Ben Gamari authored May 01, 2016 and Ben Gamari committed May 01, 2016

This fixes #11830, where the RTS would livelock if run with `-I0` due
to a regression introduced by bbdc52f3.
The reason for this is that the new codepath introduced a subtle race
condition:

 1. one thread could request that the ticker stop and would block until
    the ticker in fact stopped
 2. meanwhile, another thread could sneak in and restart the ticker

this was implemented in such a way where thread (1) would end up
blocked forever. The solution here is to simply not block. The worst
that will happen is that timer fires again, but is ignored since the
ticker is stopped.

Test Plan:
Validate, try reproduction case in #11830. Need to find a nice
testcase.

Reviewers: simonmar, erikd, hsyl20, austin

Reviewed By: erikd, hsyl20

Subscribers: erikd, thomie

Differential Revision: https://phabricator.haskell.org/D2129

GHC Trac Issues: #11830