Currently, it is not possible to determine for sure if a reader
owns a rwsem by looking at the content of the rwsem data structure.
This patch adds a new state RWSEM_READER_OWNED to the owner field
to indicate that readers currently own the lock. This enables us to
address the following 2 issues in the rwsem optimistic spinning code:

 1) rwsem_can_spin_on_owner() will disallow optimistic spinning if
    the owner field is NULL which can mean either the readers own
    the lock or the owning writer hasn't set the owner field yet.
    In the latter case, we miss the chance to do optimistic spinning.

 2) While a writer is waiting in the OSQ and a reader takes the lock,
    the writer will continue to spin when out of the OSQ in the main
    rwsem_optimistic_spin() loop as the owner field is NULL wasting
    CPU cycles if some of readers are sleeping.

Adding the new state will allow optimistic spinning to go forward as
long as the owner field is not RWSEM_READER_OWNED and the owner is
running, if set, but stop immediately when that state has been reached.

On a 4-socket Haswell machine running on a 4.6-rc1 based kernel, the
fio test with multithreaded randrw and randwrite tests on the same
file on a XFS partition on top of a NVDIMM were run, the aggregated
bandwidths before and after the patch were as follows:

  Test      BW before patch     BW after patch  % change
  ----      ---------------     --------------  --------
  randrw         988 MB/s          1192 MB/s      +21%
  randwrite     1513 MB/s          1623 MB/s      +7.3%

The perf profile of the rwsem_down_write_failed() function in randrw
before and after the patch were:

   19.95%  5.88%  fio  [kernel.vmlinux]  [k] rwsem_down_write_failed
   14.20%  1.52%  fio  [kernel.vmlinux]  [k] rwsem_down_write_failed

The actual CPU cycles spend in rwsem_down_write_failed() dropped from
5.88% to 1.52% after the patch.

The xfstests was also run and no regression was observed.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Jason Low <jason.low2@hp.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1463534783-38814-2-git-send-email-Waiman.Long@hpe.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>