ip_close() simply closes a previously opened ip file descriptor. To be more specific, ip_close() removes the ip file descriptor from its protocol-specific linked list, frees all the packets in its read queue, and marks the ip file descriptor as available.
ip_close() is called only to close an ip device file (e.g., /dev/ip) that has been directly opened. ip_close() will not be called if a higher-layer device file (e.g., /dev/udp) has been opened.