On Mon, Mar 29, 2010 at 04:26:36PM -0700, Paul E. McKenney wrote:
quoted text
> On Mon, Mar 29, 2010 at 11:59:03PM +0100, David Howells wrote:
> > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > > Only on Alpha.  Otherwise only a volatile access.
> > 
> > Whilst that is true, it's the principle of the thing.  The extra barrier
> > shouldn't be emitted on Alpha.  If Alpha's no longer important, then can we
> > scrap smp_read_barrier_depends()?
> > 
> > My point is that some of these rcu_dereference*()'s are unnecessary.  If
> > there're required for correctness tracking purposes, fine; but can we have a
> > macro that is just a dummy for the purpose of stripping the pointer Sparse
> > annotation?  One that doesn't invoke rcu_dereference_raw() and interpolate a
> > barrier, pretend or otherwise, when there's no second reference to order
> > against.
> 
> Interesting point.  Perhaps an rcu_dereference_update(p, c) for the
> cases where the data structure cannot change.  Also, such a name makes
> it more clear that this is an update-side access, and it further documents
> the update-side lock.
> 
> Something like the following, then?  Untested, probably does not even
> compile.

Scrap this one -- Arnd has it covered, under the much better name
of rcu_dereference_const().

							Thanx, Paul

quoted text
> ------------------------------------------------------------------------
> 
> rcu: Add update-side variant of rcu_dereference()
> 
> Upcoming consistency-checking features are requiring that even update-side
> accesses to RCU-protected pointers use some variant of rcu_dereference().
> Even though rcu_dereference() is quite lightweight, it does constrain the
> compiler, thus producing code that is worse than required.  This patch
> therefore adds rcu_dereference_update(), which allows lockdep-style
> checks for holding the correct update-side lock, but which does not
> constrain the compiler.
> 
> Suggested-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  rcupdate.h |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 872a98e..0a6047f 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -209,9 +209,26 @@ static inline int rcu_read_lock_sched_held(void)
>  		rcu_dereference_raw(p); \
>  	})
>  
> +/**
> + * rcu_dereference_update - rcu_dereference on structure that cannot change
> + *
> + * Do rcu_dereference() checking, but just pick up the pointer without
> + * the normal RCU read-side precautions.  These precautions are only
> + * needed if the data structure can change.  The caller is responsible
> + * for doing whatever is necessary (such as holding locks) to prevent
> + * such changes from occurring.
> + */
> +#define rcu_dereference_update(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_dereference_update(p, c)	(p)
>  
>  #endif /* #else #ifdef CONFIG_PROVE_RCU */
>  
--
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