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Date:   Sat, 24 Feb 2018 13:32:50 +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 04/17] lockdep: Introduce
 lock_list::dep
Message-ID: <20180224053250.ketrlbq4gtx573qo@tardis>
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On Fri, Feb 23, 2018 at 08:37:32PM +0800, Boqun Feng wrote:
> On Fri, Feb 23, 2018 at 12:55:20PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 03:08:51PM +0800, Boqun Feng wrote:
> > > @@ -1012,6 +1013,33 @@ static inline bool bfs_error(enum bfs_result r=
es)
> > >  	return res < 0;
> > >  }
> > > =20
> > > +#define DEP_NN_BIT 0
> > > +#define DEP_RN_BIT 1
> > > +#define DEP_NR_BIT 2
> > > +#define DEP_RR_BIT 3
> > > +
> > > +#define DEP_NN_MASK (1U << (DEP_NN_BIT))
> > > +#define DEP_RN_MASK (1U << (DEP_RN_BIT))
> > > +#define DEP_NR_MASK (1U << (DEP_NR_BIT))
> > > +#define DEP_RR_MASK (1U << (DEP_RR_BIT))
> > > +
> > > +static inline unsigned int __calc_dep_bit(int prev, int next)
> > > +{
> > > +	if (prev =3D=3D 2 && next !=3D 2)
> > > +		return DEP_RN_BIT;
> > > +	if (prev !=3D 2 && next =3D=3D 2)
> > > +		return DEP_NR_BIT;
> > > +	if (prev =3D=3D 2 && next =3D=3D 2)
> > > +		return DEP_RR_BIT;
> > > +	else
> > > +		return DEP_NN_BIT;
> > > +}
> > > +
> > > +static inline unsigned int calc_dep(int prev, int next)
> > > +{
> > > +	return 1U << __calc_dep_bit(prev, next);
> > > +}
> > > +
> > >  static enum bfs_result __bfs(struct lock_list *source_entry,
> > >  			     void *data,
> > >  			     int (*match)(struct lock_list *entry, void *data),
> > > @@ -1921,6 +1949,16 @@ check_prev_add(struct task_struct *curr, struc=
t held_lock *prev,
> > >  		if (entry->class =3D=3D hlock_class(next)) {
> > >  			if (distance =3D=3D 1)
> > >  				entry->distance =3D 1;
> > > +			entry->dep |=3D calc_dep(prev->read, next->read);
> > > +		}
> > > +	}
> > > +
> > > +	/* Also, update the reverse dependency in @next's ->locks_before li=
st */
> > > +	list_for_each_entry(entry, &hlock_class(next)->locks_before, entry)=
 {
> > > +		if (entry->class =3D=3D hlock_class(prev)) {
> > > +			if (distance =3D=3D 1)
> > > +				entry->distance =3D 1;
> > > +			entry->dep |=3D calc_dep(next->read, prev->read);
> > >  			return 1;
> > >  		}
> > >  	}
> >=20
> > I think it all becomes simpler if you use only 2 bits. Such that:
> >=20
> >   bit0 is the prev R (0) or N (1) value,
> >   bit1 is the next R (0) or N (1) value.
> >=20
> > I think this should work because we don't care about the empty set
> > (currently 0000) and all the complexity in patch 5 is because we can
> > have R bits set when there's also N bits. The concequence of that is
> > that we cannot replace ! with ~ (which is what I kept doing).
> >=20
> > But with only 2 bits, we only track the strongest relation in the set,
> > which is exactly what we appear to need.
> >=20
>=20
> But if we only have RN and NR, both bits will be set, we can not check
> whether we have NN or not. Consider we have:
>=20
> 	A -(RR)-> B
> 	B -(NR)-> C and B -(RN)-> C
> 	C -(RN)-> A
>=20
> this is not a deadlock case, but with "two bits" approach, we can not
> differ this with:
>=20
> 	A -(RR)-> B
> 	B -(NN)-> C
> 	C -(RN)-> A
>=20
> , which is a deadlock.
>=20
> But maybe "three bits" (NR, RN and NN bits) approach works, that is if
> ->dep is 0, we indicates this is only RR, and is_rx() becomes:
>=20
> 	static inline bool is_rx(u8 dep)
> 	{
> 		return !(dep & (NR_MASK | NN_MASK));
> 	}
>=20
> and is_xr() becomes:
>=20
> 	static inline bool is_xr(u8 dep)
> 	{
> 		return !(dep & (RN_MASK | NN_MASK));
> 	}
>=20
> , with this I think your simplification with have_xr works, thanks!
>=20

Ah! I see. Actually your very first approach works, except the
definitions of is_rx() and ir_xr() are wrong. In that approach, you
define
=09
	static inline bool is_rx(u8 dep)
	{
		return !!(dep & (DEP_RR_MASK | DEP_RN_MASK);
	}

, which means "whether we have a R* dependency?". But in fact, what we
need to check is "whether we _only_ have R* dependencies?", if so and
have_xr is true, that means we could only have a -(*R)-> A -(R*)-> if we
pick the next dependency, and that means we should skip. So my new
definition above works, and I think we better name it as only_rx() to
avoid confusion? Ditto for is_xr().

I also reorder bit number for each kind of dependency, so that we have a
simple __calc_dep_bit(), see the following:

	/*
	 * DEP_*_BIT in lock_list::dep
	 *
	 * For dependency @prev -> @next:
	 *
	 *   RR: both @prev and @next are recursive read locks, i.e. ->read =3D=3D=
 2.
	 *   RN: @prev is recursive and @next is non-recursive.
	 *   NR: @prev is a not recursive and @next is recursive.
	 *   NN: both @prev and @next are non-recursive.
	 *=20
	 * Note that we define the value of DEP_*_BITs so that:
	 * 	bit0 is prev->read !=3D 2
	 * 	bit1 is next->read !=3D 2
	 */
	#define DEP_RR_BIT 0
	#define DEP_RN_BIT 1
	#define DEP_NR_BIT 2
	#define DEP_NN_BIT 3

	#define DEP_RR_MASK (1U << (DEP_RR_BIT))
	#define DEP_RN_MASK (1U << (DEP_RN_BIT))
	#define DEP_NR_MASK (1U << (DEP_NR_BIT))
	#define DEP_NN_MASK (1U << (DEP_NN_BIT))

	static inline unsigned int
	__calc_dep_bit(struct held_lock *prev, struct held_lock *next)
	{
		return (prev->read !=3D 2) + ((next->read !=3D 2) << 1)
	}

	static inline u8 calc_dep(struct held_lock *prev, struct held_lock *next)
	{
		return 1U << __calc_dep_bit(prev, next);
	}

 	static inline bool only_rx(u8 dep)
 	{
 		return !(dep & (DEP_NR_MASK | DEP_NN_MASK));
 	}

 	static inline bool only_xr(u8 dep)
 	{
 		return !(dep & (DEP_NR_MASK | DEP_NN_MASK));
 	}

Note that we actually don't need DEP_RR_BIT, but I leave it there for
implementation simplicity. With this, your check and set below works.

Thoughts?

Regards,
Boqun

> >=20
> >=20
> > 	if (have_xr && is_rx(entry->dep))
> > 		continue;
> >=20
> > 	entry->have_xr =3D is_xr(entry->dep);
> >=20
> >=20
> > Or did I mess that up somewhere?


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