linux-kernel - Re: [PATCH 1/2] workqueue: skip lockdep wq dependency in cancel_work

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Message-ID: <20180822024535.GA2414@X58A-UD3R>
Date:   Wed, 22 Aug 2018 11:45:35 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        kernel-team@....com
Subject: Re: [PATCH 1/2] workqueue: skip lockdep wq dependency in
 cancel_work_sync()

On Tue, Aug 21, 2018 at 09:20:41PM +0200, Johannes Berg wrote:
> On Tue, 2018-08-21 at 10:55 -0700, Tejun Heo wrote:
> 
> > > I'm not really sure what you think we might be missing? Am I missing
> > > some case where cancel_work_sync() can possibly deadlock? Apart from the
> > > issue I addressed in the second patch, obviously.
> > 
> > Ah, that was me being slow.  I thought you were skipping the work's
> > lockdep_map.  I can almost swear we had that before (the part you're
> > adding on the second patch).  Right, fd1a5b04dfb8 ("workqueue: Remove
> > now redundant lock acquisitions wrt. workqueue flushes") removed it
> > because it gets propagated through wait_for_completion().  Did we miss
> > some cases with that change?
> 
> Hmm.
> 
> It doesn't seem to be working.
> 
> No, ok, actually it probably *does*, but the point is similar to my
> issue # 3 before - we don't do any of this unless the work is actually
> running, but we really want the lockdep annotation *regardless* of that,
> so that we catch the error unconditionally.
> 
> So perhaps that commit just needs to be reverted entirely - I'd only
> looked at a small subset of it, but the flush_workqueue() case has the
> same problem - we only get to the completion when there's something to
> flush, not when the workqueue happens to actually be empty. But again,
> for lockdep we want to catch *potential* problems, not only *actual*
> ones.
> 
> The remaining part of the patch I'm not sure I fully understand (removal
> of lockdep_init_map_crosslock()), but I suppose if we revert the other
> bits we need to revert this as well.
> 
> So please drop this patch, but revert Byungchul Park's commit
> fd1a5b04dfb8 again, I don't think the lockdep annotations there are
> really redundant as I just explained.

That should've been adjusted as well when Ingo reverted Cross-release.
It would be much easier to add each pair, acquire/release, before
wait_for_completion() in both flush_workqueue() and flush_work() than
reverting the whole commit.

What's lacking is only lockdep annotations for wait_for_completion().

Byungchul

> 
> johannes