qrwlock: Introducing a queue read/write lock implementation

v1->v2:
 - Improve lock fastpath performance.
 - Optionally provide classic read/write lock behavior for backward
   compatibility.
 - Use xadd instead of cmpxchg for fair reader code path to make it
   immute to reader contention.
 - Run more performance testing.

As mentioned in the LWN article http://lwn.net/Articles/364583/, the
classic read/write lock suffer from an unfairness problem that it is
possible for a stream of incoming readers to block a waiting writer
from getting the lock for a long time. Also, a waiting reader/writer
contending a rwlock in local memory will have a higher chance of
acquiring the lock than a reader/writer in remote node.

This patch set introduces a queue-based read/write lock implementation
that can largely solve this unfairness problem while, at the same
time, can optionally provide the classic read/write lock behavior
for those locks that need it. This new queue read/write lock is both
fairer than the classic read/write lock and will be faster in lock
contention situations.

The queue write lock can also be used as a replacement for ticket
spinlocks that are highly contended if lock size increase is not
an issue.

There is no change in the interface. By just selecting the QUEUE_RWLOCK
config parameter during the configuration phase, the classic read/write
lock will be replaced by the new queue read/write lock. This will
made the systems more deterministic and faster in lock contention
situations. In uncontended cases, the queue read/write lock may be
a bit slower than the classic one depending on the exact mix of read
and write locking primitives. Given the fact that locking overhead is
typically a very small percentage of the total CPU time in uncontended
cases, there won't be any noticeable degraduation in performance with
this replacement.

This patch set currently provides queue read/write lock support on
x86 architecture only. Support for other architectures can be added
later on once proper testing is done.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>

Waiman Long (3):
  qrwlock: A queue read/write lock implementation
  qrwlock x86: Enable x86 to use queue read/write lock
  qrwlock: Optionally enable classic read/write lock behavior

 arch/x86/Kconfig                      |    3 +
 arch/x86/include/asm/spinlock.h       |    2 +
 arch/x86/include/asm/spinlock_types.h |    4 +
 include/asm-generic/qrwlock.h         |  251 +++++++++++++++++++++++++++++++++
 include/linux/rwlock.h                |   15 ++
 include/linux/rwlock_types.h          |   12 ++-
 include/linux/spinlock_types.h        |    4 +
 lib/Kconfig                           |   23 +++
 lib/Makefile                          |    1 +
 lib/qrwlock.c                         |  194 +++++++++++++++++++++++++
 lib/spinlock_debug.c                  |   20 +++
 11 files changed, 528 insertions(+), 1 deletions(-)
 create mode 100644 include/asm-generic/qrwlock.h
 create mode 100644 lib/qrwlock.c

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