Man page 9 : seq_hlist_start_head_rcu

seq_hlist_start_head_rcu - start an iteration of a hlist protected by RCU

struct hlist_node * seq_hlist_start_head_rcu(struct hlist_head *  head , loff_t  pos );

head
    the head of the hlist

pos
    the start position of the sequence

Called at seq_file->op->start. Call this function if you want to print a header at the top of the output.

This list-traversal primitive may safely run concurrently with the _rcu list-mutation primitives such as hlist_add_head_rcu as long as the traversal is guarded by rcu_read_lock.