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

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Date:	Mon, 1 Dec 2008 22:40:36 -0800
From:	Mike Anderson <andmike@...ux.vnet.ibm.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Tejun Heo <tj@...nel.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	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

Nicholas A. Bellinger <nab@...ux-iscsi.org> wrote:
> On Tue, 2008-12-02 at 13:18 +0900, Tejun Heo wrote:
> > 
> > >>> 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.

or is it possible since elv_dequeue_request BUG_ON check of queuelist did
not trigger a request is on the queuelist with a timeout_list not empty.

It would be interesting for a debug run to change the
"BUG_ON(!list_empty(&req->timeout_list))" in blk_add_timer to print out
the cmd_flags plus req->atomic_flags and also add a
"BUG_ON(!list_empty(&rq->timeout_list))" to elv_insert to ensure a request
is never added to the queuelist with a timeout_list not empty.

> 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.
> 

-andmike
--
Michael Anderson
andmike@...ux.vnet.ibm.com
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