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

Paul E. McKenney
2023-01-20T15:32:50Z

On Fri, Jan 20, 2023 at 01:51:01PM +0100, Jonas Oberhauser wrote:
> I'm not going to get it right today, am I?

Believe me, I know that feeling! Open-source development is therefore
an extremely good character-building exercise. At least that is what
I keep telling myself. ;-)

> +let srcu-rscs = ([Srcu-lock] ; (data ; [~ Srcu-unlock] ; rfe) * ; data ;
> [Srcu-unlock]) & loc
>
> I see now that I copied the format from your message but without realizing
> the original had a `|` where I have a `;`.
> I hope this version is finally right and perhaps more natural than the (data
> | rf) version, considering rf can't actually appear in most places and this
> more closely matches carry-dep;data.
> But of course feel free to use
> +let srcu-rscs = ([Srcu-lock] ; (data  | [~ Srcu-unlock] ; rf)+ ;
> [Srcu-unlock]) & loc
> instead if you prefer.

Ah, herd7 could see an rf link between any srcu_read_unlock() and any
"later" srcu_read_lock(), couldn't it? Good catch!!!

I took this last one, adding parentheses that might well be unnecessary.
(You see, herd7 was complaining about cut-and-paste, possibly due to
alternative character sets, so I indulged in a bit of diagnostic-driven
development.)

let srcu-rscs = ([Srcu-lock] ; (data | ([~ Srcu-unlock] ; rf))+ ;
[Srcu-unlock]) & loc

The reason for favoring "rf" over "rfe" is the possibility of a litmus
test where the process containing the srcu_down_read() sometimes but
not always also has the matching srcu_up_read(). Perhaps a pair of "if"
statements control which process does the matching srcu_up_read().

With this change, all of the C-srcu-nest-*.litmus tests act as expected.

And thank you!!!

Thanx, Paul

> have fun, jonas
>
>
> On 1/20/2023 1:34 PM, Jonas Oberhauser wrote:
> > I just realized I made a mistake in my earlier response to this message;
> > you still need the rf for passing the cookie across threads.
> > Perhaps it's better to just also exclude srcu_unlock type events
> > explicitly here.
> >
> > +let srcu-rscs = ([Srcu-lock] ; (data ; [~ Srcu-unlock] ; rf) + ;
> > [Srcu-unlock]) & loc
> >
> >
> > best wishes,
> > jonas
> >
> > On 1/20/2023 4:55 AM, 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.
> > > >
> > > > With the patch below, the results were as expected:
> > > >
> > > > Test C-srcu-nest-6 Allowed
> > > > States 3
> > > > 0:r1=0; 0:r2=0; 1:r1=0;
> > > > 0:r1=0; 0:r2=1; 1:r1=0;
> > > > 0:r1=0; 0:r2=1; 1:r1=1;
> > > > No
> > > > Witnesses
> > > > Positive: 0 Negative: 3
> > > > Condition exists (1:r1=1 /\ 0:r2=0)
> > > > Observation C-srcu-nest-6 Never 0 3
> > > > Time C-srcu-nest-6 0.04
> > > > Hash=2b010cf3446879fb84752a6016ff88c5
> > > >
> > > > 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.
> > > >
> > > > Alan
> > > >
> > > >
> > > >
> > > > Index: usb-devel/tools/memory-model/linux-kernel.bell
> > > > ===================================================================
> > > > --- usb-devel.orig/tools/memory-model/linux-kernel.bell
> > > > +++ usb-devel/tools/memory-model/linux-kernel.bell
> > > > @@ -53,38 +53,30 @@ let rcu-rscs = let rec
> > > >       in matched
> > > >     (* Validate nesting *)
> > > > -flag ~empty Rcu-lock \ domain(rcu-rscs) as unbalanced-rcu-locking
> > > > -flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-locking
> > > > +flag ~empty Rcu-lock \ domain(rcu-rscs) as unbalanced-rcu-lock
> > > > +flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-unlock
> > > >     (* 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
> > > > +let srcu-rscs = ([Srcu-lock] ; (data | rf)+ ; [Srcu-unlock]) & loc
> > > >     (* Validate nesting *)
> > > > -flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
> > > > -flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
> > > > +flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-lock
> > > > +flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-unlock
> > > > +flag ~empty (srcu-rscs^-1 ; srcu-rscs) \ id as multiple-srcu-matches
> > > >     (* 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
> > > > +flag ~empty different-values(srcu-rscs) as bad-srcu-value-match
> > > >     (* Compute marked and plain memory accesses *)
> > > >   let Marked = (~M) | IW | Once | Release | Acquire |
> > > > domain(rmw) | range(rmw) |
> > > > -        LKR | LKW | UL | LF | RL | RU
> > > > +         LKR | LKW | UL | LF | RL | RU | Srcu-lock | Srcu-unlock
> > > >   let Plain = M \ Marked
> > > >     (* Redefine dependencies to include those carried through
> > > > plain accesses *)
> > > > -let carry-dep = (data ; rfi)*
> > > > +let carry-dep = (data ; [~ Srcu-unlock] ; rfi)*
> > > >   let addr = carry-dep ; addr
> > > >   let ctrl = carry-dep ; ctrl
> > > >   let data = carry-dep ; data
> > > > Index: usb-devel/tools/memory-model/linux-kernel.def
> > > > ===================================================================
> > > > --- usb-devel.orig/tools/memory-model/linux-kernel.def
> > > > +++ usb-devel/tools/memory-model/linux-kernel.def
> > > > @@ -49,8 +49,10 @@ synchronize_rcu() { __fence{sync-rcu}; }
> > > >   synchronize_rcu_expedited() { __fence{sync-rcu}; }
> > > >     // SRCU
> > > > -srcu_read_lock(X)  __srcu{srcu-lock}(X)
> > > > -srcu_read_unlock(X,Y) { __srcu{srcu-unlock}(X,Y); }
> > > > +srcu_read_lock(X) __load{srcu-lock}(*X)
> > > > +srcu_read_unlock(X,Y) { __store{srcu-unlock}(*X,Y); }
> > > > +srcu_down_read(X) __load{srcu-lock}(*X)
> > > > +srcu_up_read(X,Y) { __store{srcu-unlock}(*X,Y); }
> > > >   synchronize_srcu(X)  { __srcu{sync-srcu}(X); }
> > > >   synchronize_srcu_expedited(X)  { __srcu{sync-srcu}(X); }
> > > And for some initial tests:
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-1.litmus
> > >
> > >
> > >     "Flag multiple-srcu-matches" but otherwise OK.
> > >     As a "hail Mary" exercise, I used r4 for the second SRCU
> > >     read-side critical section, but this had no effect.
> > >     (This flag is expected and seen for #4 below.)
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-2.litmus
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-3.litmus
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-4.litmus
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-5.litmus
> > >
> > >
> > >     All as expected.
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-6.litmus
> > >
> > >
> > >     Get "Flag unbalanced-srcu-lock" and "Flag unbalanced-srcu-unlock",
> > >     but this is srcu_down_read() and srcu_up_read(), where this should
> > >     be OK.    Ah, but I need to do the release/acquire/filter
> > > trick.  Once
> > >     I did that, it works as expected.
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-7.litmus
> > >
> > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-8.litmus
> > >
> > >
> > >     Both as expected.
> > >
> > > Getting there!!!
> > >
> > > I also started a regression test, hopefully without pilot error.  :-/
> > >
> > >                             Thanx, Paul
> >
>