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

Jonas Oberhauser
2023-01-20T09:43:55Z

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