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Date: Tue, 05 Feb 2002 23:40:22 -0500
From: Douglas Gilbert <dougg@torque.net>
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To: Jens Axboe <axboe@suse.de>
CC: Ralf Oehler <R.Oehler@GDAmbH.com>, Scsi <linux-scsi@vger.kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: one-line-patch against SCSI-Read-Error-BUG()
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Jens Axboe wrote:
> 
> On Tue, Feb 05 2002, Ralf Oehler wrote:
> > Hi, List
> >
> > I think, I found a very simple solution for this annoying BUG().
> 
> You fail to understand that the BUG triggering indicates that their is a
> BUG _somewhere_ -- the triggered BUG is not the bug itself, of course,
> that would be stupid :-)
> 
> > Since at least kernel 2.4.16 there is a BUG() in pci.h,
> > that crashes the kernel on any attempt to read a SCSI-Sector
> > from an erased MO-Medium and on any attempt to read
> > a sector from a SCSI-disk, which returns "Read-Error".
> >
> > There seems to be a thinko in the corresponding code, which
> > does not take into account the case where a SCSI-READ
> > does not return any data because of a "sense code: read error"
> > or a "sense code: blank sector".
> >
> > I simply commented out this BUG() statement (see below)
> > and everything worked well from there on. The BUG()
> > seems to be inadequate.
> 
> The BUG is dangerous, because it means mapping DMA to a 0 address (plus
> offset). Naturally there is no way in hell your "fix" will be applied,
> since it a pretty bad bandaid.
> 
> A safer solution for you right now would be to just terminate the
> mapping when you encounter this. So something ala
> 
>         if (!sg[i].page && !sg[i].address)
>                 return i;
> 
> That's not a bug fix either, but at least it's safer than your version.
> Stock kernel won't be patched until the real problem is found, though.

Jens,
The sg driver was tripping that BUG() and I have submitted
a patch for that. However Ralf probably wasn't using sg
to read his MO disk.

There are several uses of "sgpnt[i].address" in scsi_lib.c that
you might like to look at. Can't see any other uses in sd, sr
or the mid level. Another possibility is a missing memset or
explicit NULL initialisation on a scatterlist array.

Doug Gilbert
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