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

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Message-ID: <20170905112448.GY3240@X58A-UD3R>
Date:   Tue, 5 Sep 2017 20:24:48 +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 12:52:36PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 05, 2017 at 07:31:44PM +0900, Byungchul Park wrote:
> > Let me show you a possible scenario with a leaf lock:
> > 
> >                         lock(A)
> >    lock(A)              wait_for_completion(B)
> >    unlock(A)            ...
> >    ...                  unlock(A)
> >    process_one_work()
> >       work->func()
> >          complete(B)
> > 
> > It's a deadlock by a lead lock A and completion B.
> 
> By having wait_for_completion() in it, A is not a leaf lock.

I see. After all, you want to force to use only leaf locks in (1), (3)
and (5) forever in future. I really don't understand why you want to
force it and use them carefully always in head, *only* for that locks,
though original code even makes it unnecessary.

But ok...

Let's discuss the issue later if necessary.

Again, I think your patches are worth nothing but avoiding the
workqueue issue. I really hope you think it more, but it's ok if you
don't want...