linux-kernel - Re: [PATCH v8 00/14] lockdep: Implement crossrelease feature

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Date:   Wed, 16 Aug 2017 18:38:36 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Boqun Feng <boqun.feng@...il.com>, mingo@...nel.org,
        peterz@...radead.org
Cc:     Thomas Gleixner <tglx@...utronix.de>, walken@...gle.com,
        kirill@...temov.name, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org, willy@...radead.org,
        npiggin@...il.com, kernel-team@....com
Subject: Re: [PATCH v8 00/14] lockdep: Implement crossrelease feature

On Wed, Aug 16, 2017 at 05:06:23PM +0900, Byungchul Park wrote:
> On Wed, Aug 16, 2017 at 04:14:21PM +0900, Byungchul Park wrote:
> > On Wed, Aug 16, 2017 at 01:58:08PM +0800, Boqun Feng wrote:
> > > > I'm not sure this caused the lockdep warning but, if they belongs to the
> > > > same class even though they couldn't be the same instance as you said, I
> > > > also think that is another problem and should be fixed.
> > > > 
> > > 
> > > My point was more like this is a false positive case, which we should
> > > avoid as hard as we can, because this very case doesn't look like a
> > > deadlock to me.
> > > 
> > > Maybe the pattern above does exist in current kernel, but we need to
> > > guide/adjust lockdep to find the real case showing it's happening.
> > 
> > As long as they are initialized as a same class, there's no way to
> > distinguish between them within lockdep.
> > 
> > And I also think we should avoid false positive cases. Do you think
> > there are many places where completions are initialized in a same place
> > even though they could never be the same instance?
> > 
> > If no, it would be better to fix it whenever we face it, as you did.
> 
> BTW, of course, the same problem would have occured when applying
> lockdep for the first time. How did you solve it?
> 
> I mean that lockdep basically identifies classes even for typical locks
> with the call site. So two locks could be the same class even though
> they should not be the same. Of course, for now, we avoid the problemaic
> cases with sub-class. Anyway, the problems certainly would have arised
             ^
             or setting a class or re-design code like what Boqun
             suggested. And so on...

> for the first time. I want to follow that solution you did.
> 
> Thanks,
> Byungchul