linux-kernel - Re: [PATCH] md: do not use ++ in rcu

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Message-ID: <20100910034603.GA2612@linux.vnet.ibm.com>
Date:	Thu, 9 Sep 2010 20:46:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Kulikov Vasiliy <segooon@...il.com>,
	kernel-janitors@...r.kernel.org, Neil Brown <neilb@...e.de>,
	Jens Axboe <jaxboe@...ionio.com>, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md: do not use ++ in rcu_dereference() argument

On Thu, Sep 09, 2010 at 05:14:43PM +0200, Arnd Bergmann wrote:
> On Tuesday 07 September 2010, Paul E. McKenney wrote:
> > On Tue, Sep 07, 2010 at 10:00:58PM +0200, Arnd Bergmann wrote:
> > > On Tuesday 07 September 2010 21:21:55 Kulikov Vasiliy wrote:
> > > > #define __rcu_dereference_check(p, c, space) \
> > > >         ({ \
> > > >                 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
> > > >                                                                    ^
> > > >                 rcu_lockdep_assert(c); \
> > > >                 (void) (((typeof (*p) space *)p) == p); \
> > > >                                       ^     ^
> > > >                 smp_read_barrier_depends(); \
> > > >                 ((typeof(*p) __force __kernel *)(_________p1)); \
> > > >         })
> > > > 
> > > > If I understand this, it is evaluated three times, right?
> > > 
> > > Yes, that looks like my own fault, I added that :(
> > > 
> > > This patch seems to fix it, but I need to think about it some more
> > > to make sure it still does everything we need.
> > 
> > Let me know when you are satisfied with it, and then I will pick it up.
> 
> I guess it would be good to put it in now. I haven't had the time
> to try out all cases, but the current code in -next is definitely
> broken, so please put the fix in now.

Hmmm...  One approach would be have a secondary macro that was:

	#define __rcu_dereference_check_sparse(p, space) \
		(void) (((typeof (*p) space *)p) == p);

when running sparse and:

	#define __rcu_dereference_check_sparse(p, space)

otherwise.

Would that do the trick?

							Thanx, Paul
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