ipc: update locking scheme comments

 *            Pavel Emelianov <xemul@openvz.org>

 *

 * General sysv ipc locking scheme:

 *  when doing ipc id lookups, take the ids->rwsem

 *	rcu_read_lock()

 *          obtain the ipc object (kern_ipc_perm)

 *          perform security, capabilities, auditing and permission checks, etc.

 *          acquire the ipc lock (kern_ipc_perm.lock) throught ipc_lock_object()

 *             perform data updates (ie: SET, RMID, LOCK/UNLOCK commands)

 *          obtain the ipc object (kern_ipc_perm) by looking up the id in an idr

 *	    tree.

 *	    - perform initial checks (capabilities, auditing and permission,

 *	      etc).

 *	    - perform read-only operations, such as STAT, INFO commands.

 *	      acquire the ipc lock (kern_ipc_perm.lock) through

 *	      ipc_lock_object()

 *		- perform data updates, such as SET, RMID commands and

 *		  mechanism-specific operations (semop/semtimedop,

 *		  msgsnd/msgrcv, shmat/shmdt).

 *	    drop the ipc lock, through ipc_unlock_object().

 *	rcu_read_unlock()

 *

 *  The ids->rwsem must be taken when:

 *	- creating, removing and iterating the existing entries in ipc

 *	  identifier sets.

 *	- iterating through files under /proc/sysvipc/

 *

 *  Note that sems have a special fast path that avoids kern_ipc_perm.lock -

 *  see sem_lock().

 */

#include <linux/mm.h>