linux-kernel - Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

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Date:   Wed, 6 Sep 2017 09:48:18 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Byungchul Park <max.byungchul.park@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, david@...morbit.com,
        Johannes Berg <johannes@...solutions.net>, oleg@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel-team@....com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

On Tue, Sep 05, 2017 at 03:46:43PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 05, 2017 at 07:58:38PM +0900, Byungchul Park wrote:
> > On Tue, Sep 05, 2017 at 07:31:44PM +0900, Byungchul Park wrote:
> > > Recursive-read and the hint I proposed(a.k.a. might) should be used for
> > > their different specific applications. Both meaning and constraints of
> > > them are totally different.
> > > 
> > > Using a right function semantically is more important than making it
> > > just work, as you know. Wrong?
> 
> > Of course, in the following cases, the results are same:
> > 
> >    recursive-read(A) -> recursive-read(A), is like nothing, and also
> >    might(A)          -> might(A)         , is like nothing.
> > 
> >    recursive-read(A) -> lock(A), end in a deadlock, and also
> >    might(A)          -> lock(A), end in a deadlock.
> 
> And these are exactly the cases we need.
> 
> > Futhermore, recursive-read-might() can be used if needed, since their
> > semantics are orthogonal so they can be used in mixed forms.
> > 
> > I really hope you accept the new semantics... I think current workqueue
> > code exactly needs the semantics.
> 
> I really don't want to introduce this extra state if we don't have to.
> And as you already noted, this 'might' thing of yours doesn't belong in
> the .read argument, since as you say its orthogonal.

Right. Of course, it can be changed to be a proper form if allowed. I
was afraid to introduce another new function instead of using an arg.

> recursive-read
> wait_for_completion()
> 			recursive-read
> 			complete()
> 
> is fundamentally not a deadlock, we don't need anything extra.

It might be ok wrt the workqueue. But, I think generally the
recursive-read is not a good option for that purpose.