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

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Date:   Wed, 30 Aug 2017 18:35:07 +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

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@...radead.org]
> Sent: Wednesday, August 30, 2017 6:14 PM
> To: Byungchul Park
> 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 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.