On Mon, Mar 29, 2010 at 04:26:36PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 29, 2010 at 11:59:03PM +0100, David Howells wrote:
> > Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> 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
> ------------------------------------------------------------------------
>
> 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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>
> 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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