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

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Date:	Tue, 7 Sep 2010 23:21:55 +0400
From:	Kulikov Vasiliy <segooon@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
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 Mon, Sep 06, 2010 at 22:10 +0200, Arnd Bergmann wrote:
> On Sunday 05 September 2010 20:32:18 Kulikov Vasiliy wrote:
> > From: Vasiliy Kulikov <segooon@...il.com>
> > 
> > rcu_dereference() is macro, so it might use its argument twice.
> > Argument must not has side effects.
> > 
> > It was found by compiler warning:
> > drivers/md/raid1.c: In function ‘read_balance’:
> > drivers/md/raid1.c:445: warning: operation on ‘new_disk’ may be undefined
> 
> I think the rcu_dereference macro should really not evaluate its argument
> twice, and I don't see where it does.
> As a general rule, we try to write macros in Linux such that they behave
> like functions and don't have surprising side-effects.
> 
> Which kernel and gcc version do you see the warning with?
> 
> 	Arnd

gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), linux-next.

#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?


-- 
Vasiliy
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