From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 2/2] dm: stay in blk_queue_bypass until queue becomes
 initialized
Date: Mon, 29 Oct 2012 17:45:15 +0100
Message-ID: <1351529115.24721.40.camel@twins>
References: <50890937.7010809@ce.jp.nec.com>
	 <20121026202105.GF24687@redhat.com> <508E572C.6000707@ce.jp.nec.com>
	 <20121029163845.GB7709@redhat.com>
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To: Vivek Goyal <vgoyal@redhat.com>
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, 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..