linux-kernel - Re: [PATCH 3/3] btrfs: refactor btrfs

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Message-ID: <20141201025205.GA14040@mew>
Date:	Sun, 30 Nov 2014 18:52:05 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
	Joe Perches <joe@...ches.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 3/3] btrfs: refactor btrfs_device->name updates

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@...ndov.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@...ndov.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
--
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