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Date:   Fri, 23 Feb 2018 00:31:20 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Andrea Parri <parri.andrea@gmail.com>
Subject: Re: [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to
 work with multiple kinds of dependencies
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On Thu, Feb 22, 2018 at 04:51:43PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 22, 2018 at 04:30:34PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 11:12:10PM +0800, Boqun Feng wrote:
> > > On Thu, Feb 22, 2018 at 03:26:14PM +0100, Peter Zijlstra wrote:
> >=20
> > > > However, I would suggest:
> > > >=20
> > > > static inline bool is_xr(u16 dep)
> > > > {
> > > > 	return !!(dep & (DEP_NR_MASK | DEP_RR_MASK));
> > > > }
> > > >=20
> > > > static inline bool is_rx(u16 dep)
> > > > {
> > > > 	return !!(dep & (DEP_RN_MASK | DEP_RR_MASK));
> > > > }
> > > >=20
> > > > 		/* Skip *R -> R* relations */
> > > > 		if (have_xr && is_rx(entry->dep))
> > > > 			continue;
> > >=20
> > > I don't think this works, if we pick a *R for previous entry, and for
> > > current entry, we have RR, NN and NR, your approach will skip the
> > > current entry, but actually we can pick NN or NR (of course, in __bfs=
(),
> > > we can greedily pick NN, because if NR causes a deadlock, so does NN).
> >=20
> > I don't get it, afaict my suggestion is identical.
> >=20
> > You skip condition: pick_dep() < 0, evaluates to:
> >=20
> > 	is_rr && (!NN_MASK && !NR_MASK) :=3D
> > 	is_rr && (RN_MASK | RR_MASK)
> >=20
> > Which is exactly what I have.
>=20
> Ooh, I think I see what I did wrong, would something like:
>=20
> 	if (have_xr && !is_nx(entry-dep))
>=20
> work? That's a lot harder to argue about though, still much better than

I think it works. Although I prefer use name "has_nx" for the fucntion.

> that tri-state pick thing.
>=20

Agree.

> > If that is satisfied, you set entry->is_rr to pick_dep(), which his
> > harder to evaluate, but is something like:
> >=20
> > 	is_rr && NR_MASK || !(NN_MASK | RN_MASK) :=3D

If is_rr is true and NN_MASK is true, pick_dep() will return 0, however,
your expression will return NR_MASK.

	entry->have_xr =3D !(has_nn(entry->dep) || (!is_rr && has_rn(entry->dep)));
	              :=3D !has_nn(entry->dep) && (is_rr || !has_rn(entry->dep))

Regards,
Boqun

> > 	is_rr && NR_MASK || (NR_MASK | RR_MASK) :=3D
> > 	(NR_MASK | RR_MASK)
> >=20
> > (because is_rr && RR_MASK will have been skipped)
> >=20
> > > >=20
> > > > 		entry->have_xr =3D is_xr(entry->dep);
>=20
> This one I think is still correct though.

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