glibc-2.5 test suite hangs/crashes the machine

Benjamin Herrenschmidt benh at
Mon Oct 30 12:47:05 EST 2006

On Fri, 2006-10-27 at 12:22 -0400, Jeff Bailey wrote:
> Le vendredi 27 octobre 2006 à 07:56 +0200, Fabio Massimo Di Nitto a
> écrit :
> > Hi everybody,
> > 
> > i am in the process of bootstrapping the new toolchain for ubuntu and I am
> > hitting a problem building glibc-2.5 on ppc.
> > 
> > This behaviour has been reproduced on 2.6.15/2.6.17 and 2.6.19-rc2 (where the
> > machine crashes) and with ppc32 and ppc64 kernels.
> > A hard reboot of the machine is required to get rid of the Zl processes hanging
> > around that keep spinning the CPU at 100%.
> > 
> > I did place sources here:
> > 
> > but i start to believe it is a kernel bug we are exploiting only now.
> > 
> > Any hint or help for what to look for would be extremely appreciated.
> Heya Fabio, just an update, it looks like the tests that are zombie'ing
> are the nptl tst-robust[1-8] tests.  According to /proc/##/wchan, the
> tasks are cheerfully spinning in do_exit.

So I've built that glibc with debian 2.6.16 kernel headers (since Fabio
says the problem doesn't happen with glibc built with 2.6.19 headers)
and have ran that with 2.6.19-rc3-git-du-jour.

The machine didn't crash, nor did I see any zombie with those
tst-robust[1-8], however, I did get as SIGBUS with tst-robustpi1. I've
tracked it down to being an alignment exception. It looks like glibc is
doing a lwarx on a non-aligned value, though I can't say precisely
what's up here. I don't know how I can get a backtrace when running
those test-cases... the test harness seems to catch signals, I suppose
it could be modified to spit one out.

At this point, it would be useful to have somebody who knows glibc to
tell us:

 - what are those tst-robust all about ? (what do they do "special" that
might trigger bad reactions with older kernels)
 - how can glibc ever do atomic operations on a non-aligned value ?


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