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Date:   Fri, 28 Dec 2018 16:00:16 +0100
From:   "Stephen R. van den Berg" <srb@cuci.nl>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: Kernel traces
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Qu Wenruo wrote:
>On 2018/12/28 ??????9:40, Stephen R. van den Berg wrote:
>>> It's caused by qgroup, and a dead lock on btrfs_drop_snapshot().

>>> This is one of the easiest way to trigger an ABBA deadlock.

>>> Please either disable qgroup or apply this patch to solve it:
>>> https://patchwork.kernel.org/patch/10725371/

Ok, booting the system in v4.20 with this patch.  I'll report back if
it does not resolve the problem.
-- 
Stephen.