Re: udevd: serialize device chain event sequence

[Kay Sievers <kay.sievers@vrfy.org>]

On Wed, 2004-12-08 at 19:07 -0800, David Brownell wrote: 
> On Wednesday 08 December 2004 3:13 pm, Kay Sievers wrote:
> > Any objections against the serialization of the event sequence of a
> > chain of devices. Currently udevd delays only events for the same
> > DEVPATH.
> 
> I'm amused ... that's what the original hotplug code achieved.
> Thing is, now serialization would be done in userspace, rather
> than offering new and fun paths to kernel deadlock nirvana!  :)

Yeah, the same applies to current userspace code, like the one we have
in HAL :) 

> > Example of an "add" event sequence:
> >   /block/sda
> >   /block/sda/sda1
> > 
> > With this change, we make sure, that the udev process handling /block/sda
> > has finished its work (waited for all attributes, created the node) before
> > we fork the udev event for /block/sda/sda1. This way the event for sda1 can
> > be sure, that the node for the main device is already created (may be
> > useful for disk labels).
> 
> But the "add" for the underlying hardware (maybe USB) would
> not be serialized.

Right, just the stream of events for the /sys/devices/-device is
serialized, but in another event sequence. The class/block device
carries from kernel 2.6.10+ on the PHYSDEVPATH variable in the
environment, which makes it easy to know what to wait for. 

> > The main motivation to do this is the program execution of the dev.d/
> > and hotplug.d/ directory. If we don't wait for the parent event to exit,
> > we can't be sure that the executed scripts are run in the right order.
> 
> Could that argument apply to the underlying hardware, too?

Good question. I never thought about that until you asked. :) It sounds
nice to let udevd serialize the complete event stream, based on
PHYSDEVPATH. I will try it today and come back...

Thanks,
Kay




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