linux-kernel - Re: linux-5.13.2: warning from kernel/rcu/tree

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Date:   Mon, 19 Jul 2021 10:27:43 +0800
From:   Zhouyi Zhou <zhouzhouyi@...il.com>
To:     paulmck@...nel.org
Cc:     Matthew Wilcox <willy@...radead.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, Chris Clayton <chris2553@...glemail.com>,
        Chris Rankin <rankincj@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        rcu <rcu@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359

On Mon, Jul 19, 2021 at 10:24 AM Zhouyi Zhou <zhouzhouyi@...il.com> wrote:
>
> On Mon, Jul 19, 2021 at 9:53 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > On Sun, Jul 18, 2021 at 11:51:36PM +0100, Matthew Wilcox wrote:
> > > On Sun, Jul 18, 2021 at 02:59:14PM -0700, Paul E. McKenney wrote:
> > > > > > https://lore.kernel.org/lkml/CAK2bqVK0Q9YcpakE7_Rc6nr-E4e2GnMOgi5jJj=_Eh_1k
> > > > > > EHLHA@...l.gmail.com/
> > > >
> > > > But this one does show this warning in v5.12.17:
> > > >
> > > >     WARN_ON_ONCE(!preempt && rcu_preempt_depth() > 0);
> > > >
> > > > This is in rcu_note_context_switch(), and could be caused by something
> > > > like a schedule() within an RCU read-side critical section.  This would
> > > > of course be RCU-usage bugs, given that you are not permitted to block
> > > > within an RCU read-side critical section.
> > > >
> > > > I suggest checking the functions in the stack trace to see where the
> > > > rcu_read_lock() is hiding.  CONFIG_PROVE_LOCKING might also be helpful.
> > >
> > > I'm not sure I see it in this stack trace.
> > >
> > > Is it possible that there's something taking the rcu read lock in an
> > > interrupt handler, then returning from the interrupt handler without
> > > releasing the rcu lock?  Do we have debugging that would fire if
> > > somebody did this?
> >
> > Lockdep should complain, but in the absence of lockdep I don't know
> > that anything would gripe in this situation.
> I think Lockdep should complain.
> Meanwhile, I examined the 5.12.17 by naked eye, and found a suspicious place
I examined 5.13.2 the unpaired rcu_read_lock is still there
> 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.
>
> However the get_swap_device has changed in the mainline tree,
> there is no rcu_read_lock anymore.
>
> >
> > Also, this is a preemptible kernel, so it is possible to trace
> > __rcu_read_lock(), if that helps.
> >
> >                                                         Thanx, Paul
> Thanx
> Zhouyi