linux-kernel - Re: USB storage no-boot regression (bisected)

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Date:	Wed, 15 Apr 2009 10:06:17 -0700
From:	VomLehn <dvomlehn@...co.com>
To:	Mark Lord <lkml@....ca>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <greg@...ah.com>,
	Jeff Garzik <jeff@...zik.org>,
	Linux USB kernel mailing list <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: USB storage no-boot regression (bisected)

On Wed, Apr 15, 2009 at 11:49:45AM -0400, Mark Lord wrote:
> Alan Stern wrote:
>> On Wed, 15 Apr 2009, Alan Cox wrote:
>>
>>> Why should every user suffer a slower boot and a poorer resume time ?
>>>
>>> Instead make the root fs mounting look like this
>>>
>>>
>>> 	while(my_rootfs_hasnt_appeared_and_i_am_sad()) {
>>> 		wait_on(&new_disk_discovery);
>>> 	}
>>>
>>> and poke the queue whenever we add a relevant device.
>>>
>>> That way if you are booting off an initrd you can finish the SATA probe
>>> in parallel to getting userspace ticking over.
>>>
>>> On what is nowdays essentially a hot plug system it all needs turning
>>> this way up - eg RAID volumes should assemble and come online as the
>>> drives are discovered not at some fixed point later in userspace.
>>
>> Indeed, something like this should also be used for  
>> resume-from-hibernation, to wait for the swap device.
> ..
>
> It just needs a way to set a finite timeout, so that server room
> equipment can auto-panic-reboot and try again if a device has died.

The problem with USB root devices is the same one I brought up a couple of
weeks ago--faster booting means that USB boot devices fail. We now have
problems with three different classes of devices:

o	Disks
o	Network devices
o	Serial consoles

Saying that we were "lucky" that things worked before is no help and
you should be aware that it ticks people off. I agree that this is not a
USB problem, but there is a *very* real problem:

     The work to decrease boot time has exposed race conditions that
     always existed, but are now making the kernel less usable.

So, instead of spending time denying that there is a USB problem, let's
focus on solve the boot device synchronization problem.  I have already
posted a (probably incomplete, possibly wrong) patch to synchronize console
initialization. We need to do the same for other boot devices, too.

David VomLehn
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