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Date: Wed, 23 Aug 2017 15:31:57 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, kernel-team@lge.com,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Dave Chinner <david@fromorbit.com>, Tejun Heo <tj@kernel.org>,
        johannes@sipsolutions.net
Subject: Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all
 part of PROVE_LOCKING
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On Wed, Aug 23, 2017 at 11:43:23AM +0900, Byungchul Park wrote:
> On Tue, Aug 22, 2017 at 03:49:22PM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 22, 2017 at 12:08:40PM +0200, Peter Zijlstra wrote:
> > 
> > > > > > I meant:
> > > > > > 
> > > > > >  	mutex_lock(&A)
> > > > > >  				<work>
> > > > > >  				lockdep_map_acquire_read(&work)
> > > > > >  				mutex_lock(&A)
> > > > > > 
> > > > > >  	lockdep_map_acquire(&work)
> > > > > >  	flush_work(&work)
> > > > > > 
> > > > > > I mean it can still be detected with a read acquisition in work.
> > > > > > Am I wrong?
> > > > > 
> > > > > Think so, although there's something weird with read locks that I keep
> > > > > forgetting. But I'm not sure it'll actually solve the problem. But I can
> > > > 
> > > > I mean, read acquisitions are nothing but ones allowing read ones to be
> > > > re-acquired legally, IOW, we want to check entrance of flush_work() and
> > > > works, not between works. That's why I suggested to use read ones in work
> > > > in that case.
> > > 
> > > Does seem to work.
> > 
> > So I think we'll end up hitting a lockdep deficiency and not trigger the
> > splat on flush_work(), see also:
> > 
> >   https://lwn.net/Articles/332801/
> > 
> > lock_map_acquire_read() is a read-recursive and will not in fact create
> > any dependencies because of this issue.
> > 
> > In specific, check_prev_add() has:
> > 
> > 	if (next->read == 2 || prev->read == 2)
> > 		return 1;
> > 
> > This means that for:
> > 
> > 	lock_map_acquire_read(W)(2)
> > 	down_write(A)		(0)
> > 
> > 			down_write(A)		(0)
> > 			wait_for_completion(C)	(0)
> > 
> > 					lock_map_acquire_read(W)(2)
> > 					complete(C)		(0)
> > 
> > All the (2) effectively go away and 'solve' our current issue, but:
> > 
> > 	lock_map_acquire_read(W)(2)
> > 	mutex_lock(A)		(0)
> > 
> > 			mutex_lock(A)		(0)
> > 			lock_map_acquire(W)	(0)
> > 
> > as per flush_work() will not in fact trigger anymore either.
> 
> It should be triggered. Lockdep code should be fixed so that it does.
> 
> > See also the below locking-selftest changes.
> > 
> > 
> > Now, this means I also have to consider the existing
> > lock_map_acquire_read() users and if they really wanted to be recursive
> > or not. When I change lock_map_acquire_read() to use
> > lock_acquire_shared() this annotation no longer suffices and the splat
> > comes back.
> > 
> > 
> > Also, the acquire_read() annotation will (obviously) no longer work to
> > cure this problem when we switch to normal read (1), because then the
> > generated chain:
> > 
> > 	W(1) -> A(0) -> C(0) -> W(1)
> 
> Please explain what W/A/C stand for.

I eventually found them in your words. Let me read this again.