Re: [PATCH] md: do not use ++ in rcu_dereference() argument

Kulikov Vasiliy
2010-09-07T19:21:55Z

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@gmail.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