Date: Wed, 15 Sep 2004 01:20:11 +0200
From: Andrea Arcangeli <andrea@novell.com>
To: Greg KH <greg@kroah.com>
Cc: "Marco d'Itri" <md@Linux.IT>, "Giacomo A. Catenazzi" <cate@pixelized.ch>,
       linux-kernel@vger.kernel.org
Subject: Re: udev is too slow creating devices
Message-ID: <20040914232011.GG3365@dualathlon.random>
References: <41473972.8010104@debian.org> <41474926.8050808@nortelnetworks.com> <20040914195221.GA21691@kroah.com> <414757FD.5050209@pixelized.ch> <20040914213506.GA22637@kroah.com> <20040914214552.GA13879@wonderland.linux.it> <20040914215122.GA22782@kroah.com> <20040914224731.GF3365@dualathlon.random> <20040914230409.GA23474@kroah.com>
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On Tue, Sep 14, 2004 at 04:04:09PM -0700, Greg KH wrote:
> On Wed, Sep 15, 2004 at 12:47:31AM +0200, Andrea Arcangeli wrote:
> > On Tue, Sep 14, 2004 at 02:51:22PM -0700, Greg KH wrote:
> > > True, so sit and spin and sleep until you see the device node.  That's
> > > how a number of distros have fixed the fsck startup issue.
> > 
> > that's more a band-aid than a fix (I can imagine a userspace hang if the
> > device isn't created for whatever reason), if there's no way to do
> > better than this if you've to run fsck (or if it's not the best to run
> > the fsck inside the dev.d scripts), then probably this needs better
> > fixing. is such a big problem to execute a sys_wait4 to wait the udev
> > userspace to return before returning from the insmod syscall?
> 
> But how do you know what to wait for?

the kernel sure can know about it, by passing a waitqueue into the
registration routine and calling wake_up once the discovery is over.

> Sitting and waiting is a band-aid, I agree.  That's why we created the
> /etc/dev.d/ notifier system to fix this issue (that is there for systems
> that don't even use udev.)

if the fsck can run from there ok, if for some reason it's not feasible
to run it from there (like if it would create a bus congestion by
running all the fsck in parallel if you attach multiple devices at the
same time), and/or you need some other seriaization, then probably
having a way to run things serially without a
spin-and-sleep-and-risk-to-hang could be needed. I guess in the worst
case one could serialize things by using file locking in /var/run
and by creating an API between the dev.d and the init.d scripts, is that
how the long term is supposed to work?

So it's more a question if the current interface is complete for all
usages, and if the fsck spin-and-sleep is just a temporary band-aid, or
if the spin-and-sleep is supposed to last longer than a few month
release cycle.

(and I'm not a udev guru, just in listen mode ;)
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