linux-kernel - Re: [PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

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Date:   Thu, 7 Sep 2017 17:21:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <max.byungchul.park@...il.com>
Cc:     Byungchul Park <byungchul.park@....com>, Tejun Heo <tj@...nel.org>,
        johannes.berg@...el.com, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, oleg@...hat.com,
        david@...morbit.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel-team@....com
Subject: Re: [PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue
 flush

On Thu, Sep 07, 2017 at 08:37:08PM +0900, Byungchul Park wrote:
> On Thu, Sep 7, 2017 at 6:41 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, Sep 07, 2017 at 09:33:16AM +0900, Byungchul Park wrote:
> >> Workqueue added manual acquisitions to catch deadlock cases. Now
> >> crossrelease was introduced, some of those are redundant because
> >> crossrelease-enabled wait_for_completeion() also does it. Removed it.
> >
> > This does not explain how it doesn't become the regular timing dependent
> > stuff cross-release normally is.
> 
> Yes, I will add more explanation for it at the next spin.
> 
> But, let me explain it a bit here to help you get it, since it's very
> simple. This has nothing to do with 'might' thing I introduced
> previously. For 'might' thing, I will start it after getting your response.
> 
> Here, I just removed acquire() because wait_for_completion()
> already includes the acquire() by LOCKDEP_COMPLETIONS.
> That's all.

Ah I see, but it does so using some fairly nasty completion init stuff.
The old code was far easier to read.