Re: [OOPS] 2.6.11 - NMI lockup with CFQ scheduler

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Jens Axboe wrote:
On Wed, Apr 06 2005, Arjan van de Ven wrote:

@@ -324,6 +334,7 @@
	issue_flush_fn		*issue_flush_fn;
	prepare_flush_fn	*prepare_flush_fn;
	end_flush_fn		*end_flush_fn;
+	release_queue_data_fn	*release_queue_data_fn;

	/*
	 * Auto-unplugging state

where does this function method actually get called?


I missed the hunk in ll_rw_blk.c, rmk pointed the same thing out not 5
minutes ago :-)

The patch would not work anyways, as scsi_sysfs.c clears queuedata
unconditionally. This is a better work-around, it just makes the queue
hold a reference to the device as well only killing it when the queue is
torn down.

Still not super happy with it, but I don't see how to solve the circular
dependency problem otherwise.


 Hello, Jens.

I've been thinking about it for a while. The problem is that we're reference counting two different objects to track lifetime of one entity. This happens in both SCSI upper and mid layers. In the upper layer, genhd and scsi_disk (or scsi_cd, ...) are ref'ed separately while they share their destiny together (not really different entity) and in the middle layer scsi_device and request_queue does the same thing. Circular dependency is occuring because we separate one entity into two and reference counting them separately. Two are actually one and necessarily want each other. (until death aparts. Wow, serious. :-)

IMHO, what we need to do is consolidate ref counting such that in each layer only one object is reference counted, and the other object is freed when the ref counted object is released. The object of choice would be genhd in upper layer and request_queue in mid layer. All ref-counting should be updated to only ref those objects. We'll need to add a release callback to genhd and make request_queue properly reference counted.

Conceptually, scsi_disk extends genhd and scsi_device extends request_queue. So, to go one step further, as what UL represents is genhd (disk device) and ML request_queue (request-based device), embedding scsi_disk into genhd and scsi_device into request_queue will make the architecture clearer. To do this, we'll need something like alloc_disk_with_udata(int minors, size_t udata_len) and the equivalent for request_queue.

I've done this half-way and then doing it without fixing the SCSI model seemed silly so got into working on the state model. (BTW, the state model is almost done, I'm about to run tests.)

 What do you think?  Jens?

--
tejun

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