linux-kernel - Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all part of PROVE

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Date:   Wed, 23 Aug 2017 12:26:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@....com, Arnaldo Carvalho de Melo <acme@...nel.org>,
        Dave Chinner <david@...morbit.com>, Tejun Heo <tj@...nel.org>,
        johannes@...solutions.net
Subject: Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all
 part of PROVE_LOCKING

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:

> > 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.

Yeah, its just something we never got around to. Once every so often I
get reminded of it, like now. But then it sits on the todo list and
never quite happens.