linux-kernel - Re: About the try to remove cross-release feature entirely by Ingo

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Message-ID: <20171230061624.GA27959@bombadil.infradead.org>
Date:   Fri, 29 Dec 2017 22:16:24 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Byungchul Park <max.byungchul.park@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, david@...morbit.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        oleg@...hat.com, kernel-team@....com, daniel@...ll.ch
Subject: Re: About the try to remove cross-release feature entirely by Ingo

On Fri, Dec 29, 2017 at 04:28:51PM +0900, Byungchul Park wrote:
> On Thu, Dec 28, 2017 at 10:51:46PM -0500, Theodore Ts'o wrote:
> > On Fri, Dec 29, 2017 at 10:47:36AM +0900, Byungchul Park wrote:
> > > 
> > >    (1) The best way: To classify all waiters correctly.
> > 
> > It's really not all waiters, but all *locks*, no?
> 
> Thanks for your opinion. I will add my opinion on you.
> 
> I meant *waiters*. Locks are only a sub set of potential waiters, which
> actually cause deadlocks. Cross-release was designed to consider the
> super set including all general waiters such as typical locks,
> wait_for_completion(), and lock_page() and so on..

I think this is a terminology problem.  To me (and, I suspect Ted), a
waiter is a subject of a verb while a lock is an object.  So Ted is asking
whether we have to classify the users, while I think you're saying we
have extra objects to classify.

I'd be comfortable continuing to refer to completions as locks.  We could
try to come up with a new object name like waitpoints though?

> > In addition, the lock classification system is not documented at all,
> > so now you also need someone who understands the lockdep code.  And
> > since some of these classifications involve transient objects, and
> > lockdep doesn't have a way of dealing with transient locks, and has a
> > hard compile time limit of the number of locks that it supports, to
> > expect a subsystem maintainer to figure out all of the interactions,
> > plus figure out lockdep, and work around lockdep's limitations
> > seems.... not realistic.
> 
> I have to think it more to find out how to solve it simply enough to be
> acceptable. The only solution I come up with for now is too complex.

I want to amplify Ted's point here.  How to use the existing lockdep
functionality is undocumented.  And that's not your fault.  We have
Documentation/locking/lockdep-design.txt which I'm sure is great for
someone who's willing to invest a week understanding it, but we need a
"here's how to use it" guide.

> > Given that once Lockdep reports a locking violation, it doesn't report
> > any more lockdep violations, if there are a large number of false
> > positives, people will not want to turn on cross-release, since it
> > will report the false positive and then turn itself off, so it won't
> > report anything useful.  So if no one turns it on because of the false
> > positives, how does the bitrot problem get resolved?
> 
> The problems come from wrong classification. Waiters either classfied
> well or invalidated properly won't bitrot.

I disagree here.  As Ted says, it's the interactions between the
subsystems that leads to problems.  Everything's goig to work great
until somebody does something in a way that's never been tried before.