linux-kernel - lockdep && recursive-read

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Message-ID: <20170829170757.GA21241@redhat.com>
Date:   Tue, 29 Aug 2017 19:07:57 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Byungchul Park <byungchul.park@....com>, 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: lockdep && recursive-read

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...

Oleg.