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

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Message-ID: <20170829064637.GC3240@X58A-UD3R>
Date:   Tue, 29 Aug 2017 15:46:38 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tj@...nel.org, boqun.feng@...il.com,
        david@...morbit.com, johannes@...solutions.net, oleg@...hat.com,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

On Wed, Aug 23, 2017 at 01:58:47PM +0200, Peter Zijlstra wrote:
> The new completion/crossrelease annotations interact unfavourable with
> the extant flush_work()/flush_workqueue() annotations.
> 
> The problem is that when a single work class does:
> 
>   wait_for_completion(&C)
> 
> and
> 
>   complete(&C)
> 
> in different executions, we'll build dependencies like:
> 
>   lock_map_acquire(W)
>   complete_acquire(C)
> 
> and
> 
>   lock_map_acquire(W)
>   complete_release(C)
> 
> which results in the dependency chain: W->C->W, which lockdep thinks
> spells deadlock, even though there is no deadlock potential since
> works are ran concurrently.
> 
> One possibility would be to change the work 'lock' to recursive-read,
> but that would mean hitting a lockdep limitation on recursive locks.
> Also, unconditinoally switching to recursive-read here would fail to
> detect the actual deadlock on single-threaded workqueues, which do
> have a problem with this.
> 
> For now, forcefully disregard these locks for crossrelease.

Eventually, you pushed this patch to tip tree without any comment.

I don't really understand you.

How does a maintainer choose a very work-around method and avoid
problems rather than fix a root cause? I am very disappointed.

But, I have nothing to do against your will.