From: "J. R. Okajima" <hooanon05g@gmail.com>
To: Jens Axboe <axboe@fb.com>
Cc: peterz@infradead.org, linux-kernel@vger.kernel.org,
darrick.wong@oracle.com, david@fromorbit.com
Subject: Re: Q: lockdep_assert_held_read() after downgrade_write()
Date: Wed, 01 Feb 2017 00:40:03 +0900 [thread overview]
Message-ID: <5873.1485877203@jrobl> (raw)
In-Reply-To: <86195df9-2a43-2a0f-38ac-68792edc41c0@fb.com>
Jens Axboe:
> 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.
Arg...
It is my bonehead mistake that I inserted up_write() before
downgrade_write(). Sorry about that.
Fortunately Peter Zijlstra reviewed downgrade_write() and sent a
patch. Thank you, it passed my first test.
Now allow me going on the second test (based upon Peter's patch)
- two rwsem, rwA and rwB.
- the locking order is rwA first, and then rwB.
- good case
down_read(rwA)
down_read(rwB)
up_read(rwB)
up_read(rwA)
down_write(rwA)
down_write(rwB)
up_write(rwB)
up_write(rwA)
- questionable case
down_write(rwA)
down_write(rwB)
downgrade_write(rwA)
downgrade_write(rwB)
up_read(rwB)
up_read(rwA)
These two downgrade_write() have their strict order? If so, what is
that?
Do the added two lines
+ rwsem_release(&sem->dep_map, 1, _RET_IP_);
+ rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
produce a traditional AB-BA deadlock warning, don't they?
J. R. Okajima
next prev parent reply other threads:[~2017-01-31 15:40 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
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 [this message]
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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