[lttng-dev] [rp] [RFC] Readiness for URCU release with RCU lock-free hash table

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> On Tue, May 01, 2012 at 10:16:09AM -0400, Mathieu Desnoyers wrote:
> > Hi!
> > 
> > After 1 year of development, with the last 6-7 months spent polishing
> > the API and testing the implementation, I think it is getting about time
> > to release the RCU lock-free hash table in a new Userspace RCU version
> > (0.7).
> > 
> > I recently described the guarantees provided by the hash table in more
> > detail, and created tests for the uniqueness guarantee for traversals
> > performed concurrently with add_unique and add_replace operations. I
> > also added test modes that create long hash chains, to test corner-cases
> > of the hash table.
> > 
> > One small thing I wonder is whether we should document that the hash
> > table update operations imply full memory barriers ?
> 
> The Linux kernel's rule seems good here -- if a hash-table operation is
> atomic and returns a value, it should imply a full barrier.  So:
> 
> cds_lfht_new(): No point in claiming barriers -- publishing the
> 	pointer to the hash table is where the barriers are important.
> 
> cds_lfht_destroy(): Ditto.
> 
> cds_lfht_lookup(): Not an update (let alone an atomic update), no barriers.
> 
> cds_lfht_next_duplicate(): Ditto.
> 
> cds_lfht_first(): Ditto.
> 
> cds_lfht_next(): Ditto.
> 
> cds_lfht_add(): Atomic update, but no return value, so no barrier
> 	implied.

Yep, makes sense. We use cmpxchg internally to perform the update, but
it could make sense to eventually use a cmpxchg that has no memory
barriers to perform this update. So I agree on not providing a memory
barrier guarantee on the "add" operation, since it does not return any
value.

> 
> cds_lfht_add_unique(): Atomic update that returns a value, so should
> 	imply a full memory barrier.

add_unique is a bit special:

- if it returns the node received as parameter, it means the add
  succeeded, which imply an update, and thus a memory barrier.

- if it returns a different node than the one received as parameter, it
  failed, and thus means that it only performed a lookup, so there is no
  guarantee that a memory barrier has been executed.


> 
> cds_lfht_add_replace(): Ditto.
> 
> cds_lfht_replace(): Ditto.
> 
> cds_lfht_del(): Ditto.

Yep, makes sense.

I'll add this documentation in the API.

Thanks!

Mathieu

> 
> cds_lfht_is_node_deleted(): Not an update (let alone an atomic update),
> 	no barriers.
> 
> cds_lfht_resize(): Atomic update, but no return value, so no barrier
> 	implied.
> 
> cds_lfht_for_each(): Not an update (let alone an atomic update),
> 	no barriers.
> 
> cds_lfht_for_each_duplicate(): Ditto.
> 
> cds_lfht_for_each_entry(): Ditto.
> 
> cds_lfht_for_each_entry_duplicate(): Ditto.
> 
> 							Thanx, Paul
> 
> > I'm personally getting confident that the hash table API is clean
> > enough, and the implementation well tested, but I'd like to have your
> > thoughts on the readiness of the hash table for production use.
> > 
> > Thanks,
> > 
> > Mathieu
> > 
> > -- 
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com
> > 
> > _______________________________________________
> > rp mailing list
> > rp at svcs.cs.pdx.edu
> > http://svcs.cs.pdx.edu/mailman/listinfo/rp
> > 
> 
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com