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

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Date:   Tue, 21 Aug 2018 21:20:41 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Tejun Heo <tj@...nel.org>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 1/2] workqueue: skip lockdep wq dependency in
 cancel_work_sync()

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.

johannes