clarification on RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF flag

[Stephen Hemminger]

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,
> or this flag should always be used with
> RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD.
> 
> 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.
> 
> 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.