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

[Jonas Oberhauser]

On 1/19/2023 10:53 PM, Paul E. McKenney wrote:
> On Thu, Jan 19, 2023 at 02:51:53PM -0500, Alan Stern wrote:
> > On Thu, Jan 19, 2023 at 10:41:07AM -0800, Paul E. McKenney wrote:
> > > In contrast, this actually needs srcu_down_read() and srcu_up_read():
> > >
> > > ------------------------------------------------------------------------
> > >
> > > C C-srcu-nest-6
> > >
> > > (*
> > >   * Result: Never
> > >   *
> > >   * Flag unbalanced-srcu-locking
> > >   * This would be valid for srcu_down_read() and srcu_up_read().
> > >   *)
> > >
> > > {}
> > >
> > > P0(int *x, int *y, struct srcu_struct *s1, int *idx)
> > > {
> > > 	int r2;
> > > 	int r3;
> > >
> > > 	r3 = srcu_down_read(s1);
> > > 	WRITE_ONCE(*idx, r3);
> > > 	r2 = READ_ONCE(*y);
> > > }
> > >
> > > P1(int *x, int *y, struct srcu_struct *s1, int *idx)
> > > {
> > > 	int r1;
> > > 	int r3;
> > >
> > > 	r1 = READ_ONCE(*x);
> > > 	r3 = READ_ONCE(*idx);
> > > 	srcu_up_read(s1, r3);
> > > }
> > >
> > > P2(int *x, int *y, struct srcu_struct *s1)
> > > {
> > > 	WRITE_ONCE(*y, 1);
> > > 	synchronize_srcu(s1);
> > > 	WRITE_ONCE(*x, 1);
> > > }
> > >
> > > locations [0:r1]
> > > exists (1:r1=1 /\ 0:r2=0)
> > I modified this litmus test by adding a flag variable with an
> > smp_store_release in P0, an smp_load_acquire in P1, and a filter clause
> > to ensure that P1 reads the flag and idx from P1.

This sounds like good style.
I suppose this is already flagged as mismatched srcu_unlock(), in case 
you accidentally read from the initial write?

> > It turns out that the idea of removing rf edges from Srcu-unlock events
> > doesn't work well.  The missing edges mess up herd's calculation of the
> > fr relation and the coherence axiom.  So I've gone back to filtering
> > those edges out of carry-dep.
> >
> > Also, Boqun's suggestion for flagging ordinary accesses to SRCU
> > structures no longer works, because the lock and unlock operations now
> > _are_ normal accesses.  I removed that check too, but it shouldn't hurt
> > much because I don't expect to encounter litmus tests that try to read
> > or write srcu_structs directly.
> Agreed.  I for one would definitely have something to say about an
> SRCU-usage patch that directly manipulated a srcu_struct structure!  ;-)

Wouldn't the point of having it being flagged be that herd (or similar 
tools) would be having something to say long before it has to reach your 
pair of eyes?
I don't think Boqun's patch is hard to repair.
Besides the issue you mention, I think it's also missing Sync-srcu, 
which seems to be linked by loc based on its first argument.

How about something like this?

let ALL-LOCKS = LKR | LKW | UL | LF | RU | Srcu-lock | Srcu-unlock | 
Sync-srcu   flag ~empty ~[ALL_LOCKS | IW] ; loc ; [ALL-LOCKS] as mixed-lock-accesses

If you're using something that isn't a lock or intial write on the same location as a lock, you get the flag.

Best wishes,
  jonas