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Date:   Mon, 19 Jul 2021 18:14:21 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     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>, Miaohe Lin <linmiaohe@huawei.com>,
        "Huang, Ying" <ying.huang@intel.com>
Subject: Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359
Message-ID: <YPVQfaamqwu1PRrK@boqun-archlinux>
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 <20210718215914.GQ4397@paulmck-ThinkPad-P17-Gen-1>
 <YPSweHyCrD2q2Pue@casper.infradead.org>
 <20210719015313.GS4397@paulmck-ThinkPad-P17-Gen-1>
 <CAABZP2yE+3vzd+LgJDJcJ2f8qttJQSUQ6efD9MaFd2iD4xPTZA@mail.gmail.com>
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On Mon, Jul 19, 2021 at 03:43:00AM +0100, Matthew Wilcox wrote:
> 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.

Right, but we need to make sure there is no sleepable function called
before put_swap_device() called, and the call trace showed the following
happened:

	do_swap_page():
	  si = get_swap_device():
	    rcu_read_lock();
	  lock_page_or_retry():
	    might_sleep(); // call a sleepable function inside RCU read-side c.s.
	    __lock_page_or_retry():
	      wait_on_page_bit_common():
	        schedule():
		  rcu_note_context_switch();
		  // Warn here
	  put_swap_device();
	    rcu_read_unlock();

, which introduced by commit 2799e77529c2a

[Copy the author]

Regards,
Boqun