Description of problem:
In several places, an epoll fd can call another file's ->f_op->poll() method
with ep->mtx held. This is in general unsafe, because that other file could
itself be an epoll fd that contains the original epoll fd.

The code defends against this possibility in its own ->poll() method using
ep_call_nested, but there are several other unsafe calls to ->poll elsewhere
that can be made to deadlock. For example, the following simple program causes
the call in ep_insert recursively call the original fd's ->poll, leading to
deadlock


References:
https://lkml.org/lkml/2011/2/5/220
http://seclists.org/oss-sec/2011/q1/337

Upstream commit:
http://git.kernel.org/linus/22bacca48a1755f79b7e0f192ddb9fbb7fc6e64e

Acknowledgements:                                                                                                                                                                            

Red Hat would like to thank Nelson Elhage for reporting this issue.