From: "Byungchul Park" <byungchul.park@lge.com>
To: "'Peter Zijlstra'" <peterz@infradead.org>
Cc: <mingo@kernel.org>, <tj@kernel.org>, <boqun.feng@gmail.com>,
        <david@fromorbit.com>, <johannes@sipsolutions.net>, <oleg@redhat.com>,
        <linux-kernel@vger.kernel.org>, <kernel-team@lge.com>
References: <20170823121432.990701317@infradead.org> <20170824021840.GC6772@X58A-UD3R> <20170824140240.t4imrpvussebfimm@hirez.programming.kicks-ass.net> <20170825011114.GA3858@X58A-UD3R> <20170829085939.ggmb6xiohw67micb@hirez.programming.kicks-ass.net> <20170830020953.GE3240@X58A-UD3R> <20170830074117.GG3240@X58A-UD3R> <20170830085333.GM32112@worktop.programming.kicks-ass.net> <004601d3216e$a3702030$ea506090$@lge.com> <20170830091223.xxnh3podtcumlabm@hirez.programming.kicks-ass.net> <20170830091409.y2d4puph3qn2jwxx@hirez.programming.kicks-ass.net>
In-Reply-To: <20170830091409.y2d4puph3qn2jwxx@hirez.programming.kicks-ass.net>
Subject: RE: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
Date: Wed, 30 Aug 2017 18:35:07 +0900
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> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@infradead.org]
> Sent: Wednesday, August 30, 2017 6:14 PM
> To: Byungchul Park
> Cc: mingo@kernel.org; tj@kernel.org; boqun.feng@gmail.com;
> david@fromorbit.com; johannes@sipsolutions.net; oleg@redhat.com; linux-
> kernel@vger.kernel.org; kernel-team@lge.com
> Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
> 
> On Wed, Aug 30, 2017 at 11:12:23AM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 30, 2017 at 06:01:59PM +0900, Byungchul Park wrote:
> > > My point is that we inevitably lose valuable dependencies by yours.
> That's
> > > why I've endlessly asked you 'do you have any reason you try those
> patches?'
> > > a ton of times. And you have never answered it.
> >
> > The only dependencies that are lost are those between the first work and
> > the setup of the workqueue thread.
> >
> > And there obviously _should_ not be any dependencies between those. A
> > work should not depend on the setup of the thread.
> 
> Furthermore, the save/restore can't preserve those dependencies. The
> moment a work exhausts xhlocks[] they are gone. So by assuming the first

They are gone _one time_ only once it has been overwritten, and
Recovered at next turn, with original code. But you made it
un-recoverable even at the next time and lose all valuable
dependencies unconditionally.

> work _will_ exhaust the history there is effectively nothing lost.