From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: 3.6-rc5 cgroups blkio throttle + md regression
Date: Thu, 20 Sep 2012 13:17:52 -0700
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To: Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Joseph Glanville <joseph.glanville-2MxvZkOi9dvvnOemgxGiVw@public.gmane.org>, cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>

Hello, Vivek.

On Thu, Sep 20, 2012 at 03:32:27PM -0400, Vivek Goyal wrote:
> > Yeah, I incorrectly assumed that bio based drivers would call
> > blk_init_allocated_queue().  I think we need to move the initial
> > bypass_end call to blk_register_queue().  Like the following.  Not
> > completely sure yet tho.
> 
> Hmm.., I am just trying to remember the details hence thinking loud.
> 
> So issue was that we did not want to call synchronize_rcu() in queue exit
> path (ex. cfq_exit_queue()) because some driver could call create/destroy
> queues very frequently during device/disk discovery.
> 
> So we started queue in bypass mode so that if it is destroyed early, it
> will be destroyed in bypass mode and we shall never have to call
> synchronize_rcu().

Yeah, that and it's desirable to ensure that all the fancy stuff is
skipped until initialization is complete.

> So assumption here is that blk_register_queue() will be called by
> add_disk() and hopefully add_disk() will be called by driver only if
> it is not planning to tear down the queue soon. If this assumption is
> right, then this patch should work, I think.

I'm wondering whether there's any use case where a queue is used
without being registered and still require elevator and all other
stuff.  I can't think of any valid use case but am not completely sure
either.

Thanks.

-- 
tejun