+/**
+ * ipc_lock_down - Lock an ipc structure with rw_sem held
+ * @ids: IPC identifier set
+ * @id: ipc id to look for
+ *
+ * Look for an id in the ipc ids idr and lock the associated ipc object.
+ *
+ * The ipc object is locked on exit.
+ *
+ * This is the routine that should be called when the rw_mutex is already
+ * held, i.e. idr tree protected.
+ */
+
+struct kern_ipc_perm *ipc_lock_down(struct ipc_ids *ids, int id)
+{
+ struct kern_ipc_perm *out;
+ int lid = ipcid_to_idx(id);
+
+ rcu_read_lock();
+ out = idr_find(&ids->ipcs_idr, lid);
+ if (out == NULL) {
+ rcu_read_unlock();
+ return ERR_PTR(-EINVAL);
+ }
+
+ spin_lock(&out->lock);
+
+ /*
+ * No need to verify that the structure is still valid since the
+ * rw_mutex is held.
+ */
+ return out;
+}
+