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

Paul E. McKenney
2023-01-19T23:12:01Z

On Thu, Jan 19, 2023 at 05:04:49PM -0500, Alan Stern wrote:
> 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.

Even better!

> > > (* 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.

Got it, thank you!

> > > (* 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.

Given that lockdep already complains about that sort of thing in the
Linux kernel, agreed, it is not worth much trouble at all.

Thanx, Paul