linux-kernel - Re: [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to work with multiple kinds of dependencies

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Date:   Fri, 23 Feb 2018 13:02:09 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to
 work with multiple kinds of dependencies

On Fri, Feb 23, 2018 at 12:31:20AM +0800, Boqun Feng wrote:
> 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:
> > > 
> > > > > However, I would suggest:
> > > > > 
> > > > > static inline bool is_xr(u16 dep)
> > > > > {
> > > > > 	return !!(dep & (DEP_NR_MASK | DEP_RR_MASK));
> > > > > }
> > > > > 
> > > > > static inline bool is_rx(u16 dep)
> > > > > {
> > > > > 	return !!(dep & (DEP_RN_MASK | DEP_RR_MASK));
> > > > > }
> > > > > 
> > > > > 		/* Skip *R -> R* relations */
> > > > > 		if (have_xr && is_rx(entry->dep))
> > > > > 			continue;
> > > > 
> > > > 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).
> > > 
> > > I don't get it, afaict my suggestion is identical.
> > > 
> > > You skip condition: pick_dep() < 0, evaluates to:
> > > 
> > > 	is_rr && (!NN_MASK && !NR_MASK) :=
> > > 	is_rr && (RN_MASK | RR_MASK)
> > > 
> > > Which is exactly what I have.
> > 
> > Ooh, I think I see what I did wrong, would something like:
> > 
> > 	if (have_xr && !is_nx(entry-dep))
> > 
> > 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.
> > 
> 
> Agree.
> 
> > > If that is satisfied, you set entry->is_rr to pick_dep(), which his
> > > harder to evaluate, but is something like:
> > > 
> > > 	is_rr && NR_MASK || !(NN_MASK | RN_MASK) :=
> 
> If is_rr is true and NN_MASK is true, pick_dep() will return 0, however,
> your expression will return NR_MASK.
> 

It was too late last night, I was meant to say, the correct
entry->have_xr should be as follow:

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

so it seems that we have to introduce is_{nn,rn,nx}(), I'm not sure
introducing three one-off helpers is a good direction to go. One benefit
of using pick_dep() is that we can keep the whole logic in one function.
Thoughts?

Regards,
Boqun

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



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