On Fri, May 04, 2012 at 12:53:12PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paul...@linux.vnet.ibm.com) wrote:
> > On Thu, May 03, 2012 at 01:13:30PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paul...@linux.vnet.ibm.com) wrote:
> [...]
> > > >
> > > > A write barrier would be sufficient in the case where there were only
> > > > two threads observing each other. A full memory barrier would be needed
> > > > to prevent the assertion from firing in this sort of case (not sure that
> > > > this is exactly right, but something like this):
> > > >
> > > > Initial contents: B, C
> > > >
> > > > T0: add A; del B
> > > > T1: if (!lookup B) { add B; del C }
> > > > T2: r1 = lookup C; smp_mb(); r2 = lookup A
> > > >
> > > > assert(lookup C || lookup A);
> > >
> > > What you are bringing here as counter-example is, I think, transitivity.
> >
> > Yep!
> >
> > > I'm trying to figure out how your example could fail, and I cannot see
> > > how. Follows a detail of the closest scenario I get to failing is the
> > > following, but it does not fail. After that, I'm proposing a different
> > > scenario, which I think will be more appropriate for the current topic.
> > >
> > > Attempted detail of your scenario:
> > >
> > > T0 T1 T2
> > >
> > > add A
> > > wmb
> > > (add A globally
> > > observable)
> >
> > Almost. All that this really guarantees is that if someone sees
> > the "add B", they will also see the "add A".
> >
> > > del B
> > > (del B globally
> > > observable)
> > > (add A NOT brought
> > > into cache)
> > > (del B brought into
> > > cache)
> > > read B
> > > (test false)
> > > add B
> > > wmb
> > > (add B globally
> > > observable)
> > > del C
> > > (del C globally
> > > observable)
> >
> > Here, if someone sees the "del C", they will see the "add B", and
> > they also will have lost the opportunity to modify B before T1
> > reads from it and modifies it.
> >
> > > (add A NOT brought
> > > into cache)
> > > (del C brought into
> > > cache)
> > > read C -> not there.
> > > mb
> > > (add A brought
> > > into cache)
> > > read A -> there -> success.
> >
> > So we see that C is not there. We know that B would be there if
> > we looked at it. But we don't look at B, we look at A. But the
> > ordering back to T0's "add A" requires transitivity, which wmb
> > does not guarantee.
>
> OK, got it!
>
> >
> > > If I look at the "transitivity" section in Linux memory-barriers.txt, I
> > > notice that the example is mainly around using read barrier around loads
> > > rather than general barrier. Let's see if I can modify that example to
> > > come up with an example error case:
> > >
> > > Initial content: empty
> > >
> > > T0: add X
> > > T1: r1 = lookup X; smp_mb; r2 = lookup Y
> > > T2: add Y; r3 = lookup X
> > >
> > > assert( !(r1 && !r2 && !r3) )
> > >
> > > The key thing here is that if the barrier in T2 after "add Y" is a
> > > smp_wmb rather than a smp_mb, this could allow the "r3 = lookup X" to be
> > > reordered before add Y, thus allowing the assertion to fail.
> >
> > Your example is simpler, and demonstrates the need just as well, so
> > let's go with your example.
> >
> > > I think it would be more intuitive for users if lookups vs updates
> > > performed on the same thread are ordered with full memory barriers.
> > > Given that we don't want to add extra barriers in read operations, it
> > > would make sense to guarantee full memory barriers before and after
> > > updates.
> > >
> > > So how about we use full memory barriers before and after each of: add,
> > > del (success), add_unique (success), replace, and add_replace ? If we
> > > ever want to relax those ordering guarantees, then we can always add new
> > > update APIs with a "weaker" ordering.
> > >
> > > Thoughts ?
> >
> > That line of reasoning makes a lot of sense to me!
>
> Sounds good.
>
> Here is what I propose, thoughts ?
Just to make sure I understand -- the reason that the "del" functions
say "no memory barrier" instead of "acts like rcu_dereference()" is
that the "del" functions don't return anything.
Assuming my understanding is correct:
Reviewed-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Thanx, Paul
> diff --git a/urcu/rculfhash.h b/urcu/rculfhash.h
> index 2d8a310..2938e5e 100644
> --- a/urcu/rculfhash.h
> +++ b/urcu/rculfhash.h
> @@ -203,6 +203,7 @@ void cds_lfht_count_nodes(struct cds_lfht *ht,
> *
> * Call with rcu_read_lock held.
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function acts as a rcu_dereference() to read the node pointer.
> */
> void cds_lfht_lookup(struct cds_lfht *ht, unsigned long hash,
> cds_lfht_match_fct match, const void *key,
> @@ -226,6 +227,7 @@ void cds_lfht_lookup(struct cds_lfht *ht, unsigned long
> hash,
> * node returned by a previous cds_lfht_next.
