[systemd-devel] [PATCH 2/2] domain: grab the domain's parent lock only when needed

[Kay Sievers]

On Sat, Mar 22, 2014 at 3:43 PM, Daniel Mack <daniel at zonque.org> wrote:
> On 03/21/2014 12:35 PM, Djalal Harouni wrote:
>> I've two questions:
>>
>> 1) Can we improve it in ordre to reduce the lock hold time ?
>
> We don't consider that a very frequent operation, so we never thought
> about optimizing this code path. What use case do you have in mind so
> that performance would matter much?
>
>> currently creating a domain will make create/disconnect/open... buses
>> and endpoints on the parent of that domain block, these are separated
>> operations on different domains.
>
> I don't quite follow. Can you elaborate?

Creating domains and buses are relatively expensive system operations,
they involve udev, device node creation, sysfs/driver-core
interaction.  If you think it's worth, please try it, we just took the
easy road so far, not sure how much we can really optimize here.

>> Also it seems that now there is only support for one level of nested
>> domains? will this be increased?
>
> I don't think so. What's your use case here?

We never tried, but the code was supposed to allow stacking of them.
I'll fix it.

>> 2) What about creating custom endpoints on the bus that was already
>> unrefed ? IMHO this is the same scenario!
>
> That shouldn't happen of course. We've been dealing with locking in that
> area quite a bit, but we might have overlooked something. Please send a
> patch if you see such an unsafe locking scenario.
>
>> Hmm perhaps this can be improved by taking a ref ASAP and revalidate
>> objects by checking the "*->disconnected" ?
>
> ->disconnected isn't so much of an issue, and we do check for it where
> necessary. Apart from that, users that store a pointer to any object
> should take a reference, so it can't disappear underneath them. But
> again, if you think we've overlooked anything, let us know. Reviewing
> all these details is certainly much appreciated.

I'll add a few more "disconnected checks" before we link into the
parent objects.

Kay