Commit f0bab73c ("locking/lockdep: Restrict the use of recursive
read_lock() with qrwlock") changed lockdep to try and conform to the
qrwlock semantics which differ from the traditional rwlock semantics.

In particular qrwlock is fair outside of interrupt context, but in
interrupt context readers will ignore all fairness.

The problem modeling this is that read and write side have different
lock state (interrupts) semantics but we only have a single
representation of these. Therefore lockdep will get confused, thinking
the lock can cause interrupt lock inversions.

So revert it for now; the old rwlock semantics were already imperfectly
modeled and the qrwlock extra won't fit either.

If we want to properly fix this, I think we need to resurrect the work
by Gautham did a few years ago that split the read and write state of
locks:

   http://lwn.net/Articles/332801/

FWIW the locking selftest that would've failed (and was reported by
Borislav earlier) is something like:

  RL(X1);	/* IRQ-ON */
  LOCK(A);
  UNLOCK(A);
  RU(X1);

  IRQ_ENTER();
  RL(X1);	/* IN-IRQ */
  RU(X1);
  IRQ_EXIT();

At which point it would report that because A is an IRQ-unsafe lock we
can suffer the following inversion:

	CPU0		CPU1

	lock(A)
			lock(X1)
			lock(A)
	<IRQ>
	 lock(X1)

And this is 'wrong' because X1 can recurse (assuming the above lock are
in fact read-lock) but lockdep doesn't know about this.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: ego@linux.vnet.ibm.com
Cc: bp@alien8.de
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20140930132600.GA7444@worktop.programming.kicks-ass.netSigned-off-by: Ingo Molnar <mingo@kernel.org>