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

Jonas Oberhauser
2023-01-20T12:51:46Z

I'm not going to get it right today, am I?

+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.

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
>