On Wed, Jul 02, 2014 at 09:39:18PM -0700, Jason Low wrote:
> And due to padding, the additional modification below reduces the
> size of struct rw_semaphore by 64 bits on my machine  :)
> 
> 
>  struct rw_semaphore {
>  	long count;
> -	raw_spinlock_t wait_lock;
>  	struct list_head wait_list;
> +	raw_spinlock_t wait_lock;
>  #ifdef CONFIG_SMP
> +	struct optimistic_spin_tail osq; /* spinner MCS lock */
>  	/*
>  	 * Write owner. Used as a speculative check to see
>  	 * if the owner is running on the cpu.
>  	 */
>  	struct task_struct *owner;
> -	struct optimistic_spin_tail osq; /* spinner MCS lock */
>  #endif
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map	dep_map;
> 

Right, that might make sense.