Re: lockdep vs rmmod loop

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2008-10-14 at 19:14 +0400, Alexey Dobriyan wrote:
> On Tue, Oct 14, 2008 at 01:51:38PM +0200, Peter Zijlstra wrote:
> > On Mon, 2008-10-13 at 21:57 +0400, Alexey Dobriyan wrote:
> > > This exists for quite some time, IIRC.
> > > 
> > > 	# rmmod loop
> > 
> > I tried to reproduce with -git from today but failed.
> > 
> > v2.6.27-3976-g7591103
> > 
> > I build a x86_64 kernel with modular loop and lockdep enabled, then I
> > did: modprobe loop; rmmod loop
> > 
> > Is there anything else to reproducing this?
> 
> No, modprobe,rmmod is enough here even with minimal debugging.

Yep can reproduce with your config.

Its:

static void wait_on_work(struct work_struct *work)
{
	...
	lock_map_acquire(&work->lockdep_map);
	lock_map_release(&work->lockdep_map);


that triggers this. Which would suggest the work-let wasn't properly
initialized. Now on to figuring out where it came from.

Loop doesn't appear to have anything workqueue related in it, which
suggests the block layer.

And indeed, it appears blk_queue_make_request() initializes
q->unplug_work, and by modprobe loop; rmmod loop you never get to run
that, therefore q->unplug_work gets canceled without ever having been
initialized -> bang!

And indeed, the below patch fixes it:

---
Subject: block: move q->unplug_work initialization
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu Oct 16 13:44:57 CEST 2008

modprobe loop; rmmod loop effectively creates a blk_queue and destroys it
which results in q->unplug_work being canceled without it ever being
initialized.

Therefore, move the initialization of q->unplug_work from 
blk_queue_make_request() to blk_alloc_queue*().

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 block/blk-core.c     |    1 +
 block/blk-settings.c |    2 --
 2 files changed, 1 insertion(+), 2 deletions(-)

Index: linux-2.6/block/blk-core.c
===================================================================
--- linux-2.6.orig/block/blk-core.c
+++ linux-2.6/block/blk-core.c
@@ -501,6 +501,7 @@ struct request_queue *blk_alloc_queue_no
 	init_timer(&q->unplug_timer);
 	setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
 	INIT_LIST_HEAD(&q->timeout_list);
+	INIT_WORK(&q->unplug_work, blk_unplug_work);
 
 	kobject_init(&q->kobj, &blk_queue_ktype);
 
Index: linux-2.6/block/blk-settings.c
===================================================================
--- linux-2.6.orig/block/blk-settings.c
+++ linux-2.6/block/blk-settings.c
@@ -141,8 +141,6 @@ void blk_queue_make_request(struct reque
 	if (q->unplug_delay == 0)
 		q->unplug_delay = 1;
 
-	INIT_WORK(&q->unplug_work, blk_unplug_work);
-
 	q->unplug_timer.function = blk_unplug_timeout;
 	q->unplug_timer.data = (unsigned long)q;