> * Call with rcu_read_lock held.
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function acts as a rcu_dereference() to read the node pointer.
> */
> void cds_lfht_next_duplicate(struct cds_lfht *ht,
> cds_lfht_match_fct match, const void *key,
> @@ -239,6 +241,7 @@ void cds_lfht_next_duplicate(struct cds_lfht *ht,
> * Output in "*iter". *iter->node set to NULL if table is empty.
> * Call with rcu_read_lock held.
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function acts as a rcu_dereference() to read the node pointer.
> */
> void cds_lfht_first(struct cds_lfht *ht, struct cds_lfht_iter *iter);
>
> @@ -252,6 +255,7 @@ void cds_lfht_first(struct cds_lfht *ht, struct
> cds_lfht_iter *iter);
> * pointing to the last table node.
> * Call with rcu_read_lock held.
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function acts as a rcu_dereference() to read the node pointer.
> */
> void cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_iter *iter);
>
> @@ -264,6 +268,8 @@ void cds_lfht_next(struct cds_lfht *ht, struct
> cds_lfht_iter *iter);
> * This function supports adding redundant keys into the table.
> * Call with rcu_read_lock held.
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function issues a full memory barrier before and after its
> + * atomic commit.
> */
> void cds_lfht_add(struct cds_lfht *ht, unsigned long hash,
> struct cds_lfht_node *node);
> @@ -288,6 +294,12 @@ void cds_lfht_add(struct cds_lfht *ht, unsigned long
> hash,
> * to add keys into the table, no duplicated keys should ever be
> * observable in the table. The same guarantee apply for combination of
> * add_unique and add_replace (see below).
> + *
> + * Upon success, this function issues a full memory barrier before and
> + * after its atomic commit. Upon failure, this function acts like a
> + * simple lookup operation: it acts as a rcu_dereference() to read the
> + * node pointer. The failure case does not guarantee any other memory
> + * barrier.
> */
> struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht *ht,
> unsigned long hash,
> @@ -321,6 +333,9 @@ struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht
> *ht,
> * schemes will never generate duplicated keys. It also allows us to
> * guarantee that a combination of add_replace and add_unique updates
> * will never generate duplicated keys.
> + *
> + * This function issues a full memory barrier before and after its
> + * atomic commit.
> */
> struct cds_lfht_node *cds_lfht_add_replace(struct cds_lfht *ht,
> unsigned long hash,
> @@ -352,6 +367,10 @@ struct cds_lfht_node *cds_lfht_add_replace(struct
> cds_lfht *ht,
> *
> * The semantic of replacement vs lookups is the same as
> * cds_lfht_add_replace().
> + *
> + * Upon success, this function issues a full memory barrier before and
> + * after its atomic commit. Upon failure, this function does not issue
> + * any memory barrier.
> */
> int cds_lfht_replace(struct cds_lfht *ht,
> struct cds_lfht_iter *old_iter,
> @@ -377,6 +396,9 @@ int cds_lfht_replace(struct cds_lfht *ht,
> * After successful removal, a grace period must be waited for before
> * freeing the memory reserved for old node (which can be accessed with
> * cds_lfht_iter_get_node).
> + * Upon success, this function issues a full memory barrier before and
> + * after its atomic commit. Upon failure, this function does not issue
> + * any memory barrier.
> */
> int cds_lfht_del(struct cds_lfht *ht, struct cds_lfht_node *node);
>
> @@ -391,6 +413,7 @@ int cds_lfht_del(struct cds_lfht *ht, struct
> cds_lfht_node *node);
> * function.
> * Call with rcu_read_lock held.
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function does not issue any memory barrier.
> */
> int cds_lfht_is_node_deleted(struct cds_lfht_node *node);
>
> @@ -400,6 +423,7 @@ int cds_lfht_is_node_deleted(struct cds_lfht_node *node);
> * @new_size: update to this hash table size.
> *
> * Threads calling this API need to be registered RCU read-side threads.
> + * This function does not (necessarily) issue memory barriers.
> */
> void cds_lfht_resize(struct cds_lfht *ht, unsigned long new_size);
>
> @@ -407,6 +431,7 @@ void cds_lfht_resize(struct cds_lfht *ht, unsigned long
> new_size);
> * Note: it is safe to perform element removal (del), replacement, or
> * any hash table update operation during any of the following hash
> * table traversals.
> + * These functions act as rcu_dereference() to read the node pointers.
> */
> #define cds_lfht_for_each(ht, iter, node) \
> for (cds_lfht_first(ht, iter), \
>
> Thanks,
>
> Mathieu
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>
> _______________________________________________
> rp mailing list
> r...@svcs.cs.pdx.edu
> http://svcs.cs.pdx.edu/mailman/listinfo/rp
>
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