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:   Thu, 22 Feb 2018 17:08:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
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 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:
> > On Thu, Feb 22, 2018 at 03:08:52PM +0800, Boqun Feng wrote:
> > > Now we have four kinds of dependencies in the dependency graph, and not
> > > all the pathes carry strong dependencies, for example:
> > > 
> > > 	Given lock A, B, C, if we have:
> > > 
> > > 	CPU1			CPU2
> > > 	=============		==============
> > > 	write_lock(A);		read_lock(B);
> > > 	read_lock(B);		write_lock(C);
> > > 
> > > 	then we have dependencies A--(NR)-->B, and B--(RN)-->C, (NR and
> > > 	RN are to indicate the dependency kind), A actually doesn't have
> > > 	strong dependency to C(IOW, C doesn't depend on A), to see this,
> > > 	let's say we have a third CPU3 doing:
> > > 
> > > 	CPU3:
> > > 	=============
> > > 	write_lock(C);
> > > 	write_lock(A);
> > > 
> > > 	, this is not a deadlock. However if we change the read_lock()
> > > 	on CPU2 to a write_lock(), it's a deadlock then.
> > > 
> > > 	So A --(NR)--> B --(RN)--> C is not a strong dependency path but
> > > 	A --(NR)--> B --(NN)-->C is a strong dependency path.
> > 
> > I'm not really satisfied with the above reasoning. I don't disagree, but
> > if possible it would be nice to have something a little more solid.
> > 
> 
> What do you mean by "solid"? You mean "A --(NR)--> B --(NN)-->C" is too
> abstract, and want something like the below instead:

The above description mostly leaves it as an exercise to the reader to
'proof' ignoring *R -> R* is both safe and complete while that is the
main argument.