rculist: Add hlists_swap_heads_rcu

Using the struct pid to refer to two tasks in de_thread was a clever
idea and ultimately too clever, as it has lead to proc_flush_task
being called inconsistently.

To support rectifying this add hlists_swap_heads_rcu.  An hlist
primitive that just swaps the hlist heads of two lists.  This is
exactly what is needed for exchanging the pids of two tasks.

Only consideration of correctness of the code has been given,
as the caller is expected to be a slowpath.

Link: https://lore.kernel.org/lkml/87mu6vajnq.fsf_-_@x220.int.ebiederm.org/Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/rculist.h

@@ -506,6 +506,27 @@ static inline void hlist_replace_rcu(struct hlist_node *old,
 	WRITE_ONCE(old->pprev, LIST_POISON2);
 }
 
+/**
+ * hlists_swap_heads_rcu - swap the lists the hlist heads point to
+ * @left:  The hlist head on the left
+ * @right: The hlist head on the right
+ *
+ * The lists start out as [@left  ][node1 ... ] and
+                          [@right ][node2 ... ]
+ * The lists end up as    [@left  ][node2 ... ]
+ *                        [@right ][node1 ... ]
+ */
+static inline void hlists_swap_heads_rcu(struct hlist_head *left, struct hlist_head *right)
+{
+	struct hlist_node *node1 = left->first;
+	struct hlist_node *node2 = right->first;
+
+	rcu_assign_pointer(left->first, node2);
+	rcu_assign_pointer(right->first, node1);
+	WRITE_ONCE(node2->pprev, &left->first);
+	WRITE_ONCE(node1->pprev, &right->first);
+}
+
 /*
  * return the first or the next element in an RCU protected hlist
  */