Date: Tue, 19 Dec 2017 10:17:43 -0600
From: Bruno Wolff III <bruno@wolff.to>
To: weiping zhang <zwp10758@gmail.com>
Cc: Laura Abbott <labbott@redhat.com>, Jan Kara <jack@suse.cz>,
        Jens Axboe <axboe@kernel.dk>, linux-mm@kvack.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        regressions@leemhuis.info, linux-block@vger.kernel.org
Subject: Re: Regression with a0747a859ef6 ("bdi: add error handle for
 bdi_debug_register")
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On Sun, Dec 17, 2017 at 21:43:50 +0800,
  weiping zhang <zwp10758@gmail.com> wrote:
>Hi, thanks for testing, I think you first reproduce this issue(got WARNING
>at device_add_disk) by your own build, then add my debug patch.

The problem is still in rc4. Reverting the commit still fixes the problem. 
I tested that warning level messages should appear using lkdtm. While 
there could be something weird relating to the WARN_ON macro, more likely 
there is something different about the boots with the kernels I build 
(the exact way initramfs is built is probably different) and probably 
that (WARN_ON) code is not getting executed.