clarification on RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF flag

[Honnappa Nagarahalli]

<snip>

> On Wed, 6 Jul 2022 19:07:54 +0530
> venkatesh bs <venki.bsv at gmail.com> wrote:
> 
> > Hi All,
> >
> > In multithreaded/Multicore  environment can we use
> > RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF
> > independently,
This flag is about reader-writer concurrency (not writer-writer concurrency). Reader-writer concurrency will use lock-free algorithm allowing for the data plane to scale well.

> > or this flag should always be used with
> > RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD.
If you have multiple writers, you need to enable this flag. This is unrelated to RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF.

> >
> > We are trying to create and access the hash table with
> > RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF
> > only.
> > We are getting crashes in multi core environments , we debugged
> > nothing wrong in the application , everything looks good.
If the crash is happening while adding/deleting keys and there are multiple writers, enabling RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD might solve the problem.

> >
> > We call rte_hash_del_key() first and from the returned position we are
> > calling rte_hash_free_key_with_position().
> >
> > Please let me know if we missed something.
> >
> > Thanks,
> > Venkatesh.
> 
> Repeating same question doesn't get answer faster.
> 
> Read the code, it is fairly straightforward.
> 
> The multi-writer add means that writers take a lock.
> If doing lock free support then:
>   1. It is up to your application to use a single writer and/or
>      wrap writer calls in a lock.
> 
>   2. You need to use RCU mechanism to guarantee that no reader
>      will access a deleted entry. Something like:
> 
>      rte_hash_del_key()
>      synchronize_rcu()
>      rte_hash_free_key_with_position()
> 
>      You can use either the DPDK RCU library or the userspace RCU library.
>      Read that documentation, RCU is non-trivial change.
RCU is integrated in the hash library. You could use that as well. Look at ' rte_hash_rcu_qsbr_add' API for more details.

>