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SubjectRe: 2.6.17-rc5-mm3 - crash in cfq_queue_empty() after iosched change
On Mon, Jun 05 2006, Valdis.Kletnieks@vt.edu wrote:
> I've been hitting this about once every two weeks for a while now,
> probably back to a 2.6.16-rc or so. It always bites at the same time
> while my laptop was at a point very late in bootup. I finally caught
> one when I had pen, paper, *and* time to chase it a bit rather than
> reboot. Sorry for the very partial traceback, it's not a good CTS day
> and I didn't have a digital camera handy.
>
> BUG: Unable to handle kernel NULL pointer dereference at 0x0000005c
> EIP at cfq_queue_empty+0x9/0x15
> call trace:
> elv_queue_empty+0x20/0x22
> ide_do_request+0xa4/0x788
> ide_intr+0x1ec/0x236
> handle_IRQ_eent+0x27/0x52
> handle_level_IRQ+0xb6
> do_IRQ+0x5d/0x78
> common_interrupt+0x1a/0x20
>
> In my .config:
>
> CONFIG_IOSCHED_NOOP=y
> CONFIG_IOSCHED_AS=y
> CONFIG_IOSCHED_DEADLINE=y
> CONFIG_IOSCHED_CFQ=y
> CONFIG_DEFAULT_IOSCHED="anticipatory"
>
> This happened very soon (within a few milliseconds or two) after my /etc/rc.local did:
>
> echo cfq >| /sys/block/hda/queue/scheduler
>
> (The next executable statement in /etc/rc.local is this:
> echo noop >| /sys/block/hdb/queue/scheduler and 'last sysfs file' still
> pointed at /dev/hda).
>
> It *looks* like the problem is in elevator_switch() in block/elevator.c:
>
> while (q->rq.elvpriv) {
> blk_remove_plug(q);
> q->request_fn(q);
> spin_unlock_irq(q->queue_lock);
> msleep(10);
> spin_lock_irq(q->queue_lock);
> elv_drain_elevator(q);
> }
>
> this--> spin_unlock_irq(q->queue_lock);
>
> /*
> * unregister old elevator data
> */
> elv_unregister_queue(q);
> old_elevator = q->elevator;
>
> /*
> * attach and start new elevator
> */
> if (elevator_attach(q, e))
> goto fail;
>
> should be down here someplace, after elevator_attach(), I suspect?
> Looks like the disk popped an IRQ after we had installed the
> iosched_cfq.ops[] but q->elevator->elevator_data hadn't been
> initialized yet...
>
> (I'd attach a patch, except I'm not positive I have the diagnosis
> right?)

I think your analysis is pretty good, there's definitely a period there
where we don't want the queueing invoked. Does this help?

diff --git a/block/elevator.c b/block/elevator.c
index 8768a36..429702a 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -806,8 +806,6 @@ static int elevator_switch(request_queue
elv_drain_elevator(q);
}

- spin_unlock_irq(q->queue_lock);
-
/*
* unregister old elevator data
*/
@@ -823,6 +821,8 @@ static int elevator_switch(request_queue
if (elv_register_queue(q))
goto fail_register;

+ spin_unlock_irq(q->queue_lock);
+
/*
* finally exit old elevator and turn off BYPASS.
*/
@@ -831,16 +831,19 @@ static int elevator_switch(request_queue
return 1;

fail_register:
+ spin_unlock_irq(q->queue_lock);
/*
* switch failed, exit the new io scheduler and reattach the old
* one again (along with re-adding the sysfs dir)
*/
elevator_exit(e);
e = NULL;
+ spin_lock_irq(q->queue_lock);
fail:
q->elevator = old_elevator;
elv_register_queue(q);
clear_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
+ spin_unlock_irq(q->queue_lock);
if (e)
kobject_put(&e->kobj);
return 0;
--
Jens Axboe

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