ipc,shm: introduce lockless functions to obtain the ipc object

This is the third and final patchset that deals with reducing the amount
of contention we impose on the ipc lock (kern_ipc_perm.lock).  These
changes mostly deal with shared memory, previous work has already been
done for semaphores and message queues:

  http://lkml.org/lkml/2013/3/20/546 (sems)
  http://lkml.org/lkml/2013/5/15/584 (mqueues)

With these patches applied, a custom shm microbenchmark stressing shmctl
doing IPC_STAT with 4 threads a million times, reduces the execution
time by 50%.  A similar run, this time with IPC_SET, reduces the
execution time from 3 mins and 35 secs to 27 seconds.

Patches 1-8: replaces blindly taking the ipc lock for a smarter
combination of rcu and ipc_obtain_object, only acquiring the spinlock
when updating.

Patch 9: renames the ids rw_mutex to rwsem, which is what it already was.

Patch 10: is a trivial mqueue leftover cleanup

Patch 11: adds a brief lock scheme description, requested by Andrew.

This patch:

Add shm_obtain_object() and shm_obtain_object_check(), which will allow us
to get the ipc object without acquiring the lock.  Just as with other
forms of ipc, these functions are basically wrappers around
ipc_obtain_object*().
Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -124,6 +124,26 @@ void __init shm_init (void)
 				IPC_SHM_IDS, sysvipc_shm_proc_show);
 }
 
+static inline struct shmid_kernel *shm_obtain_object(struct ipc_namespace *ns, int id)
+{
+	struct kern_ipc_perm *ipcp = ipc_obtain_object(&shm_ids(ns), id);
+
+	if (IS_ERR(ipcp))
+		return ERR_CAST(ipcp);
+
+	return container_of(ipcp, struct shmid_kernel, shm_perm);
+}
+
+static inline struct shmid_kernel *shm_obtain_object_check(struct ipc_namespace *ns, int id)
+{
+	struct kern_ipc_perm *ipcp = ipc_obtain_object_check(&shm_ids(ns), id);
+
+	if (IS_ERR(ipcp))
+		return ERR_CAST(ipcp);
+
+	return container_of(ipcp, struct shmid_kernel, shm_perm);
+}
+
 /*
  * shm_lock_(check_) routines are called in the paths where the rw_mutex
  * is not necessarily held.