From: Davide Libenzi <davidel@xmailserver.org>

A potential race can happen in epoll_ctl(EPOLL_CTL_MOD) where an event can
happen in between f_op->poll() and the lock on ep->lock (we cannot call
f_op->poll() inside a lock, and the f_op->poll() callback does not carry
any info at the current time - missing wake_up_info() already ;).  In that
case the event would be removed.  We can easily leave the event inside the
ready list and have the ep_send_events() logic do the job for us at later
time.  (Thanks to david.lee@teracruz.com for reporting the thing, since it
shouldn't have been a nice one ;)