Message-ID: <20120406221139.GA22854@kroah.com>
Date:	Fri, 6 Apr 2012 15:11:39 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
Cc:	James Bottomley <JBottomley@...allels.com>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sysfs: handle 'parent deleted before child added'

On Fri, Apr 06, 2012 at 02:44:37PM -0700, Williams, Dan J wrote:
> On Fri, Apr 6, 2012 at 2:17 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > On Fri, Apr 06, 2012 at 02:06:50PM -0700, Williams, Dan J wrote:
> >> On Fri, Apr 6, 2012 at 1:45 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> >> > On Fri, Apr 06, 2012 at 01:41:06PM -0700, Dan Williams wrote:
> >> >> In scsi at least two cases of the parent device being deleted before the
> >> >> child is added have been observed.
> >> >>
> >> >> 1/ scsi is performing async scans and the device is removed prior to the
> >> >>    async can thread running (can happen with an in-opportune / unlikely
> >> >>    unplug during initial scan).
> >> >
> >> > That sounds like a bug in the scsi code, doesn't it?
> >> >
> >> >> 2/ libsas discovery event running after the parent port has been torn
> >> >>    down (this is a bug in libsas).
> >> >
> >> > Is this fixed somewhere?
> >>
> >> Yes, these two issues have pending fixes that are posted to linux-scsi:
> >>
> >> http://marc.info/?l=linux-scsi&m=133239707903443&w=2
> >> http://marc.info/?l=linux-scsi&m=133239709603452&w=2
> >>
> >> > I don't want to paper over bugs like this by changing the sysfs core.
> >> > We went through this a lot years ago when scsi changed to use the driver
> >> > core, and I thought we had fixed all of these types of errors properly.
> >>
> >> Hotplug lifetime rules are still transport specific.  So in this case
> >> scsi-core is innocent these are bugs from libsas and
> >> scsi_transport_sas.
> >
> > Ok, thanks for the explaination.
> >
> >> > So, any chance to fix these properly as well?
> >>
> >> This patch doesn't really paper over anything.  It turns a NULL
> >> pointer crash into an explicit warning from kobject_add_internal.  For
> >> the libsas/scsi case this device_add() failure is still fatal.
> >> Regardless of whether sysfs changes the above two fixes are still
> >> required.
> >>
> >> Since the -EEXIST case is just a KERN_ERR and not a BUG_ON I figured
> >> it was worthwhile to post a patch to do the same for this 'parent
> >> deleted' case.  But if crashing is the expectation then this patch can
> >> be dropped.
> >
> > No, crashing is not the expectation :)
> 
> I thought not, but sometimes the kernel likes to teach people that
> bollix an api a hard lesson :).
> 
> >
> > But, without that crash, would the above fixes ever have been noticed
> > and fixed?  The device_add() most likely would have quietly failed and
> > no one would have been the wiser.
> >
> > Or would something else have caused this to be an obvious problem?
> >
> 
> We still have the big red flag dump_stack() in kobject_add_internal()
> (which patch 2 turns into a real WARN()), and for scsi our hotplug
> tests still crash later on because libsas makes assumptions about the
> device path.  I understand the paranoia here,  "check for NULL" is
> usually a band-aid, but in this case this is just a softer
> introduction to a debug session.  No less vocal than before as far as
> I can see.

Ok, that's reasonable, I'll queue this up for 3.5.

thanks,

greg k-h
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