linux-kernel - Re: Q: lockdep_assert_held_read() after downgrade

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Message-ID: <86195df9-2a43-2a0f-38ac-68792edc41c0@fb.com>
Date:   Mon, 30 Jan 2017 14:30:45 -0700
From:   Jens Axboe <axboe@...com>
To:     "J. R. Okajima" <hooanon05g@...il.com>, <peterz@...radead.org>
CC:     <linux-kernel@...r.kernel.org>, <darrick.wong@...cle.com>,
        <david@...morbit.com>
Subject: Re: Q: lockdep_assert_held_read() after downgrade_write()

On 01/30/2017 02:25 PM, J. R. Okajima wrote:
> 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)

I don't think you understand how it works. downgrade_write() turns a write
lock into read held. To make that last sequence valid, you'd need:

	down_write(&rw);
	downgrade_write(&rw);
	lockdep_assert_held_read(&rw)
	up_read(&rw);

or just not drop up_write() from the last section.

-- 
Jens Axboe