Re: [Patch 2/2] tools/memory-model: Provide exact SRCU semantics

[Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>]

On 1/25/2023 11:52 PM, Alan Stern wrote:
> On Wed, Jan 25, 2023 at 10:04:29PM +0100, Jonas Oberhauser wrote:
>>
>> On 1/25/2023 9:21 PM, Alan Stern wrote:
>>>    (* Validate nesting *)
>>>    flag ~empty Srcu-lock \ domain(srcu-rscs) as unmatched-srcu-lock
>>>    flag ~empty Srcu-unlock \ range(srcu-rscs) as unmatched-srcu-unlock
>>> +flag ~empty (srcu-rscs^-1 ; srcu-rscs) \ id as multiple-srcu-matches
>> [...] 
>>>    // 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); }
>> How do you feel about introducing Srcu-up and Srcu-down with this patch?
> Why invent new classes for them?  They are literally the same operation
> as Srcu-lock and Srcu-unlock; the only difference is how the kernel's
> lockdep checker treats them.
I don't think they're necessarily implemented in a compatible way, so

r = srcu_lock(s);
srcu_up(s,r);

might not actually work, but would currently be ok'ed by LKMM. With 
different classes you could state
   flag ~empty [Srcu-lock];srcu-rscs;[Srcu-up] as srcu-mismatch-lock-to-up
   flag ~empty [Srcu-down];srcu-rscs;[Srcu-unlock] as 
srcu-mismatch-down-to-unlock

I think with the current implementation this code might work, but I 
don't feel like this is inherently true.

You could then also go ahead and define the "same CPU" requirement as a 
flag for lock and unlock specifically, like
   flag ~empty [Srcu-lock];srcu-rscs & ext as srcu-lock-unlock-mismatch-CPU
or so.

Best wishes, jonas