[CRIU] Thread safety of C API

[Pavel Emelyanov]

On 06/09/2016 01:56 AM, Tycho Andersen wrote:
> 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?

Yup! The criu_local_ set of functions was introduced exactly for that.
Michal, do you know exact places in the code that are not thread safe?

> My understanding is that libprotobuf is threadsafe, so as long as
> you're not using the global struct it should be okay.

Agreed.

> Tycho

-- Pavel

>> 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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