[Bug c++/46096] Code produces two different outputs when optimized respectively with -O2 and without it.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46096

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2010-10-20 
14:29:09 UTC ---
Using a mutex around the reads and writes of shared data will make it work as
expected, the compiler won't optimise away the read and will re-read the value
every time.