> >
> > 2. They try LKMM on it, see that LKMM allows it, but cannot find
> > something else that works just as well. They complain to us,
> > and we either show them how to get the same results some other
> > way or adjust LKMM (and perhaps the implementations) accordingly.
> > These are also fine.
> >
> > 3. They don't try LKMM on it, see that it works when they test it,
> > and they send it upstream. The use case is entangled deeply
> > enough in other code that no one spots it on review. The Linux
> > kernel unconditionally prohibits the cycle. This too is fine.
> >
> > 4. They don't try LKMM on it, see that it works when they test it,
> > and they send it upstream. The use case is entangled deeply
> > enough in other code that no one spots it on review. Because RCU
> > grace periods incur tens of microseconds of latency at a minimum,
> > all tests (almost) always pass, just due to delays and unrelated
> > accesses and memory barriers. Even in kernels built with some
> > future SRCU equivalent of CONFIG_RCU_STRICT_GRACE_PERIOD=y.
> > But the Linux kernel allows the cycle when there is a new moon
> > on Tuesday during a triple solar eclipse of Jupiter, a condition
> > that is eventually met, and at the worst possible time and place.
> >
> > This is absolutely the opposite of fine.
> >
> > I don't want to deal with #4. So this is an RCU-maintainer use case
> > that I would like to avoid. ;-)
>
> Since it is well known that the non-RCU barriers in the Linux kernel, C,
> and C++ do not enforce ordering in n+nW, and seeing as how your litmus
> test relies on an smp_store_release() at one point, I think it's
> reasonable to assume people won't expect it to provide ordering.

The presence of that grace period, which is well known to have super-heavy
ordering properties, will likely reduce the number of people whose
expectations are aligned with LKMM. :-/

Plus it is not easy to create something that meets the LKMM grace-period
requirements without also making it provide this additional ordering on
real systems.

> Ah, but what about a litmus test that relies solely on RCU?
>
> rcu_read_lock Wy=2 rcu_read_lock Wv=2
> Wx=2 synchronize_rcu Wu=2 synchronize_rcu
> Wy=1 Wu=1 Wv=1 Wx=1
> rcu_read_unlock rcu_read_unlock
>
> exists (x=2 /\ y=2 /\ u=2 /\ v=2)
>
> Luckily, this _is_ forbidden by the LKMM. So I think you're okay.

Some times I get lucky! ;-)

The reader-free counterpart of your test is also forbidden, which is no
surprise given that smp_mb() also suffices.

Thanx, Paul