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Date:   Mon, 19 Jul 2021 12:22:17 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>, paulmck@kernel.org,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        stable@vger.kernel.org, Chris Clayton <chris2553@googlemail.com>,
        Chris Rankin <rankincj@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        rcu <rcu@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        "Huang, Ying" <ying.huang@intel.com>, gregkh@linuxfoundation.org
Subject: Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359
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On Mon, Jul 19, 2021 at 07:12:58PM +0800, Miaohe Lin wrote:
> When in the commit 2799e77529c2a, we're using the percpu_ref to serialize against
> concurrent swapoff, i.e. there's percpu_ref inside get_swap_device() instead of
> rcu_read_lock(). Please see commit 63d8620ecf93 ("mm/swapfile: use percpu_ref to
> serialize against concurrent swapoff") for detail.

Oh, so this is a backport problem.  2799e77529c2 was backported without
its prerequisite 63d8620ecf93.  Greg, probably best to just drop
2799e77529c2 from all stable trees; the race described is not very
important (swapoff vs reading a page back from that swap device).