linux-kernel - Re: [PATCH tip/urgent] rcu: add rcu_access_pointer and rcu_dereference

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Message-ID: <1270658332.8141.37.camel@edumazet-laptop>
Date:	Wed, 07 Apr 2010 18:38:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	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,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com
Subject: Re: [PATCH tip/urgent] rcu: add rcu_access_pointer and
 rcu_dereference_protected

Le mercredi 07 avril 2010 à 09:26 -0700, Paul E. McKenney a écrit :
> This patch adds variants of rcu_dereference() that handle situations
> where the RCU-protected data structure cannot change, perhaps due to
> our holding the update-side lock, or where the RCU-protected pointer is
> only to be fetched, not dereferenced.  These are needed due to some
> performance concerns with using rcu_dereference() where it is not
> required, aside from the need for lockdep/sparse checking.
> 
> The new rcu_access_pointer() primitive is for the case where the pointer
> is be fetch and not dereferenced.  This primitive may be used without
> protection, RCU or otherwise, due to the fact that it uses ACCESS_ONCE().
> 
> The new rcu_dereference_protected() primitive is for the case where updates
> are prevented, for example, due to holding the update-side lock.  This
> primitive does neither ACCESS_ONCE() nor smp_read_barrier_depends(), so
> can only be used when updates are somehow prevented.
> 
> Suggested-by: David Howells <dhowells@...hat.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Acked-by: Eric Dumazet <eric.dumazet@...il.com>


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