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

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Date:   Tue, 5 Sep 2017 11:36:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
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 05:57:27PM +0900, Byungchul Park wrote:
> On Tue, Sep 05, 2017 at 09:19:30AM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 05, 2017 at 09:08:25AM +0200, Peter Zijlstra wrote:
> > > So you worry about max_active==1 ? Or you worry about pool->lock or
> > > about the thread setup? I'm still not sure.
> > 
> > So the thing about pool->lock is that its a leaf lock, we take nothing
> 
> I think the following sentence is a key, I hope...
> 
> Leaf locks can also create dependecies with *crosslocks*. These
> dependencies are not built between holding locks like typical locks.

They can create dependencies, but they _cannot_ create deadlocks. So
there's no value in those dependencies.

> > And the whole setup stuff isn't properly preserved between works in any
> > case, only the first few works would ever see that history, so why
> > bother.
> 
> As I said in another reply, what about (1), (3) and (5) in my example?

So for single-threaded workqueues, I'd like to get recursive-read sorted
and then we can make the lockdep_invariant_state() conditional.

Using recurisve-read lock for the wq lockdep_map's has the same effect
as your might thing without having to introduce new magic.