Re: [linux-next:master 6618/6917] kernel/sched/psi.c:1230:13: sparse: error: incompatible types in comparison expression (different address spaces)

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Feb 11, 2019 at 05:36:06PM -0800, Matthew Wilcox wrote:
> On Fri, Feb 08, 2019 at 11:44:07PM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 08, 2019 at 03:14:41PM -0800, Andrew Morton wrote:
> > > On Fri, 8 Feb 2019 02:29:33 +0800 kbuild test robot <lkp@intel.com> wrote:
> > > >   1223	static __poll_t psi_fop_poll(struct file *file, poll_table *wait)
> > > >   1224	{
> > > >   1225		struct seq_file *seq = file->private_data;
> > > >   1226		struct psi_trigger *t;
> > > >   1227		__poll_t ret;
> > > >   1228	
> > > >   1229		rcu_read_lock();
> > > > > 1230		t = rcu_dereference(seq->private);
> 
> So the problem here is the opposite of what we think it is -- seq->private
> is not marked as being RCU protected.
>
> > If you wish to opt into this checking, you need to mark the pointer
> > definitions (in this case ->private) with __rcu.  It may also
> > be necessary to mark function parameters as well, as is done for
> > radix_tree_iter_resume().  If you do not wish to use this checking,
> > you should ignore these sparse warnings.

We cannot make struct seq_file->private generally __rcu, but the
cgroup code has a similar thing with kernfs, where it's doing rcu for
its particular use of struct kernfs_node->private. This is how it does
the dereference:

	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);

We could do this here as well.

It's ugly, though. I'd also be fine with ignoring the sparse warning.