Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359

[Zhouyi Zhou]

On Mon, Jul 19, 2021 at 9:53 AM Paul E. McKenney <paulmck@xxxxxxxxxx> 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@xxxxxxxxxxxxxx/
> > >
> > > 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
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