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

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Date:	Mon, 6 Sep 2010 22:10:55 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Kulikov Vasiliy <segooon@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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 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
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