linux-kernel - Q: lockdep_assert_held_read() after downgrade

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Date:   Tue, 31 Jan 2017 06:25:42 +0900
From:   "J. R. Okajima" <hooanon05g@...il.com>
To:     peterz@...radead.org
cc:     linux-kernel@...r.kernel.org, axboe@...com,
        darrick.wong@...cle.com, david@...morbit.com
Subject: Q: lockdep_assert_held_read() after downgrade_write()

Peter Zijlstra,

May I ask you a question?
v4.10-rc1 got a commit
	f831948 2016-11-30 locking/lockdep: Provide a type check for lock_is_held
I've tested a little and lockdep splat a stack trace.

{
	DECLARE_RWSEM(rw);
	static struct lock_class_key key;
	lockdep_set_class(&rw, &key);

	down_read(&rw);
	lockdep_assert_held_read(&rw);
	up_read(&rw);

	down_write(&rw);
	lockdep_assert_held_exclusive(&rw);
	up_write(&rw);

	downgrade_write(&rw);
	lockdep_assert_held_read(&rw);	<-- here
	up_read(&rw);
}

I was expecting that lockdep_assert_held_read() splat nothing after
downgrade_write(). Is this warning an intentional behaviour?

Also the final up_read() gives me a warning too. It is produced at
	lockdep.c:3514:lock_release(): DEBUG_LOCKS_WARN_ON(depth <= 0)

As an additional information, I increased some lockdep constants.
Do you think this is related?

include/linux/lockdep.h
+#define MAX_LOCKDEP_SUBCLASSES		(8UL + 4)
+#define MAX_LOCKDEP_KEYS_BITS		(13 + 3)

kernel/locking/lockdep_internals.h
+#define MAX_LOCKDEP_ENTRIES	(32768UL << 5)
+#define MAX_LOCKDEP_CHAINS_BITS	(16 + 5)
+#define MAX_STACK_TRACE_ENTRIES	(524288UL << 5)


J. R. Okajima