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

[Alan Stern <stern@rowland.harvard.edu>]

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.

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.

Alan