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

Paul E. McKenney
2023-01-20T15:39:39Z

On Fri, Jan 20, 2023 at 10:43:10AM +0100, Jonas Oberhauser wrote:
>
>
> 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 might, except that a filter clause excludes this case. Here is the
updated test:

C C-srcu-nest-6

(*
* Result: Never
*
* This would be valid for srcu_down_read() and srcu_up_read().
*)

{}

P0(int *x, int *y, struct srcu_struct *s1, int *idx, int *f)
{
int r2;
int r3;

r3 = srcu_down_read(s1);
WRITE_ONCE(*idx, r3);
r2 = READ_ONCE(*y);
smp_store_release(f, 1);
}

P1(int *x, int *y, struct srcu_struct *s1, int *idx, int *f)
{
int r1;
int r3;
int r4;

r4 = smp_load_acquire(f);
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]
filter (1:r4=1)
exists (1:r1=1 /\ 0:r2=0)

> > > 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?

That would of course be even better.

> 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.

Wouldn't that unconditionally complain about the first srcu_read_lock()
in a given process? Or am I misreading those statements?

Thanx, Paul