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

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