linux-kernel - Re: [RESEND PATCH v2 2/2] lockdep: Remove unnecessary acquisitions wrt workqueue flush

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Date:   Fri, 13 Oct 2017 16:56:33 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, johannes.berg@...el.com,
        mingo@...nel.org, tglx@...utronix.de, oleg@...hat.com,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [RESEND PATCH v2 2/2] lockdep: Remove unnecessary acquisitions
 wrt workqueue flush

On Thu, Oct 12, 2017 at 05:56:35PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 12, 2017 at 08:38:17AM -0700, Tejun Heo wrote:
> > 
> > As long as we have the same level of protection, simpler code is of
> > course preferable.  That said, I haven't followed the discussion
> > closely and don't want to apply it without Peter's ack.  Peter?
> 
> I'm really tied up atm; and feel we should be addressing the false
> positives generated by the current code before we start doing new stuff
> on top.

We can never avoid adding false dependencies as long as we use
acquisitions in that way the workqueue code does, even though you
successfully replace write acquisitions with recursive-read ones after
making them work, as you know.

Speaking a bit more about the reason, it's because all write locks used
in every work->func() obviously generate false dependencies(links) with
'work' lockdep_map and 'wq' lockdep_map, when they do not involve flush.
This is why I used a word, 'speculative', whenever we were talking.

At the beginning of this issue, I suggested to use recursive-read
acquisitions instead of write ones, which you are working on, since
anyway it reduces false ones.

But, if it's allowed to add a new primitive that just suits that purpose,
I want to propose it to be used instead, which makes false ones reduced
more. A read acuisition is a real acquisition used for a read lock.
Semantics are similar to but not same as what we need for that
speculative purpose.