Re: Serious regression caused by fix for [BUG 1/3] bsg queue oops with iscsi logout

[Mike Christie <michaelc@cs.wisc.edu>]

Mike Christie wrote:
> FUJITA Tomonori wrote:
>> On Wed, 26 Mar 2008 07:36:26 -0700
>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>
>>> On Wed, 2008-03-26 at 23:22 +0900, FUJITA Tomonori wrote:
>>>> On Sat, 22 Mar 2008 11:06:00 -0500
>>>> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>>>
>>>>> On Tue, 2008-03-11 at 00:36 -0500, Mike Christie wrote:
>>>>>> Mike Christie wrote:
>>>>>>> Pete Wyckoff wrote:
>>>>>>>> I think this used not to happen; not sure.  But I changed two 
>>>>>>>> things
>>>>>>> This most likely did not happen before 2.6.25-rc* or it broke in 
>>>>>>> slightly different ways, because iscsi used to try and do
>>>>>>>
>>>>>>> echo 1 > /sys/block/sdX/device/delete
>>>>>>>
>>>>>>> from userspace instead of calling scsi_remove_target from the 
>>>>>>> kernel.
>>>>>>>
>>>>>>> As you know around 2.6.21, the behavior of doing the echo to the 
>>>>>>> delete file changed due to a driver model and scsi change and 
>>>>>>> that broke the iscsi tools. The iscsi tools userspace removal was 
>>>>>>> sort of hack in the first place and was racey, so we switched to 
>>>>>>> removing devices/target like the FC class.
>>>>>>>
>>>>>>>
>>>>>>>> lately.  2.6.25-rc1 to -rc4 and fedora 8 iscsi-initiator-utils 
>>>>>>>> (865) to
>>>>>>>> fedora devel (868).  Bidi and varlen patches always too.
>>>>>>>>
>>>>>>>> I'll follow with some more variations on this theme.  Looks like 
>>>>>>>> bsg
>>>>>>>> needs to protect more carefully against the device going away.  Any
>>>>>>>> ideas how best to do this?  What was the approach in sg?
>>>>>>>>
>>>>>>> I think sg is broken in similar ways. The iser guys have some 
>>>>>>> tests cases that have broken sg while IO is outstanding. I am 
>>>>>>> ccing Erez.
>>>>>> Actually one of the problems looks a little different than some of 
>>>>>> the problems hit with sg and are caused because we remove the bsg 
>>>>>> device too soon. I think we want to wait until all the references 
>>>>>> from the commands/requests are released. The attached patch 
>>>>>> (untested) moves the bsg unreg call to the scsi device release fn.
>>>>> Well, this fix is now upstream.  However, it's causing all our
>>>>> scsi_devices never to get released, which is a serious regression.
>>>>> We're also doing spurious bsg_unregister_queue() for things that never
>>>>> actually registered one (all scan devices that return DID_NO_CONNECT),
>>>>> but bsg doesn't seem to be complaining about this.
>>>>>
>>>>> The essence of the problem is that bsg_register_queue() takes a ref to
>>>>> the sdev_gendev, so you can't move bsg_unregister_queue() into the
>>>>> release function because nothing ever puts bsg's device ref and so
>>>>> release is never called.
>>>>>
>>>>> Options for fixing this before 2.6.25 are
>>>>>
>>>>>      1. revert the patch
>>>>>      2. Do an additional put for the bsg reference in
>>>>>         __scsi_remove_device (patch below).  It's nasty but it 
>>>>> preserves
>>>>>         the semantics and does what you want
>>>> After some investigation, this patch doesn't fix the bug that Pete
>>>> reported (I'll send a new patch shortly).
>>>>
>>>> Can you revert the commit 4b6f5b3a993cbe34b4280f252bccc76967c185c8
>>>> instead of merging this?
>>> Sure ... I didn't like the hack either.  As long as iSCSI is fine with
>>> the reversion it's the quickest way to fix the problem.
>>
>> How about this? With the commit reversion, I confirmed that this patch
>> fixes the first bug that Pete reported:
>>
>> http://marc.info/?l=linux-scsi&m=120508166505141&w=2
>>
>> I suspect that this could fix the rest too.
>>
>> =
>> From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
>> Subject: [PATCH] bsg: takes a ref to struct device in fops->open
>>
>> bsg_register_queue() takes a ref to struct device that a caller
>> passes. For example, it takes a ref to the sdev_gendev with scsi
>> devices. However, bsg doesn't takes a ref to it in fops->open. So
>> while an application opens a bsg device, the scsi device that the bsg
>> device holds can go away (bsg also takes a ref to a queue, but it
>> doesn't prevent the device from going away).
>>
>> With this, bsg takes a ref to struct device in fops->open and frees it
>> in fops->release.
>>
>> Note that bsg doesn't need to takes a ref to a queue for SCSI devices
>> at least. I think that it would be better to remove the code but I let
>> it alone for now.
>>
> 
> Why does bsg_add_device do kobject_get instead of blk_get_queue?
> 
> It seems like if we added a blk_qet_queue when we opened the device and 
> a blk_put_queue when bsg_release is called we could remove the 
> get/put_device calls. I am not sure if that is cleaner or not. I was 
> just thinking that bsg goes from bsg->request_queue->scsi_device so 
> maybe it should not worry about the device.

Doh, I guess we sort of do this today. It looks like the blk_execute 
functions are bypassing the QUEUE_FLAG_DEAD checks, so 
scsi_device_dev_release_usercontext could have called scsi_free_queue, 
but if bsg calls blk_execute at the same time then it could stick a 
request into the queue and end up calling the scsi_request_fn (maybe 
this is what happens in #2 and when scsi_request_fn calls get_device we 
get that weird error since the refcount on the device is zero).