+ rcu_read_lock();
+ ipcp = ipc_obtain_object_idr(&shm_ids(ns), id);
+ if (IS_ERR(ipcp))
+ goto err;
+
+ ipc_lock_object(ipcp);
+ /*
+ * ipc_rmid() may have already freed the ID while ipc_lock_object()
+ * was spinning: here verify that the structure is still valid.
+ * Upon races with RMID, return -EIDRM, thus indicating that
+ * the ID points to a removed identifier.
+ */
+ if (ipc_valid_object(ipcp)) {
+ /* return a locked ipc object upon success */
+ return container_of(ipcp, struct shmid_kernel, shm_perm);
+ }
+
+ ipc_unlock_object(ipcp);
+err:
+ rcu_read_unlock();