rts/ThreadLabels.c has a global hashtable for each
running haskell thread. It's not synchronized across
OS threads.

Was discovered when ran -debug build of ghc itself as:

    $ ghc-stage2 -j8 +RTS -A256M -l

and glibc detected double-free corruption:

    #2  in __libc_message (do_abort=do_abort@entry=2,
        fmt=fmt@entry=0x7fe0bcebf368 "*** Error in `%s': %s: 0x%s ***\n")
    #3  in malloc_printerr (action=3, str=0x7fe0bcebf4c0 "double free or corruption (fasttop)",
        ptr=<optimized out>)
    #4  in _int_free (av=<optimized out>, p=<optimized out>, have_lock=0)
    #5  in stgFree (p=0x7fe060001820) at rts/RtsUtils.c:108
    #6  in freeHashTable (table=0x5929320, freeDataFun=0x36374df <stgFree>) at rts/Hash.c:360
    #7  in freeThreadLabelTable () at rts/ThreadLabels.c:37
    #8  in hs_exit_ (wait_foreign=rtsFalse) at rts/RtsStartup.c:403
    #9  in shutdownHaskellAndExit (n=0, fastExit=0) at rts/RtsStartup.c:481
    #10 in hs_main (...) at rts/RtsMain.c:91
    #11 in main (...) at ghc/hschooks.c:63

Exposed itself after commit:

> commit f6866824


> Author: Sergei Trofimovich <slyfox@gentoo.org>
> Date:   Mon Aug 4 08:10:33 2014 -0500
>
>     ghc --make: add nicer names to RTS threads (threaded IO manager, make workers)

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Reviewers: austin, simonmar, ezyang, bgamari

Reviewed By: ezyang, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1146