[OE-core] [PATCH 08/20] udev-cache: Create cache asynchronously

[Otavio Salvador]

On Mon, Aug 4, 2014 at 6:37 PM, Richard Tollerton <rich.tollerton at ni.com> wrote:
> Otavio Salvador <otavio at ossystems.com.br> writes:
>
>> On Mon, Aug 4, 2014 at 6:18 PM, Richard Tollerton <rich.tollerton at ni.com> wrote:
>>> Otavio Salvador <otavio at ossystems.com.br> writes:
>>>
>>>> I am not sure about this one. I see the value you are adding here but
>>>> I worry how often something can be connected during this process and
>>>> change the contents along the way. Did you see something as that
>>>> during your tests?
>>>
>>> No, but point taken. I didn't really test "physical" hotplug during
>>> bootup -- and in particular, for this to be a persisting issue, the
>>> added/removed device must be removed/added during this boot -- but I
>>> suppose that is totally possible.
>>>
>>> This reflects an underlying race condition, starting at the computation
>>> of NEWDATA/OLDDATA in the udev initscript, and ending in the tarball
>>> creation in udev-cache. I suppose that we can mitigate this by spinning
>>> in udev-cache until the devcache stops changing...? Cheesy, untested,
>>> dirty RFC follows:
>>
>> You need to check the list of files taken by 'tar' and compare. This
>> mitigates it but does not really solves the issue.
>
> Hmm, I'm confused. If we relied entirely on /dev contents, how would we
> detect (and correct for) if a device was removed between the execution
> of udev and of udev-cache?
>
>> Maybe we could "copy" the nodes to a tmp area and use this as a
>> snapshot? The copy should be fast and makes it mostly "atomic".
>
> I agree that the copy of the contents of /dev needs to be as fast as
> possible -- your about using tmpfs as a staging area is well taken. But
> beyond that, it seems to me that the root issue you articulated is about
> ensuring adequate synchronization of udev.cache and the tarball;
> rereading CMP_FILE_LIST in udev-cache thus seems unavoidable.

Yes, that's why I think we need:

 - copy contents to a staging area
 - generate the tarball

This way we avoid the following steps:

 - generate the list of contents
 - generate the tarball
 - generate tarball's contents
 - compare, loop if does not match.

Makes sense?

-- 
Otavio Salvador                             O.S. Systems
http://www.ossystems.com.br        http://code.ossystems.com.br
Mobile: +55 (53) 9981-7854            Mobile: +1 (347) 903-9750