Re: docker crashes rcuos in __blkg_release_rcu

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Jun 10, 2014 at 02:39:06PM -0400, Joe Lawrence wrote:

[..]
> > I am wondering if we need to take a reference on the queue
> > (blk_get_queue()) in blkg_alloc(), to make sure request queue is
> > still around when blkg is being freed.
> 
> I experimented with this and the crash does go away (and the docker
> invocation completes successfully).  I wasn't sure where the
> accompanying blk_put_queue() should go.  If I put it in blkg_free, the
> kref accounting doesn't seem to even out, ie they never fall to zero.

I think blkg_free() is logical place to put down queue reference. I am
not sure why it does not even out. Will spend some time on this. 

> 
> > I will try to reproduce the issue locally.
> 
> Any luck?  I found that slub_debug was required to draw out the crash,
> otherwise the use-after-free silently goes about its business.

I had tried it yesterday and saw the crash out of 3 attempts. I am
right now busy in something. Give me 1-2 days and I will be able to
spend more time on this problem.

Thanks
Vivek