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

["Paul E. McKenney" <paulmck@kernel.org>]

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