Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po

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

On Sun, Jan 29, 2023 at 03:09:00PM -0800, Paul E. McKenney wrote:
> On Sun, Jan 29, 2023 at 01:43:53PM -0800, Boqun Feng wrote:
> > in lock.cat: 
> > 
> > 	(* Allow up to one unmatched LKW per location; more must deadlock *)
> > 	let UNMATCHED-LKW = LKW \ domain(critical)
> > 	empty ([UNMATCHED-LKW] ; loc ; [UNMATCHED-LKW]) \ id as unmatched-locks
> > 
> > we rule out deadlocks from the execution candidates we care about.
> 
> Thank you, Boqun!

Actually that's only part of it.  The other part is rather obscure:

(* Generate all co relations, including LKW events but not UL *)
let co0 = co0 | ([IW] ; loc ; [LKW]) |
	(([LKW] ; loc ; [UNMATCHED-LKW]) \ [UNMATCHED-LKW])

Implicitly this says that any lock with no corresponding unlock must 
come last in the coherence order, which implies the unmatched-locks rule 
(since only one lock event can be last).  By itself, the unmatched-locks 
rule would not prevent P3 from executing before P1, provided P1 executes 
both its lock and unlock.

Alan