On Tue, 2012-01-17 at 17:11 -0800, Tejun Heo wrote:
> On Wed, Jan 18, 2012 at 09:05:26AM +0800, Shaohua Li wrote:
> > > Vivek is seeing the problem while switching elevators.  Are you too?
> > > Or is it during normal operation?
> > same here. I had some problems when I debug my ioscheduler, but
> > eventually found even switching cfq and noop can trigger oops.
> 
> Hmmm... maybe quiescing isn't working as expected and kmem cache is
> being destroyed with live icq's.  I'll try to reproduce it.
this debug patch seems to fix for me.

diff --git a/block/blk-core.c b/block/blk-core.c
index e6c05a9..c6a8ef5 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -872,11 +872,11 @@ retry:
 	spin_unlock_irq(q->queue_lock);
 
 	/* create icq if missing */
-	if (unlikely(et->icq_cache && !icq))
+	if (unlikely(et->icq_cache && !icq && (rw_flags & REQ_ELVPRIV)))
 		icq = ioc_create_icq(q, gfp_mask);
 
 	/* rqs are guaranteed to have icq on elv_set_request() if requested */
-	if (likely(!et->icq_cache || icq))
+	if (likely(!et->icq_cache || icq || !(rw_flags & REQ_ELVPRIV)))
 		rq = blk_alloc_request(q, icq, rw_flags, gfp_mask);
 
 	if (unlikely(!rq)) {