Re: [Bugme-new] [Bug 8940] New: BUG: unable to handle kernel NULL pointer dereference at virtual address 00000000

On Sun, 26 Aug 2007, Matthew Wilcox wrote:

> On Sun, Aug 26, 2007 at 12:52:07AM -0700, Andrew Morton wrote:
> > On Sat, 25 Aug 2007 09:47:40 -0700 (PDT) [email protected] wrote:
> > > Problem Description: I got this error message:
> > > BUG: unable to handle kernel NULL pointer dereference at virtual address
> > > 00000000
> > > in dmesg output when connecting a USB device. Turning off
> > > CONFIG_SCSI_SCAN_ASYNC fixed the problem. Apparently there is a bug in SCSI
> > > core code.
> 
> I don't think SCSI_SCAN_ASYNC is the problem.  It's probably a
> coincidence.  SCSI_SCAN_ASYNC doesn't touch the call-path reported in
> the backtrace.

It's not a coincidence.  The oops occurred because of the way the async
scanning routine registers new devices.  See the explanation and
discussion in this thread:

	http://marc.info/?l=linux-scsi&m=118650567017151&w=2

> In any case, if it is SCSI_SCAN_ASYNC-related, there's an outstanding
> patch to fix the locking, which is slated for inclusion in 2.6.24.
> http://git.kernel.org/?p=linux/kernel/git/jejb/scsi-misc-2.6.git;a=commit;h=a93a091df8232fad60867d41fbc3be855a0b78f2

I have seen exactly this same problem, and it also shows up in Bugzilla
entries #8840 and #8846.  The patch mentioned above did fix it.

I thought (and still do think!) that the patch should go into 2.6.23 
and 2.6.22-stable.  Why wait for 2.6.24 for a serious bugfix?

Alan Stern


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