linux-kernel - Re: acpi dsts loading and populate

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Date:	Mon, 11 Feb 2008 16:47:19 +0300
From:	Sergey Vlasov <vsu@...linux.ru>
To:	Eric Piel <eric.piel@...mplin-utc.net>
Cc:	Christoph Hellwig <hch@....de>, dsdt@...gusch.at,
	len.brown@...el.com, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>, trenn@...e.de
Subject: Re: acpi dsts loading and populate_rootfs

On Sun, 10 Feb 2008 12:58:09 +0100 Eric Piel wrote:

> (adding some CC's)
> Christoph Hellwig wrote:
> > On Sun, Feb 10, 2008 at 08:12:26AM +0100, Christoph Hellwig wrote:
> >> Folks, moving this call around hidden behing in completely unreviewed
> >> acpi junk is not acceptable.
> >>
> >> Either populate_rootfs _is_ safe to be called earlier and then we should
> >> do it always or it's not.  Either way such a change should be posted
> >> separately and reviewd on lkml.
> Hi,
>
> I agree with you, the order of the initialization should not be changed
> by some config options. Either it works, or it doesn't! So the question
> now is : "is fine to move populate_rootfs() earlier?"
>
> Some data points:
> * It used to be called earlier. It was moved to be called later about a
> year ago by Linus (8d610dd52dd1da696e199e4b4545f33a2a5de5c6) with this log:
> >     Make sure we populate the initroot filesystem late enough
> >
> >     We should not initialize rootfs before all the core initializers have
> >     run.  So do it as a separate stage just before starting the regular
> >     driver initializers.
> So there must be some good reasons to keep it late enough...
>
> * This patch is used in SuSE, and it seems noone has ever had a problem
> with the initramfs early init.
>
> I guess the problem that Linus solved by moving populate_rootfs()
> happens only rarely or on only few configurations. Linus, do you
> remember what kind of problem it was? How can I reproduce it?

AFAIR the problem was that the kernel was trying to exec /sbin/hotplug
before some kernel subsystems (e.g., pipe support) were completely
initialized, which caused oopses during boot.

Initramfs images generated by distro tools usually do not contain
/sbin/hotplug, and therefore avoid the problem.  (The kernel might
also call /sbin/modprobe by itself, but it either does not do it
during early boot, or /sbin/modprobe does not use kernel features
which had not been initialized yet).

> One different solution that I implemented [1] was to have an
> "early_populate_rootfs()" before the acpi_early_init(), leaving
> populate_rootfs() at the normal place. If the early version fails, it's
> ok: we can't override the DSDT, but the later version will work as usual
> anyway. This solution also seems to work quite often (it's used in
> Ubuntu, Mandriva and PCLinuxOS)

This would not solve the problem, because /sbin/hotplug (if present in
the initramfs image) will exist too early.

> Would that seem an acceptable solution? Or what other way exists?

Disabling call_usermodehelper() until all core initializers had
completed would fix the problem too; will such change be acceptable?

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