locking/rwsem: Reduce the size of struct rw_semaphore

Recent optimistic spinning additions to rwsem provide significant performance
benefits on many workloads on large machines. The cost of it was increasing
the size of the rwsem structure by up to 128 bits.

However, now that the previous patches in this series bring the overhead of
struct optimistic_spin_queue to 32 bits, this patch reorders some fields in
struct rw_semaphore such that we can reduce the overhead of the rwsem structure
by 64 bits (on 64 bit systems).

The extra overhead required for rwsem optimistic spinning would now be up
to 8 additional bytes instead of up to 16 bytes. Additionally, the size of
rwsem would now be more in line with mutexes.

Signed-off-by: Jason Low <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Scott Norton <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Aswin Chandramouleeswaran <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Josef Bacik <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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