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

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Message-ID: <20100907205015.GJ2448@linux.vnet.ibm.com>
Date:	Tue, 7 Sep 2010 13:50:15 -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 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.

							Thanx, Paul

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 89414d6..743bc3f 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -313,21 +313,21 @@ extern int rcu_my_thread_group_empty(void);
>  #define __rcu_access_pointer(p, space) \
>  	({ \
>  		typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
> -		(void) (((typeof (*p) space *)p) == p); \
> +		(void) (((typeof (*p) space *)NULL) == (typeof(p))NULL); \
>  		((typeof(*p) __force __kernel *)(_________p1)); \
>  	})
>  #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); \
> +		(void) (((typeof (*p) space *)NULL) == (typeof(p))NULL); \
>  		smp_read_barrier_depends(); \
>  		((typeof(*p) __force __kernel *)(_________p1)); \
>  	})
>  #define __rcu_dereference_protected(p, c, space) \
>  	({ \
>  		rcu_lockdep_assert(c); \
> -		(void) (((typeof (*p) space *)p) == p); \
> +		(void) (((typeof (*p) space *)NULL) == (typeof(p))NULL); \
>  		((typeof(*p) __force __kernel *)(p)); \
>  	})
> 
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