linux-kernel - Re: [RFC] workqueue: remove manual lockdep uses to detect deadlocks

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Date:   Wed, 30 Aug 2017 10:53:39 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Byungchul Park <max.byungchul.park@...il.com>,
        Tejun Heo <tj@...nel.org>, johannes.berg@...el.com,
        Ingo Molnar <mingo@...nel.org>, tglx@...utronix.de,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel-team@....com
Subject: Re: [RFC] workqueue: remove manual lockdep uses to detect deadlocks

On Tue, Aug 29, 2017 at 08:57:27PM +0200, Peter Zijlstra wrote:
> On Sat, Aug 26, 2017 at 12:49:26AM +0900, Byungchul Park wrote:
> > > However, how would it distinguish things like flushing another work
> > 
> > I think it must be distinguished with what it actually waits for, e.i.
> > completion
> > variables instead of work or wq. I will make it next week and let you know.
> 
> So no. The existing annotations are strictly better than relying on
> cross-release.

Thank you for exaplanation but, as I already said, this is why I said
"I don't think it's the same level currently. But, I can make it with
some modification." to TJ:

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1479560.html

And also I mentioned we might need the current code inevitably but, the
existing annotations are never good and why here:

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1480173.html

> As you know the problem with cross-release is that it is timing
> dependent. You need to actually observe the problematic sequence before
> it can warn, and only the whole instance->class mapping saves us from
> actually hitting the deadlock.

Of course.

> The same would be true for using cross-release for workqueues as well,
> something like:
> 
> 					W:
> 					mutex_lock(A)
> 
> 	mutex_lock(A)
> 	flush_work(W)
> 
> would go unreported whereas the current workqueue annotation will
> generate a splat.

Of course.

That's why I said we need to work on it. But it should be modified so
that the wq code becomes more clear instead of abusing weird acquire()s.