[SSSD] [PATCH] Release mutex if a thread gets cancelled

Sumit Bose sbose at redhat.com
Wed Aug 15 12:07:12 UTC 2012


On Mon, Aug 13, 2012 at 06:13:24PM +0200, Jakub Hrozek wrote:
> On Mon, Aug 13, 2012 at 04:40:03PM +0200, Jakub Hrozek wrote:
> > On Mon, Aug 13, 2012 at 01:30:21PM +0200, Sumit Bose wrote:
> > > On Fri, Aug 10, 2012 at 06:40:35PM +0200, Jakub Hrozek wrote:
> > > > https://fedorahosted.org/sssd/ticket/1460
> > > > 
> > > > Please see the commit. I'm wondering if there is still a (small) race
> > > > condition between the call to pthread_cleanup_pop() and unlocking the
> > > > mutex. Would it be better to i.e. always call the cleanup handler with
> > > > pthread_cleanup_pop(1) and disconnect from the fd based on some other
> > > > condition?
> > > 
> > > this patch works as expected for me on F17. I have some issues on F16,
> > > but this might have other reason I still try to investigate. While
> > > reading a bit about pthread I can across robust mutexes
> > > http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_getrobust.html
> > > I wonder if we can better avoid races with those and maybe even make the
> > > implementation cleaner? E.g. if we get EOWNERDEAD we can call
> > > sss_cli_close_socket(); pthread_mutex_consistent(); run the request
> > > which finally will call pthread_mutex_unlock().
> > > 
> > 
> > As discussed briefly on IRC, I agree. I like any approach that removes
> > the pthread_cleanup_push/pop hack.
> > 
> > > There are also pthread_mutexattr_getrobust_np() and
> > > pthread_mutex_consistent_np() in glibc. I do not have an idea how they
> > > differ and which one should be preferred. But the *_np version only need
> > > __USE_GNU while the others need __USE_XOPEN2K8.
> > > 
> > > bye,
> > > Sumit
> > 
> > RHEL5 only contains the _np versions, but they still seem to work. If
> > the approach is deemed OK, I'll detect the available versions during
> > configure.
> > 
> > Traditionally the _np suffix means "non-portable GNU extension".
> > 
> > Please see the attached patch. Would that work for you? If so, I'll
> > extend it to work on all supported RHEL releases.
> 
> Sumit nacked the patch on IRC. I was missing the disconnect logic when
> lock returns EOWNERDEAD. I also moved the mutexattr to sss_mt_init, I
> don't think there is any reason to keep it separate. It doesn't have to
> live as long as the mutex itself.

This patch works as expected on F17, so ACK.

I still have some strange behaviour on F16 and strangely in the case
where the fast cache files in the /var/lib/sss/mc directory are
available. Here the reproducer get stuck in a futex() call with or
without the patch. I looks that glibc itself also uses a mutex here
which is neither robust nor unlocked if the tread holding it is killed.
I would be nice if someone with a F16 system can verify this.

bye,
Sumit



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