linux-kernel - Re: Changes to Linux/SCSI target mode infrastructure for v2.6.28

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Date:	Mon, 01 Dec 2008 21:05:35 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Anderson <andmike@...ux.vnet.ibm.com>,
	Mike Christie <michaelc@...wisc.edu>,
	Christoph Hellwig <hch@....de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...l.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Hannes Reinecke <hare@...e.de>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Linux-iSCSI.org Target Dev" 
	<linux-iscsi-target-dev@...glegroups.com>
Subject: Re: Changes to Linux/SCSI target mode infrastructure for v2.6.28

On Tue, 2008-12-02 at 13:18 +0900, Tejun Heo wrote:
> Nicholas A. Bellinger wrote:
> >>> So far during my initial testing, I am running into a two different
> >>> exceptions.  One NULL pointer deference OOPS after half dozen Open/iSCSI
> >>> login/logouts in block/elevator.c:elv_dequeue_request().   Here is the
> >>> trace from SCSI softirq context:
> >>>
> >>> http://linux-iscsi.org/builds/user/nab/2.6.28-rc6-oops-0.png
> >>> http://linux-iscsi.org/builds/user/nab/2.6.28-rc6-oops-1.png
> 
> Can you build with debug info and find out which line is the offending
> one?

(gdb) list *(elv_dequeue_request+0x1d)
0x1320 is in elv_dequeue_request (include/linux/list.h:92).
87	 * This is only for internal list manipulation where we know
88	 * the prev/next entries already!
89	 */
90	static inline void __list_del(struct list_head * prev, struct
list_head * next)
91	{
92		next->prev = prev;
93		prev->next = next;
94	}
95	
96	/**
(gdb) list *(elv_dequeue_request+0x19)
0x131c is in elv_dequeue_request (block/elevator.c:836).
831	EXPORT_SYMBOL(elv_next_request);
832	
833	void elv_dequeue_request(struct request_queue *q, struct request
*rq)
834	{
835		BUG_ON(list_empty(&rq->queuelist));
836		BUG_ON(ELV_ON_HASH(rq));
837	
838		list_del_init(&rq->queuelist);
839	
840		/*


> 
> >>> The other one is a BUG_ON in blk/blk-timeout.c:177 in blk_add_timeout()
> >>> that happens after a few hundred MB of READ_10 traffic, which also
> >>> appears to pass through elv_dequeue_request() at some point:
> >>>
> >>> http://linux-iscsi.org/builds/user/nab/2.6.28-rc6-oops-2.png
> >>> http://linux-iscsi.org/builds/user/nab/2.6.28-rc6-oops-4.png
>
> Hmmm... this means blk_add_timer() is being called after the request
> is already completed.  All the problem discovered till now have to do
> with timeout going off without the low level driver knowing about the
> request.  I don't have much idea and it'll probably be best to trace
> what's going on using blktrace or printks.

<nod>, OK.

>   Maybe this is caused by
> list corruption as with the first issue or request completion races
> with requeueing?

Hrmm, yeah, perhaps the use of scsi_req_map_sg() (which obviously still
has struct bio behind it) is causing the breakage..  I will wait for
Tomo, Boaz and co to have a look at the original patch to
lio-core-2.6.git/drivers/lio-core/target_core_pscsi.c and see if I am
missing something obvious.

Also, with the previous patch to drivers/scsi/scsi_lib.c(), I am able to
move a few GB of bulk I/O and not hit the BUG_ON in
blk/blk-timeout.c:177 in blk_add_timeout() mentioned above when feeding
struct request into struct scsi_device->request_queue with
blk_execute_rq_nowait() with use_sg > 0 CDBs.  However, I am still
running into another reproducable BUG_ON in
block/cfq-iosched.c:cfq_find_next_rq() after extended bulk I/O tests.

Many thanks for your comments,

--nab

> 

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