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Date:   Mon, 19 Jul 2021 03:43:00 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Zhouyi Zhou <zhouzhouyi@gmail.com>
Cc:     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>
Subject: Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359
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On Mon, Jul 19, 2021 at 10:24:18AM +0800, Zhouyi Zhou wrote:
> Meanwhile, I examined the 5.12.17 by naked eye, and found a suspicious place
> that could possibly trigger that problem:
> 
> struct swap_info_struct *get_swap_device(swp_entry_t entry)
> {
>      struct swap_info_struct *si;
>      unsigned long offset;
> 
>      if (!entry.val)
>              goto out;
>     si = swp_swap_info(entry);
>     if (!si)
>        goto bad_nofile;
> 
>    rcu_read_lock();
>   if (data_race(!(si->flags & SWP_VALID)))
>      goto unlock_out;
>   offset = swp_offset(entry);
>   if (offset >= si->max)
>    goto unlock_out;
> 
>   return si;
> bad_nofile:
>   pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
> out:
>   return NULL;
> unlock_out:
>   rcu_read_unlock();
>   return NULL;
> }
> I guess the function "return si" without a rcu_read_unlock.

Yes, but the caller is supposed to call put_swap_device() which
calls rcu_read_unlock().  See commit eb085574a752.