llist: add llist_del_first_this()

llist_del_first_this() deletes a specific entry from an llist, providing
it is at the head of the list. Multiple threads can call this
concurrently providing they each offer a different entry.

This can be uses for a set of worker threads which are on the llist when
they are idle. The head can always be woken, and when it is woken it
can remove itself, and possibly wake the next if there is an excess of
work to do.

Signed-off-by: NeilBrown <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

include/linux/llist.h

···
 		init_llist_node(n);
 	return n;
 }
+
+extern bool llist_del_first_this(struct llist_head *head,
+				 struct llist_node *this);
+
 struct llist_node *llist_reverse_order(struct llist_node *head);
 
 #endif /* LLIST_H */

lib/llist.c

···
 EXPORT_SYMBOL_GPL(llist_del_first);
 
 /**
+ * llist_del_first_this - delete given entry of lock-less list if it is first
+ * @head:	the head for your lock-less list
+ * @this:	a list entry.
+ *
+ * If head of the list is given entry, delete and return %true else
+ * return %false.
+ *
+ * Multiple callers can safely call this concurrently with multiple
+ * llist_add() callers, providing all the callers offer a different @this.
+ */
+bool llist_del_first_this(struct llist_head *head,
+			  struct llist_node *this)
+{
+	struct llist_node *entry, *next;
+
+	/* acquire ensures orderig wrt try_cmpxchg() is llist_del_first() */
+	entry = smp_load_acquire(&head->first);
+	do {
+		if (entry != this)
+			return false;
+		next = READ_ONCE(entry->next);
+	} while (!try_cmpxchg(&head->first, &entry, next));
+
+	return true;
+}
+EXPORT_SYMBOL_GPL(llist_del_first_this);
+
+/**
  * llist_reverse_order - reverse order of a llist chain
  * @head:	first item of the list to be reversed
  *