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

From: Laurent Dufour <[email-masked]>
Date: Mon, 23 Nov 2015 16:20:39 +0100

On 23/11/2015 16:13, Pratyush Anand wrote:
> 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 'b
> ~Pratyush
> 
vprv'.
>>>>
>>>> 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;bdced438acd8
>>>>  			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.

That's what I mentioned earlier ;)

Now Ming send an additional patch with seems to fix the bug introduced
through the commit bdced438acd8. When testing with this new patch I
can't get the panic anymore, but Mark reported he is still hitting it.