Re: [dpdk-dev] rte_hash thread safe

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 23 Apr 2018 17:21:15 -0700
Jim Murphy <jmurphy@arista.com> wrote:

> Has anyone seen performance data comparing rte_hash (perhaps with r/w
> locks) versus URCU hash?
> 
> 
> On Mon, Apr 23, 2018 at 4:50 PM, Stephen Hemminger <
> stephen@networkplumber.org> wrote:  
> 
> > On Mon, 23 Apr 2018 12:40:41 -0700
> > Brijesh Singh <brijesh.s.singh@gmail.com> wrote:
> >  
> > > A gentle reminder,
> > >
> > > I am curious to know if/how rte_hash is thread safe for lookups.It is
> > > not obvious to me how following code is thread safe:
> > >
> > > _rte_hash_lookup_with_hash(const struct rte_hash *h, const void *key,
> > >
> > >                                         hash_sig_t sig, void **data)
> > >
> > > {
> > >
> > >
> > >
> > > …
> > >
> > >                         if (rte_hash_cmp_eq(key, k->key, h) == 0) {
> > >
> > >                                 if (data != NULL)
> > >
> > >                                         *data = k->pdata;
> > >
> > > }
> > >
> > > a key could be deleted and another key inserted in its slot while the
> > > lookup is happening. For example, in the following sequence of events:
> > > The slot has Key1,V1
> > > Lookup Thread T1 compares the input key to Key1 and it matches. The
> > > thread gets context switched out
> > > Thread T2 deletes Key1.
> > > Thread T2 inserts Key2 with value V2.
> > > T1 reads the data from the slot and returns V2. This is incorrect.
> > >
> > >
> > > Regards,
> > > Brijesh
> > >
> > > On Wed, Apr 11, 2018 at 9:12 PM, Brijesh Singh
> > > <brijesh.s.singh@gmail.com> wrote:  
> > > > Hello,
> > > >
> > > > I  want to use DPDK's rte_hash library to keep track of tcp flows. The
> > > > lookups will be done by multiple threads but inserts will be done only
> > > > on one thread.
> > > >
> > > > As per the documentation rte_hash library has thread safe lookups. Key
> > > > /data inserts should be done on single thread, since those operations
> > > > are not thread safe. Is this documentation still correct?
> > > >
> > > > The lookup code compares the key and returns the data if the key
> > > > matches, this doesn't look like thread safe. Am I missing something?
> > > >
> > > > _rte_hash_lookup_with_hash(const struct rte_hash *h, const void *key,
> > > >
> > > >                                         hash_sig_t sig, void **data)
> > > >
> > > > {
> > > >
> > > >
> > > >
> > > > …
> > > >
> > > >                         if (rte_hash_cmp_eq(key, k->key, h) == 0) {
> > > >
> > > >                                 if (data != NULL)
> > > >
> > > >                                         *data = k->pdata;
> > > >
> > > > }
> > > >
> > > > Regards,
> > > > Brijesh  
> >
> > The best way to handle this is to do some kind of Read Copy Update.
> > Unfortunately, this is not possible in scope of DPDK since it requires
> > cooperation from application threads.
> >
> > If you need thread safe hash table, my recommendation would be to
> > skip the DPDK hash library and use userspace RCU instead:
> >   http://liburcu.org/
> >
> > Note: URCU is LGPL versus BSD licensed. But then any non trivial
> > Linux application needs to use LGPL libraries already.
> >  

No. But R/W locks are really slow. Look at one of Paul Mc Kenney's many RCU
talks and papers.