rcu: force alignment on struct callback_head/rcu_head

 * struct callback_head - callback structure for use with RCU and task_work

 * @next: next update requests in a list

 * @func: actual update function to call after the grace period.

 *

 * The struct is aligned to size of pointer. On most architectures it happens

 * naturally due ABI requirements, but some architectures (like CRIS) have

 * weird ABI and we need to ask it explicitly.

 *

 * The alignment is required to guarantee that bits 0 and 1 of @next will be

 * clear under normal conditions -- as long as we use call_rcu(),

 * call_rcu_bh(), call_rcu_sched(), or call_srcu() to queue callback.

 *

 * This guarantee is important for few reasons:

 *  - future call_rcu_lazy() will make use of lower bits in the pointer;

 *  - the structure shares storage spacer in struct page with @compound_head,

 *    which encode PageTail() in bit 0. The guarantee is needed to avoid

 *    false-positive PageTail().

 */

struct callback_head {

	struct callback_head *next;

	void (*func)(struct callback_head *head);

};

} __attribute__((aligned(sizeof(void *))));

#define rcu_head callback_head

typedef void (*rcu_callback_t)(struct rcu_head *head);