linux-kernel - Re: [PATCH tip/core/rcu 5/8] rcu: only one evaluation of arg in rcu_dereference

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Date:	Thu, 23 Sep 2010 15:13:39 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH tip/core/rcu 5/8] rcu: only one evaluation of arg in rcu_dereference_check() unless sparse

On Thursday 23 September 2010, Paul E. McKenney wrote:
> The current version of the __rcu_access_pointer(), __rcu_dereference_check(),
> and __rcu_dereference_protected() macros evaluate their "p" argument
> three times, not counting typeof()s.  This is bad news if that argument
> contains a side effect.  This commit therefore evaluates this argument
> only once in normal kernel builds.  However, the straightforward approach
> defeats sparse's RCU-pointer checking, so when __CHECKER__ is defined,
> the additional pair of evaluations of the "p" argument are performed in
> order to permit sparse to detect misuse of RCU-protected pointers.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Arnd Bergmann <arnd@...db.de>

Sorry for not following up with the alternative approach yet. We can always
do that later, for your patch should go into 2.6.36 to fix any problems
arising from unintended macro side-effects.

Acked-by: Arnd Bergmann <arnd@...db.de>
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