From: Peter Zijlstra <peterz@infradead.org>
To: Jens Axboe <axboe@fb.com>
Cc: "J. R. Okajima" <hooanon05g@gmail.com>,
linux-kernel@vger.kernel.org, darrick.wong@oracle.com,
david@fromorbit.com
Subject: Re: Q: lockdep_assert_held_read() after downgrade_write()
Date: Tue, 31 Jan 2017 11:36:20 +0100 [thread overview]
Message-ID: <20170131103620.GM6515@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <86195df9-2a43-2a0f-38ac-68792edc41c0@fb.com>
On Mon, Jan 30, 2017 at 02:30:45PM -0700, Jens Axboe wrote:
> 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:
Correct, and I'm surprised that didn't explode in different ways.
>
> down_write(&rw);
> downgrade_write(&rw);
> lockdep_assert_held_read(&rw)
> up_read(&rw);
>
> or just not drop up_write() from the last section.
Right, but also, there seems to be a missing lockdep annotation to make
that work. That is, downgrade_write() doesn't have a lockdep annotation,
so it (lockdep) will still think its a write lock.
Let me try and fix both issues.
next prev parent reply other threads:[~2017-01-31 11:04 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-30 21:25 Q: lockdep_assert_held_read() after downgrade_write() J. R. Okajima
2017-01-30 21:30 ` Jens Axboe
2017-01-31 10:36 ` Peter Zijlstra [this message]
2017-01-31 11:25 ` Peter Zijlstra
2017-01-31 14:23 ` Waiman Long
2017-01-31 14:26 ` Peter Zijlstra
2017-01-31 15:40 ` J. R. Okajima
2017-01-31 16:32 ` Peter Zijlstra
2017-02-02 16:33 ` J. R. Okajima
2017-02-02 16:38 ` [PATCH 1/3] lockdep: consolidate by new find_held_lock() J. R. Okajima
2017-02-02 16:38 ` [PATCH 2/3] lockdep: consolidate by new validate_held_lock() J. R. Okajima
2017-02-14 12:09 ` Peter Zijlstra
2017-03-16 11:25 ` [tip:locking/core] locking/lockdep: Factor out the validate_held_lock() helper function tip-bot for J. R. Okajima
2017-02-02 16:38 ` [PATCH 3/3] lockdep: new annotation lock_downgrade() J. R. Okajima
2017-02-02 17:59 ` kbuild test robot
2017-02-02 18:45 ` Peter Zijlstra
2017-02-02 21:05 ` J. R. Okajima
2017-02-14 12:11 ` Peter Zijlstra
2017-03-16 11:25 ` [tip:locking/core] locking/lockdep: Add new check to lock_downgrade() tip-bot for J. R. Okajima
2017-03-16 11:24 ` [tip:locking/core] locking/lockdep: Factor out the find_held_lock() helper function tip-bot for J. R. Okajima
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