Re: [gentoo-user] udev + /usr

[Joost Roeleveld]

On Thursday, September 15, 2011 10:32:50 AM Michael Mol wrote:
> On Thu, Sep 15, 2011 at 10:11 AM, Joost Roeleveld <joost@××××××××.org>
wrote:
> >> I'm not entirely convinced this is the case, because it feels like
> >> some situations like network devices (nbd, iSCSI) or loopback would
> >> require userland tools to bring up once networking is up.
> >
> > Yes, but the kernel-events referencing those devices won't appear untill
> > after the networking is brought up.
> > The scripts that "udev" starts are run *after* a device-event is
> > created. If the device itself has not been spotted by the kernel (for
> > instance, the networking doesn't exist yet), the event won't be
> > triggered yet.
> >
> > This situation does not require udev to start all these tools when
> > network- devices appear.
> >
> > I hope the following would make my thoughts a bit clearer:
> >
> > 1) kernel boots
> >
> > 2) kernel detects network device and places "network-device-event" in
> > the
> > queue
> >
> > 3) further things happen and kernel places relevant events in the queue
> > (some other events may also already be in the queue before step 2)
> >
> > 4) udev starts and starts processing the queue
> >
> > 5) For each event, udev creates the corresponding device-entry and
> > places
> > action-entries in a queue
> >
> > 6) system-init-scripts mount all local filesystems
> >
> > 7) udev-actions starts (I made up this name)
> >
> > 8) udev-actions processes all the entries in the action-queue
> >
> > From step 4, udev will keep processing further events it gets, which
> > means that if any action taken by "udev-actions" causes further devices
> > to become available, "udev" will create the device-entries and place
> > the action in the action-queue.
>
> So, if I read this correctly, there are two classes of processing
> events. kernel events and scripted actions. Here's rough pseudocode
> describing what I think you're saying. (Or perhaps what I'm hoping
> you're saying)
>
> while(wait_for_event())
> {
>   kevent* pkEvent = NULL;
>   if(get_waiting_kernel_event(pkEvent)) // returns true if an event was
> waiting {
>     process_kernel_event(pkEvent);
>   }
>   else
>   {
>     aevent* pAction = NULL;
>     if(get_waiting_action(pAction)) // Returns true if there's an
> action waiting.
>     {
>       process_action(pAction);
>     }
>   }
> }

This is, sort-of, what I feel should happen. But currently, in pseudo-code, 
the following seems to happen:
while(wait_for_event())
{
  kevent* pkEvent = NULL;
  if(get_waiting_kernel_event(pkEvent)) // returns true if an event was 
waiting {
    process_kernel_event(pkEvent);
  }
}

I would prefer to see 2 seperate processes:

--- process 1 ---
while(wait_for_event())
{
  kevent* pkEvent = NULL;
  if(get_waiting_kernel_event(pkEvent)) // returns true if an event was 
waiting
  {
    action_event = process_kernel_event(pkEvent);
    if (action_event != NULL)
    {
      put_action_event(pkEvent);
    }
  }
}

------

--- process 2 ---
while(wait_for_event())
{
  aevent* paEvent = NULL;
  if(get_waiting_action_event(paEvent)) // returns true if an event was 
waiting
  {
    process_action_event(paEvent);
  }
}
-------

> So, udev processes one event at a time, and always processes kernel
> events with a higher priority than resulting scripts. This makes a
> certain amount of sense; an action could launch, e.g. nbdclient, which
> would cause a new kernel event to get queued.

Yes, except that udev ONLY handles kernel-events and doesn't process any 
"actions" itself.
These are placed on a seperate queue for a seperate process.

> > If anyone has a setup where /usr can not be mounted easily, it won't
> > work
> > currently either and a init* would be necessary anyway.
> > (Am thinking of NFS, CIFS, iSCSI, NBD, special raid-drivers,.... hosting
> > /usr or other required filesystems)
>
> I don't see how this is relevant to actually fixing udev. (See below)
>
> > But anyone with a currently working environment should be able to expect
> > a currently working environment. If it fails to boot with only updating
> > versions, it's a regression. And one of the worst kinds of all.
>
> I agree that the direction udev is going is a regression. There aren't
> very many people active in this thread who would disagree with that
> point. So let's just drop it and focus on what a good, general
> solution would look like. (And anyone who says something amounting to
> 'status quo' for udev needs another explanation of why the udev
> developer sees the current scenario as broken. And he's right; the
> current scenario is architecturally unsound. I just think he's wrong
> about the solution.)

I agree he is wrong about the solution as well.

I have actually just posted my idea to the gentoo-dev list to see how the 
developers actually feel about possible splitting udev into 2 parts.

I'm not a good enough programmer to do this myself. But if anyone who can code 
and who also agrees with me that my idea for a solution is actually a good 
idea, please let me know and lets see how far we can get with implementing 
this solution.

--
Joost