linux-kernel - Re: Regression - SATA disks behind USB ones on v4.8-rc1, breaking boot. [Re: Who reordered my disks (probably v4.8-rc1 problem)]

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Date:	Sun, 14 Aug 2016 13:29:58 +0200
From:	Greg KH <gregkh@...uxfoundation.org>
To:	David Lang <david@...g.hm>
Cc:	Tom Yan <tom.ty89@...il.com>, Pavel Machek <pavel@....cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	james harvey <jamespharvey20@...il.com>,
	regressions@...mhuis.info, hdegoede@...hat.com,
	Tejun Heo <tj@...nel.org>, linux-ide@...r.kernel.org,
	linux-usb@...r.kernel.org, Oliver Neukum <oliver@...kum.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: Regression - SATA disks behind USB ones on v4.8-rc1, breaking
 boot. [Re: Who reordered my disks (probably v4.8-rc1 problem)]

On Sun, Aug 14, 2016 at 03:26:45AM -0700, David Lang wrote:
> On Sun, 14 Aug 2016, Tom Yan wrote:
> 
> > On 14 August 2016 at 18:07, Tom Yan <tom.ty89@...il.com> wrote:
> > > On 14 August 2016 at 18:01, Pavel Machek <pavel@....cz> wrote:
> > > > 
> > > > Since SATA support was merged, certainly since v2.4, and from way
> > > > before /dev/disk/by-id existed.
> > > 
> > > I have no idea how "SATA before USB" had been done in the past (if it
> > > was ever a thing in the kernel), but that has not been the case since
> > > at least v3.0 AFAIR.
> > > 
> > > > 
> > > > People may not run udev, and you can't use /dev/disk/by-id on kernel
> > > > command line.
> > > > 
> > > 
> > > No, but you can always use root=PARTUUID=, that's built into the
> > > kernel. (root=UUID= requires udev or so though).
> > 
> > Silly me. root=UUID= has nothing to do with udev, but `blkid` in
> > util-linux. At least that's how it's done in Arch/mkinitcpio.
> > 
> 
> The rule is "don't break working systems", not "but we are allowed to break
> systems, see it says here not to depend on this"
> 
> Drive ordering has been stable since the 0.1 kernel [1]

Drive probing order of USB has always been non-deterministic, so while I
agree that it is not good to break existing systems at all, perhaps this
is on the edge of what works vs. doesn't work?

I know my USB drives always seem to come up in random order, which is
why tools like udev were invented :)

> It takes a lot longer to detect USB drives, why in the world would they be
> detected before hard-wired drives?

Depends, some hard-wired drives take much longer to find than USB ones.

That being said, it would be great if the original reporter could use
'git bisect' and let the linux-usb and linux-scsi mailing list know what
the offending patch is, and we can take it from there.

thanks,

greg k-h