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

Alan Stern
2023-01-19T22:22:37Z

On Thu, Jan 19, 2023 at 01:53:04PM -0800, Paul E. McKenney wrote:
> On Thu, Jan 19, 2023 at 02:51:53PM -0500, Alan Stern wrote:
> > 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
>
> This renaming makes sense to me.

But I'll put it in a separate patch, since it's not related to the main
purpose of this change.

>
> > (* 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
>
> The point of the "+" instead of the "*" is to avoid LKMM being confused by
> an srcu_read_lock() immediately preceding an unrelated srcu_read_unlock(),
> right? Or am I missing something more subtle?

No, and it's not to avoid confusion. It merely indicates that there has
to be at least one instance of data or rf here; we will never have a
case where the lock and the unlock are the same event.

>
> > (* 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)*
>
> The "[~ Srcu-unlock]" matches the store that bridges the data and rfi",
> correct?

Right.

>
> > 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); }
>
> And here srcu_down_read() and srcu_up_read() are synonyms for
> srcu_read_lock() and srcu_read_unlock(), respectively, which I believe
> should suffice.
>
> > synchronize_srcu(X) { __srcu{sync-srcu}(X); }
> > synchronize_srcu_expedited(X) { __srcu{sync-srcu}(X); }
>
> So this looks quite reasonable to me.

Okay, good. In theory we could check for read_lock and read_unlock on
different CPUs, but I don't think it's worth the trouble.

Alan