Re: [syzbot] possible deadlock in del_gendisk

From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Stefan Haberland <sth@linux.ibm.com>,
	Jan Hoeppner <hoeppner@linux.ibm.com>
Cc: linux-block <linux-block@vger.kernel.org>,
	syzbot <syzbot+61e04e51b7ac86930589@syzkaller.appspotmail.com>,
	Tyler Hicks <tyhicks@linux.microsoft.com>,
	Petr Vorel <pvorel@suse.cz>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [syzbot] possible deadlock in del_gendisk
Date: Thu, 10 Jun 2021 01:31:17 +0900	[thread overview]
Message-ID: <7b8c9eeb-789d-e5e6-04d6-130ee8be7305@i-love.sakura.ne.jp> (raw)
In-Reply-To: <1435f266-9f6d-22ef-ba7d-f031c616aede@I-love.SAKURA.ne.jp>

Hello, Christoph.

I'm currently trying full bisection.

  # bad: [fc0586062816559defb14c947319ef8c4c326fb3] Merge tag 'for-5.13/drivers-2021-04-27' of git://git.kernel.dk/linux-block
  # good: [42dec9a936e7696bea1f27d3c5a0068cd9aa95fd] Merge tag 'perf-core-2021-04-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
  # good: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
  # good: [a800abd3ecb9acc55821f7ac9bba6c956b36a595] net: enetc: move skb creation into enetc_build_skb
  # good: [6cc8e7430801fa238bd7d3acae1eb406c6e02fe1] loop: scale loop device by introducing per device lock
  git bisect start 'fc0586062816559defb14c947319ef8c4c326fb3' '42dec9a936e7696bea1f27d3c5a0068cd9aa95fd' '68a32ba14177d4a21c4a9a941cf1d7aea86d436f' 'a800abd3ecb9acc55821f7ac9bba6c956b36a595' '6cc8e7430801fa238bd7d3acae1eb406c6e02fe1'
  # good: [2958a995edc94654df690318df7b9b49e5a3ef88] block/rnbd-clt: Support polling mode for IO latency optimization
  git bisect good 2958a995edc94654df690318df7b9b49e5a3ef88

I think we will bisect this problem to

  commit c76f48eb5c084b1e ("block: take bd_mutex around delete_partitions in del_gendisk")

because that commit introduced new locking dependency bdev_lookup_sem => disk->part0->bd_mutex
which matches the lockdep's report.

  ======================================================
  WARNING: possible circular locking dependency detected
  5.12.0-rc6-next-20210409-syzkaller #0 Not tainted
  ------------------------------------------------------
  syz-executor.4/10285 is trying to acquire lock:
  ffff8881423245a0 (&bdev->bd_mutex){+.+.}-{3:3}, at: del_gendisk+0x250/0x9e0 block/genhd.c:618

  but task is already holding lock:
  ffffffff8c7d9430 (bdev_lookup_sem){++++}-{3:3}, at: del_gendisk+0x222/0x9e0 block/genhd.c:616

Do we need to revert "partition iteration simplifications" work?

On 2021/06/07 19:56, Tetsuo Handa wrote:
> Hello.
> 
> syzbot is reporting "possible deadlock in del_gendisk" problem.
> 
> I guess this is caused by commit 6cc8e7430801fa23 ("loop: scale loop device
> by introducing per device lock") because it touches loop_ctl_mutex usage
> between v5.11 and v5.12-rc1. Please have a look.
> 
> On 2021/04/14 2:33, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    e99d8a84 Add linux-next specific files for 20210409
>> git tree:       linux-next
>> console output: https://syzkaller.appspot.com/x/log.txt?x=13b01681d00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=7cd69574979bfeb7
>> dashboard link: https://syzkaller.appspot.com/bug?extid=61e04e51b7ac86930589
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=148265d9d00000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16a981a1d00000
> 