Wound/wait mutexes are used when other multiple lock
acquisitions of a similar type can be done in an arbitrary
order. The deadlock handling used here is called wait/wound in
the RDBMS literature: The older tasks waits until it can acquire
the contended lock. The younger tasks needs to back off and drop
all the locks it is currently holding, i.e. the younger task is
wounded.

For full documentation please read Documentation/ww-mutex-design.txt.

References: https://lwn.net/Articles/548909/Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@canonical.com>
Acked-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: NRob Clark <robdclark@gmail.com>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: rostedt@goodmis.org
Cc: daniel@ffwll.ch
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/51C8038C.9000106@canonical.comSigned-off-by: NIngo Molnar <mingo@kernel.org>