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

[Alan Stern]

On Fri, Jan 13, 2023 at 12:32:41PM -0800, Paul E. McKenney wrote:
> > Making LKMM correctly model all of this has been on my todo list for an
> > embarrassingly long time.
> 
> But there is no time like the present...
> 
> Here is what mainline has to recognize SRCU read-side critical sections:
> 
> ------------------------------------------------------------------------
> 
> (* Compute matching pairs of nested Srcu-lock and Srcu-unlock *)
> let srcu-rscs = let rec
> 	    unmatched-locks = Srcu-lock \ domain(matched)
> 	and unmatched-unlocks = Srcu-unlock \ range(matched)
> 	and unmatched = unmatched-locks | unmatched-unlocks
> 	and unmatched-po = ([unmatched] ; po ; [unmatched]) & loc
> 	and unmatched-locks-to-unlocks =
> 		([unmatched-locks] ; po ; [unmatched-unlocks]) & loc
> 	and matched = matched | (unmatched-locks-to-unlocks \
> 		(unmatched-po ; unmatched-po))
> 	in matched
> 
> (* Validate nesting *)
> flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
> flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
> 
> (* Check for use of synchronize_srcu() inside an RCU critical section *)
> flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
> 
> (* Validate SRCU dynamic match *)
> flag ~empty different-values(srcu-rscs) as srcu-bad-nesting
> 
> ------------------------------------------------------------------------
> 
> And here is what I just now tried:
> 
> ------------------------------------------------------------------------
> 
> (* Compute matching pairs of Srcu-lock and Srcu-unlock *)
> let srcu-rscs = ([Srcu-lock] ; rfi ; [Srcu-unlock]) & loc

This doesn't make sense.  Herd treats srcu_read_lock() as a load
operation (it takes a pointer as argument and returns a value) and
srcu_read_unlock() as a store operation (it takes both a pointer and a
value as arguments and returns nothing).  So you can't connect them
with an rfi link; stores don't "read-from" loads.

I suppose you might be able to connect them with a data dependency,
though.  But then how would you handle situations where two unlock
calls both use the value returned from a single lock call?  You'd have
to check explicitly that srcu-rscs connected each lock with only one
unlock.

Alan

> (* Validate nesting *)
> flag empty srcu-rscs as no-srcu-readers
> flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
> flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
> 
> (* Check for use of synchronize_srcu() inside an RCU critical section *)
> flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
> 
> (* Validate SRCU dynamic match *)
> flag ~empty different-values(srcu-rscs) as srcu-bad-nesting
> 
> ------------------------------------------------------------------------
> 
> This gets me "Flag no-srcu-readers" when running this litmus test:
> 
> ------------------------------------------------------------------------
> 
> C C-srcu-nest-1
> 
> (*
>  * Result: Never
>  *)
> 
> {}
> 
> P0(int *x, int *y, struct srcu_struct *s)
> {
> 	int r1;
> 	int r2;
> 	int r3;
> 
> 	r3 = srcu_read_lock(s);
> 	r1 = READ_ONCE(*x);
> 	srcu_read_unlock(s, r3);
> 	r3 = srcu_read_lock(s);
> 	r2 = READ_ONCE(*y);
> 	srcu_read_unlock(s, r3);
> }
> 
> P1(int *x, int *y, struct srcu_struct *s)
> {
> 	WRITE_ONCE(*y, 1);
> 	synchronize_srcu(s);
> 	WRITE_ONCE(*x, 1);
> }
> 
> locations [0:r1]
> exists (0:r1=1 /\ 0:r2=0)
> 
> ------------------------------------------------------------------------
> 
> So what did I mess up this time?  ;-)
> 
> 							Thanx, Paul