Re: [dpdk-dev] rte_hash thread safe

[Stephen Hemminger <stephen@networkplumber.org>]

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.