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

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Message-Id: <201009072200.59509.arnd@arndb.de>
Date:	Tue, 7 Sep 2010 22:00:58 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Kulikov Vasiliy <segooon@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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.

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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