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

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Message-ID: <20100923172337.GF2406@linux.vnet.ibm.com>
Date:	Thu, 23 Sep 2010 10:23:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
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 Thu, Sep 23, 2010 at 03:13:39PM +0200, Arnd Bergmann wrote:
> 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>

No problem, when you get me a better approach, I will happily queue it!

							Thanx, Paul
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