I did this

if (events & EPOLLIN) {
            //noblock peek read the socket m_handle_fd, "HTTP/1.1 200 OK ....." in it
	m_master->multi_socket_action(CURL_CSELECT_IN, m_handle_fd);
            //"HTTP/1.1 200 OK ....." still in it ，libcurl not read not read the message
} else if (events & EPOLLOUT) {
	m_master->multi_socket_action(CURL_CSELECT_OUT, m_handle_fd);
}
    ............................
   void Master::multi_socket_action(curl_socket_t socket, int ev_bitmask)
    {
int running_handles = 0;
curl_multi_socket_action(m_curl_multi, socket, ev_bitmask, &running_handles);
    .............................

1，EPOLL event alive
2，libcurl CURL_CSELECT_IN with socket handl，but not read the socketbuffer
3，EPOLL again，and event deadloop
4，a 15000 ms timer timeout，libcurl turn to ok，socketbuffer be received
5，bug in centos 7.2 libcurl.x86_64 7.29.0-35.el7.centos
build curl-7.54.0 and static link is ok

I expected the following

libcurl read the socket，no event deadloop。

curl/libcurl version

curl -V output
curl 7.29.0 (x86_64-redhat-linux-gnu) libcurl/7.29.0 NSS/3.21 Basic ECC zlib/1.2.7 libidn/1.28 libssh2/1.4.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp scp sftp smtp smtps telnet tftp
Features: AsynchDNS GSS-Negotiate IDN IPv6 Largefile NTLM NTLM_WB SSL libz unix-sockets

operating system

more /proc/version
CentOS 7.2
Linux version 3.10.0-514.16.1.el7.x86_64 ([email protected]) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC) ) #1 SMP Wed Apr 12 15:04:24 UTC 2017