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

From: Nicholas A. Bellinger
Date: Tue Dec 02 2008 - 00:05:58 EST

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:
> >>>
> >>>
> >>>
> 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 }
96 /**
(gdb) list *(elv_dequeue_request+0x19)
0x131c is in elv_dequeue_request (block/elevator.c:836).
831 EXPORT_SYMBOL(elv_next_request);
833 void elv_dequeue_request(struct request_queue *q, struct request
834 {
835 BUG_ON(list_empty(&rq->queuelist));
836 BUG_ON(ELV_ON_HASH(rq));
838 list_del_init(&rq->queuelist);
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:
> >>>
> >>>
> >>>
> 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,



To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at