[Devel] Re: [RFC] Transactional CGroup task attachment

[Paul Menage]

On Fri, Jul 11, 2008 at 5:03 PM, Matt Helsley <matthltc at us.ibm.com> wrote:
>> struct cgroup_attach_state {
>
> nit: How about naming it cgroup_attach_request or
> cgroup_attach_request_state? I suggest this because it's not really
> "state" that's kept beyond the prepare-then-(commit|abort) sequence.

State doesn't have to be long-lived to be state. But I'm not too
worried about the exact name for it, if people have other preferences.

>
> What about the task->alloc_lock? Might that need to be taken by multiple
> subsystems? See my next comment.

My thought was that cgroups would take that anyway prior to calling
prepare_attach_nosleep(), since it's a requirement for changing
task->cgroups anyway.

>
> Rather than describing what might be called later for each API entry
> separately it might be simpler to prefix the whole API/protocol
> description with something like:
> ======
> A successful return from prepare_X will cause abort_X to be called if
> any of the prepatory calls fail. (where X is either sleep or nosleep)
>
> A successful return from prepare_X will cause commit to be called if all
> of the prepatory calls succeed. (where X is either sleep or nosleep)
>
> Otherwise no calls to abort_X or commit will be made. (where X is either
> sleep or nosleep)

I'll play with working that into the description.

> I think that's correct based on your descriptions. Of course changing
> this only makes sense if this proposal will go into Documentation/ in
> some form..

Yes, we'd definitely need to document this in some detail.

>
>        Allowing prepare_X to hold locks when it has exitted seems ripe for
> introducing two separate subsystems that inadvertently take locks out of
> order.

Yes, but I'm not sure that there's much that we can do about that. If
we want to guarantee to be able to rollback one subsystem when a later
subsystem fails then we have to let the earlier subsystems continue to
hold locks. Or is this too ambitious a goal to support?

Paul
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