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

[Paul E. McKenney]

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.

In kernels built with CONFIG_PROVE_LOCKING=y (AKA built with lockdep
enabled), lockdep would complain about having an srcu_read_lock() with
no matching srcu_read_unlock().  Kernels built without lockdep (that is,
kernels actually used in production) would be happy with this.

So as Jonas suspects, this should be classified as not actually working.

>                                                                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
>