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Date: Tue, 29 Aug 2017 19:30:56 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Byungchul Park <byungchul.park@lge.com>, 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: lockdep && recursive-read
Message-ID: <20170829173056.GT32112@worktop.programming.kicks-ass.net>
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On Tue, Aug 29, 2017 at 07:07:57PM +0200, Oleg Nesterov wrote:
> On 08/29, Oleg Nesterov wrote:
> >
> > Plus process_one_work() does lock_map_acquire_read(), I don't really
> > understand this too.
> 
> and in fact I don't understand lock_map_acquire_read() itself. I mean, read == 2
> and this code in check_prevs_add()
> 
> 		/*
> 		 * Only non-recursive-read entries get new dependencies
> 		 * added:
> 		 */
> 		if (hlock->read != 2 && hlock->check) {
> 			if (!check_prev_add(curr, hlock, next,
> 						distance, &stack_saved))
> 
> 
> Well, I forgot everything I ever knew about lockdep, unlikely I understand what
> the code above actually does. But I verified that this code
> 
> 		static DEFINE_SPINLOCK(exlk);
> 		static DEFINE_RWLOCK(rwlk);
> 
> 		spin_lock(&exlk);
> 		write_lock(&rwlk);
> 		write_unlock(&rwlk);
> 		spin_unlock(&exlk);
> 
> 		read_lock(&rwlk);
> 		spin_lock(&exlk);
> 		spin_unlock(&exlk);
> 		read_unlock(&rwlk);
> 
> runs without any warning from lockdep. Doesn't look right or I am totally
> confused...

Long standing lockdep issue that you used to know about ;-)

Boqun recently started working on it:

https://lkml.kernel.org/r/20170828151608.19636-1-boqun.feng@gmail.com