TCP send in Linux 2.6

tcp_sendmsg() copies the data from userspace by buiding socket buffers and calling skb_entail() on each packet. skb_entail() calls tcp_add_write_queue_tail() for adding the buffer to the sk_write_queue of the socket and setting sk_send_head to the packet if it's not yet set.
Note the difference between sk_write_queue and sk_send_head, send_head denotes the first package which has not been requested for transmitting through the lower layer (IP) while all that the packages remain on the write_queue (until they are acked, check tcp_ack() during receiving)
In case the last packet added is the only one waiting for transmission (i.e. skb == skb_send_head) tcp_push_one() is called in order to advance sk_send_head and to call tcp_transmit_skb() on the package, otherwise it is simply enqueued.
Before returning the number of bytes copied from userspace tcp_sendmsg() calls tcp_push(), which calls __tcp_push_pending_frames(), which calls tcp_write_xmit(), the general function for iterating packets from sk_send_head and calling tcp_transmit_skb() for each of them.
Both tcp_push_one() and tcp_write_xmit() call tcp_transmit_skb() for the actuall transmission of a package through the socket's icsk_af_ops->queue_xmit() function.
Both tcp_push_one() and tcp_write_xmit() call tcp_event_new_data(), which advances sk_send_head and sets up the TCP_TIME_RETRANS timer of the socket if it's not set yet.