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From: Mathieu Desnoyers on 7 Apr 2010 12:50
* Paul E. McKenney (paulmck(a)linux.vnet.ibm.com) wrote:
> 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(a)redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck(a)linux.vnet.ibm.com>

Acked-by: Mathieu Desnoyers <mathieu.desnoyers(a)efficios.com>

We should probably pull this in liburcu eventually too (renaming
ACCESS_ONCE() into LOAD_SHARED()).

Thanks,

Mathieu

>
> rcupdate.h | 38 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 38 insertions(+)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 872a98e..3f06b3d 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -209,9 +209,47 @@ static inline int rcu_read_lock_sched_held(void)
> rcu_dereference_raw(p); \
> })
>
> +/**
> + * rcu_access_pointer - fetch RCU pointer with no dereferencing
> + *
> + * Return the value of the specified RCU-protected pointer, but omit the
> + * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful
> + * when the value of this pointer is accessed, but the pointer is not
> + * dereferenced, for example, when testing an RCU-protected pointer against
> + * NULL. This may also be used in cases where update-side locks prevent
> + * the value of the pointer from changing, but rcu_dereference_protected()
> + * is a lighter-weight primitive for this use case.
> + */
> +#define rcu_access_pointer(p, c) \
> + ({ \
> + if (debug_lockdep_rcu_enabled() && !(c)) \
> + lockdep_rcu_dereference(__FILE__, __LINE__); \
> + ACCESS_ONCE(p); \
> + })
> +
> +/**
> + * rcu_dereference_protected - fetch RCU pointer when updates prevented
> + *
> + * Return the value of the specified RCU-protected pointer, but omit
> + * both the smp_read_barrier_depends() and the ACCESS_ONCE(). This
> + * is useful in cases where update-side locks prevent the value of the
> + * pointer from changing. Please note that this primitive does -not-
> + * prevent the compiler from repeating this reference or combining it
> + * with other references, so it should not be used without protection
> + * of appropriate locks.
> + */
> +#define rcu_dereference_protected(p, c) \
> + ({ \
> + if (debug_lockdep_rcu_enabled() && !(c)) \
> + lockdep_rcu_dereference(__FILE__, __LINE__); \
> + (p); \
> + })
> +
> #else /* #ifdef CONFIG_PROVE_RCU */
>
> #define rcu_dereference_check(p, c) rcu_dereference_raw(p)
> +#define rcu_access_pointer(p, c) ACCESS_ONCE(p)
> +#define rcu_dereference_protected(p, c) (p)
>
> #endif /* #else #ifdef CONFIG_PROVE_RCU */
>

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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