Subject: Re: kernel BUG at drivers/scsi/scsi_lib.c:1096!

From: Pratyush Anand <[email-masked]>
Date: Mon, 23 Nov 2015 20:43:30 +0530

On 23/11/2015:02:57:19 PM, Laurent Dufour wrote:
> On 23/11/2015 00:20, Mark Salter wrote:
> > On Sun, 2015-11-22 at 00:56 +0800, Ming Lei wrote:
> >> On Sat, 21 Nov 2015 12:30:14 +0100
> >> Laurent Dufour <[email-masked]> wrote:
> >>
> >>> On 20/11/2015 13:10, Michael Ellerman wrote:
> >>>> On Thu, 2015-11-19 at 00:23 -0800, Christoph Hellwig wrote:
> >>>>
> >>>>> It's pretty much guaranteed a block layer bug, most likely in the
> >>>>> merge bios to request infrastucture where we don't obey the merging
> >>>>> limits properly.
> >>>>>
> >>>>> Does either of you have a known good and first known bad kernel?
> >>>>
> >>>> Not me, I've only hit it one or two times. All I can say is I have hit it in
> >>>> 4.4-rc1.
> >>>>
> >>>> Laurent, can you narrow it down at all?
> >>>
> >>> It seems that the panic is triggered by the commit bdced438acd8 ("block:
> >>> setup bi_phys_segments after splitting") which has been pulled by the
> >>> merge d9734e0d1ccf ("Merge branch 'for-4.4/core' of
> >>> git://git.kernel.dk/linux-block").
> >>>
> >>> My system is panicing promptly when running a kernel built at
> >>> d9734e0d1ccf, while reverting the commit bdced438acd8, it can run hours
> >>> without panicing.
> >>>
> >>> This being said, I can't explain what's going wrong.
> >>>
> >>> May Ming shed some light here ?
> >>
> >> Laurent, looks there is one bug in blk_bio_segment_split(), would you
> >> mind testing the following patch to see if it fixes your issue?
> >>
> >> ---
> >> From 6fc701231dcc000bc8bc4b9105583380d9aa31f4 Mon Sep 17 00:00:00 2001
> >> From: Ming Lei <[email-masked]>
> >> Date: Sun, 22 Nov 2015 00:47:13 +0800
> >> Subject: [PATCH] block: fix segment split
> >>
> >> Inside blk_bio_segment_split(), previous bvec pointer('bvprvp')
> >> always points to the iterator local variable, which is obviously
> >> wrong, so fix it by pointing to the local variable of 'bvprv'.
> >>
> >> Signed-off-by: Ming Lei <[email-masked]>
> >> ---
> >>  block/blk-merge.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >> index de5716d8..f2efe8a 100644
> >> --- a/block/blk-merge.c
> >> +++ b/block/blk-merge.c
> >> @@ -98,7 +98,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> >>  
> >>  			seg_size += bv.bv_len;
> >>  			bvprv = bv;
> >> -			bvprvp = &bv;
> >> +			bvprvp = &bvprv;
> >>  			sectors += bv.bv_len >> 9;
> >>  			continue;
> >>  		}
> >> @@ -108,7 +108,7 @@ new_segment:
> >>  
> >>  		nsegs++;
> >>  		bvprv = bv;
> >> -		bvprvp = &bv;
> >> +		bvprvp = &bvprv;
> >>  		seg_size = bv.bv_len;
> >>  		sectors += bv.bv_len >> 9;
> >>  	}
> > 
> > I'm still hitting the BUG even with this patch applied on top of 4.4-rc1.
> 
> On my side, with the patch applied on top of 4.4-rc1, I can't get the
> panic anymore.

git bisect shows:

bdced438acd83ad83a6c6fc7f50099b820245ddb is the first bad commit
commit bdced438acd83ad83a6c6fc7f50099b820245ddb
Author: Ming Lei <[email-masked]>
Date:   Tue Oct 20 23:13:52 2015 +0800 

    block: setup bi_phys_segments after splitting

Reverting above commit on top if 4.4-rc1 seems to fix the problem for me.

~Pratyush