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

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Date:   Tue, 5 Sep 2017 12:52:36 +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 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.