Re: [patch] Regression in 2.6.19-rc microcode driver

[Andrew Morton]

On Tue, 07 Nov 2006 09:20:27 +0800
Shaohua Li <shaohua.li@xxxxxxxxx> wrote:

> On Mon, 2006-11-06 at 15:15 +0100, Arjan van de Ven wrote:
> > Hi,
> > 
> > if the microcode driver is built in (rather than module) there are some,
> > ehm, interesting effects happening due to the new "call out to
> > userspace" behavior that is introduced.. and which runs too early. The
> > result is a boot hang; which is really nasty.
> > 
> > The patch below is a minimally safe patch to fix this regression for
> > 2.6.19 by just not requesting actual microcode updates during early
> > boot. (That is a good idea in general anyway)
> > 
> > The "real" fix is a lot more complex given the entire cpu hotplug
> > scenario (during cpu hotplug you normally need to load the microcode as
> > well); but the interactions for that are just really messy at this
> > point; this fix at least makes it work and avoids a full detangle of
> > hotplug.
> Yes, this is an issue which I documented in my patch. It's not a hang,
> but a long delay if you have many cpus.

Due to the timeout?  So it should come back after 10*num_online_cpus seconds?

Does Arjan have a lot of CPUs?

> Other drivers with firmware
> request have the same issue if they are built-in. Maybe we should fix
> the firmware request mechanism itself. I hope no distribution has
> microcode driver built-in.

But what would a fix look like?  I think things would work OK if all the
appropriate stuff is present in initramfs, yes?  We wouldn't want to break
that.

hm.  kobject_uevent() stupidly returns void.  If we were to fix that, is
there any reason why _request_firmware() should still wait for ten seconds
if kobject_uevent() returned a synchronous error?  (ie:
__call_usermodehelper failed?)

Answer: yes.  That won't work because request_firmware() uses
call_usermodehelper(wait=0) (iirc this bad thing was done because of
deadlock problems which were hard to fix properly).

But all it not lost - because call_usermodehelper() will use CLONE_VFORK I
_think_ we can still work out whether the child thread successfully exec'ed
a new program.  It'd take a bit of hacking on the fork() code to make that
work though.

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