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

[Jonas Oberhauser]

On 1/26/2023 5:02 PM, Alan Stern wrote:
> On Thu, Jan 26, 2023 at 12:30:14PM +0100, Jonas Oberhauser wrote:
> > 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.
> I'll let Paul answer this.
>
> >   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.
> Bear in mind that the herd7 model is not obliged to find and warn about
> all possible bugs in a litmus test.  Especially if the same code would
> generate a warning or error when run in the kernel.

I  agree, and indeed it may not even possible to do so (even under a 
hypothetical formal proof that the list of bugs is complete, some might 
not be formalizable in a satisfactory way in herd).
Mostly I'm just trying to see if you have considered the possibilities 
and decided against them intentionally, or if you just hadn't considered 
it. I don't have a strong opinion either way and would be ok with the 
patch as written.

That said, I do like to think of the model as a kind of compact formal 
specification/documentation, including the assumptions the kernel makes 
about how these APIs are to be used.

Best wishes, jonas