From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
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
Date: Thu, 22 Feb 2018 17:08:11 +0100 [thread overview]
Message-ID: <20180222160811.GT25201@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180222151210.jwxjchywk4jfecyf@tardis>
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.
next prev parent reply other threads:[~2018-02-22 16:08 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-22 7:08 [RFC tip/locking/lockdep v5 00/17] lockdep: Support deadlock detection for recursive read locks Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 01/17] lockdep: Demagic the return value of BFS Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 02/17] lockdep: Make __bfs() visit every dependency until a match Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 03/17] lockdep: Redefine LOCK_*_STATE* bits Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 04/17] lockdep: Introduce lock_list::dep Boqun Feng
2018-02-23 11:55 ` Peter Zijlstra
2018-02-23 12:37 ` Boqun Feng
2018-02-24 5:32 ` Boqun Feng
2018-02-24 6:30 ` Boqun Feng
2018-02-24 8:38 ` Peter Zijlstra
2018-02-24 9:00 ` Boqun Feng
2018-02-24 9:26 ` Boqun Feng
2018-02-26 9:00 ` Peter Zijlstra
2018-02-26 10:15 ` Boqun Feng
2018-02-26 10:20 ` Boqun Feng
2018-02-24 7:31 ` Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to work with multiple kinds of dependencies Boqun Feng
2018-02-22 14:26 ` Peter Zijlstra
2018-02-22 15:12 ` Boqun Feng
2018-02-22 15:30 ` Peter Zijlstra
2018-02-22 15:51 ` Peter Zijlstra
2018-02-22 16:31 ` Boqun Feng
2018-02-23 5:02 ` Boqun Feng
2018-02-23 11:15 ` Peter Zijlstra
2018-02-22 16:08 ` Peter Zijlstra [this message]
2018-02-22 16:34 ` Boqun Feng
2018-02-22 16:32 ` Peter Zijlstra
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 06/17] lockdep: Support deadlock detection for recursive read in check_noncircular() Boqun Feng
2018-02-22 14:54 ` Peter Zijlstra
2018-02-22 15:16 ` Peter Zijlstra
2018-02-22 15:44 ` Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 07/17] lockdep: Adjust check_redundant() for recursive read change Boqun Feng
2018-02-22 17:29 ` Peter Zijlstra
2018-03-16 8:20 ` Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 08/17] lockdep: Fix recursive read lock related safe->unsafe detection Boqun Feng
2018-02-22 17:41 ` Peter Zijlstra
2018-02-22 17:46 ` Peter Zijlstra
2018-02-23 8:21 ` Boqun Feng
2018-02-23 8:58 ` Boqun Feng
2018-02-23 11:36 ` Peter Zijlstra
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 09/17] lockdep: Add recursive read locks into dependency graph Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 10/17] lockdep/selftest: Add a R-L/L-W test case specific to chain cache behavior Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 11/17] lockdep: Take read/write status in consideration when generate chainkey Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 12/17] lockdep/selftest: Unleash irq_read_recursion2 and add more Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 13/17] lockdep/selftest: Add more recursive read related test cases Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 14/17] Revert "locking/lockdep/selftests: Fix mixed read-write ABBA tests" Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 15/17] lockdep: Reduce the size of lock_list Boqun Feng
2018-02-23 11:38 ` Peter Zijlstra
2018-02-23 12:40 ` Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 16/17] lockdep: Documention for recursive read lock detection reasoning Boqun Feng
2018-02-24 22:53 ` Andrea Parri
2018-02-27 2:32 ` Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 17/17] MAINTAINERS: Add myself as a LOCKING PRIMITIVES reviewer Boqun Feng
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