[CRIU] Thread safety of C API

[Tycho Andersen]

On Wed, Jun 08, 2016 at 03:44:10PM -0700, Andrew Vagin wrote:
> On Wed, Jun 08, 2016 at 03:14:25PM +0200, Michal Privoznik wrote:
> > Dear list,
> > 
> > forgive me if this question has been asked already, but I didn't find
> > anything about that. Katerina (CCed) and me are working on implementing
> > migration support for our LXC driver [1] in libvirt [2]. And we wanted
> > to use the C API, however reading the documentation to that [3] and
> > checking out the sources we've found that C API is not threat safe. This
> > is a bummer, since libvirt is multi-threaded app where each thread is
> > allowed to execute an API over distant VMs (in this case containers) at
> > the same time. So we are unable to use the C API unless we serialize the
> > callers wanting to migrate on a mutex. This would, however, have
> > unfortunate effect on our performance.
> > 
> > My question here is, are there any plans on making the APIs thread safe?
> > I don't know what's the policy on changing APIs (for instance in Libvirt
> > we don't allow that and have to invent a new one for the flawed ones). I
> > can provide the patches if there's not enough bandwidth, but I'd rather
> > check with you before devoting my time to it.
> 
> What do we need to do to make it thread-safe?

I think if you use the criu_local_* options it is thread safe, isn't
it? My understanding is that libprotobuf is threadsafe, so as long as
you're not using the global struct it should be okay.

Tycho

> Actually I can't say anything about changing API of the library,
> it's up to Pavel.
> 
> But if you think that the library doesn't cover your needs,
> you can use our RPC protocol directly:
> 
> https://criu.org/RPC
> 
> Here is an example how we do this in runc:
> https://github.com/opencontainers/runc/blob/master/libcontainer/container_linux.go#L799
> 
> Thanks,
> Andrew
> 
> > 
> > Thanks!
> > 
> > Michal
> > 
> > 1: https://www.redhat.com/archives/libvir-list/2016-May/msg00565.html
> > 2: http://libvirt.org/internals/eventloop.html#worker_pool
> > 3: https://criu.org/C_API
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