repl_queue is the head of the linked list (repl_queue_tail is the tail) of REVIVE messages that had been previously (unsuccessfully) sent to the file system (FS) but had not been delivered due to deadlock. Before any new messages are processed by the network service, a second (and last) attempt is made by send_repl_queue() to send out these REVIVE messages in repl_queue.
For a detailed description of what circumstances lead to a deadlock, click here.