From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH 2/2] dm: stay in blk_queue_bypass until queue becomes
 initialized
Date: Mon, 29 Oct 2012 13:13:46 -0400
Message-ID: <20121029171346.GC3308@redhat.com>
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, device-mapper development <dm-devel@redhat.com>, Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>, Alasdair G Kergon <agk@redhat.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
List-Id: dm-devel.ids

On Mon, Oct 29, 2012 at 05:45:15PM +0100, Peter Zijlstra wrote:
> On Mon, 2012-10-29 at 12:38 -0400, Vivek Goyal wrote:
> > Ok, so the question is what's wrong with calling synchronize_rcu() inside
> > a mutex with CONFIG_PREEMPT=y. I don't know. Ccing paul mckenney  and
> > peterz.
> 
> int blkcg_activate_policy(struct request_queue *q,
> 
> {
> 
> 	...
> 
>         preloaded = !radix_tree_preload(GFP_KERNEL);
> 
>         blk_queue_bypass_start(q);
> 
> 
> 
> 
> where:
> 
> int radix_tree_preload(gfp_t gfp_mask)
> {
>         struct radix_tree_preload *rtp;
>         struct radix_tree_node *node;
>         int ret = -ENOMEM;
>         
>         preempt_disable();
> 
> 
> Seems obvious why it explodes..

Oh right. Thanks Peter. So just calling blk_queue_bypass_start() before
radix_tree_preload() should fix it. Junichi, can you please give it
a try.

Thanks
Vivek