hlist_add_head_rcu — adds the specified element to the specified hlist,
void hlist_add_head_rcu (struct hlist_node * n, struct hlist_head * h);
The caller must take whatever precautions are necessary
(such as holding appropriate locks) to avoid racing
with another list-mutation primitive, such as hlist_add_head_rcu
or hlist_del_rcu, running on this same list.
However, it is perfectly legal to run concurrently with
the _rcu list-traversal primitives, such as
hlist_for_each_entry_rcu, used to prevent memory-consistency
problems on Alpha CPUs. Regardless of the type of CPU, the
list-traversal primitive must be guarded by rcu_read_lock.
The caller must take whatever precautions are necessary
(such as holding appropriate locks) to avoid racing
with another list-mutation primitive, such as hlist_add_head_rcu
or hlist_del_rcu, running on this same list.
However, it is perfectly legal to run concurrently with
the _rcu list-traversal primitives, such as
hlist_for_each_entry_rcu, used to prevent memory-consistency
problems on Alpha CPUs. Regardless of the type of CPU, the
list-traversal primitive must be guarded by rcu_read_lock.