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From: "J. R. Okajima" <hooanon05g@gmail.com>
Subject: Re: Q: lockdep_assert_held_read() after downgrade_write()
To: Jens Axboe <axboe@fb.com>
Cc: peterz@infradead.org, linux-kernel@vger.kernel.org,
        darrick.wong@oracle.com, david@fromorbit.com
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References: <18295.1485811542@jrobl> <86195df9-2a43-2a0f-38ac-68792edc41c0@fb.com>
Date: Wed, 01 Feb 2017 00:40:03 +0900
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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