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

[Boqun Feng]

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