Re: [gentoo-user] udev + /usr

[Canek Peláez Valdés]

On Thu, Sep 15, 2011 at 10:48 AM, Joost Roeleveld <joost@××××××××.org> wrote:
> 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.

Now we are talking. I am really, *REALLY* interested to know the devs
saying in the matter.

Regards.
-- 
Canek Peláez Valdés
Posgrado en Ciencia e Ingeniería de la Computación
Universidad Nacional Autónoma de México