Re: Remove scsi_wait_scan module

[James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>]

On Wed, 2012-05-30 at 11:26 -0700, Dan Williams wrote:
> On Mon, May 28, 2012 at 5:07 AM, James Bottomley
> <James.Bottomley-JuX6DAaQMKPCXq6kfMZ53/egYHeGw8Jk@public.gmane.org> wrote:
> > On Mon, 2012-05-28 at 10:00 +0000, maximilian attems wrote:
> >> On Sun, May 27, 2012 at 10:13:46AM +0100, James Bottomley wrote:
> >> > scsi_wait_scan was introduced with asynchronous host scanning as a hack
> >> > for distributions that weren't using proper udev based wait for root to
> >> > appear in their initramfs scripts.  In 2.6.30 Commit
> >>
> >> > c751085943362143f84346d274e0011419c84202
> >> > Author: Rafael J. Wysocki <rjw-KKrjLPT3xs0@public.gmane.org>
> >> > Date:   Sun Apr 12 20:06:56 2009 +0200
> >> >
> >> >     PM/Hibernate: Wait for SCSI devices scan to complete during resume
> >> >
> >> > Actually broke scsi_wait_scan because it renders
> >> > scsi_complete_async_scans() a nop for modular SCSI if you include
> >> > scsi_scans.h (which this module does).
> >> >
> >> > The lack of bug reports is sufficient proof that this module is no
> >> > longer used.
> >>
> >> We do use it in initramfs-tools.
> >>
> >> There is quite a number of bug reports moaning about having to boot with
> >> `scsi_mod.scan=sync'. I didn't pass them on, because I didn't knew that
> >> the module itself got broken, for example:
> >> http://bugs.debian.org/616689
> >
> > OK, so what these bugs show is the breakage ... basically scsi_wait_scan
> > isn't really waiting for the scans to complete.  I can fix it in stable
> > so you can close your bug reports, but if I do, can you also transition
> > away from using it so I can remove it in 3.5?
> 
> Is there some other method whereby userspace can sync all driver
> probing actions?

No,  but then there never really was.  The theory is you know all the
disks you need (/ /usr and so on) and you just wait for them to appear
before mounting them and proceeding with boot.

> We won't need scsi_complete_async_scans() after:
> 
>   http://marc.info/?l=linux-scsi&m=133840132007532&w=2
> 
> ...but won't initramfs environments still need a way to trigger
> wait_for_device_probe()?  Something like echo "flush" >
> /sys/devices/async_probe. and maybe reading that file indicates if
> some async probing is still in-flight?

Why?  The job of an initramfs is to mount root.  All it has to do is
wait for root to appear via udev and then proceed.  The whole reason for
doing stuff async initially was to speed boot, so probing can still be
ongoing even after the initrd exits.

If you think about it, most modern fabrics are hot plug.  Just because
the initial scan has completed there's no guarantee that all the devices
have appeared yet.

James