Re: [PATCH 3/3] btrfs: refactor btrfs_device->name updates

[Omar Sandoval <osandov@osandov.com>]

On Sun, Nov 30, 2014 at 10:26:43AM -0500, Pranith Kumar wrote:
> On Sun, Nov 30, 2014 at 3:26 AM, Omar Sandoval <osandov@osandov.com> wrote:
> > The rcu_string API introduced some new sparse errors but also revealed existing
> > ones. First of all, the name in struct btrfs_device should be annotated as
> > __rcu to prevent unsafe reads. Additionally, updates should go through
> > rcu_dereference_protected to make it clear what's going on. This introduces
> > some helper functions that factor out this functionality.
> >
> > Signed-off-by: Omar Sandoval <osandov@osandov.com>
> > diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> > index 6e04f27..2298a70 100644
> > --- a/fs/btrfs/volumes.h
> > +++ b/fs/btrfs/volumes.h
> > @@ -54,7 +54,7 @@ struct btrfs_device {
> >
> >         struct btrfs_root *dev_root;
> >
> > -       struct rcu_string *name;
> > +       struct rcu_string __rcu *name;
> >
> >         u64 generation;
> >
> 
> Since rcu_strings are rcu specific, why not annotate the char pointer
> in 'struct rcu_string' with __rcu annotation? That should catch all
> error-prone users of rcu_string.
> 
Because the whole structure is RCU'd, not just the str part of it. If str is
annotated as __rcu, when we (correctly) rcu_dereference an rcu_string and then
access the str member, we'll still get sparse warnings.

In any case, the above code does what I want it to do. See the following
(non-sense but illustrative) example:

#include <linux/rcustring.h>

static void example_func(void)
{
	struct rcu_string __rcu *example;
	char *str;
	str = example->str;
}

  CHECK   /home/osandov/linux/example/example.c
/home/osandov/linux/example/example.c:7:13: warning: incorrect type in assignment (different address spaces)
/home/osandov/linux/example/example.c:7:13:    expected char *str
/home/osandov/linux/example/example.c:7:13:    got char [noderef] <asn:4>*<noident>

-- 
Omar