Re: [dpdk-dev] [PATCH] hash: document breakage with multi-writer thread

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> > <snip>
> >
> > > Subject: [PATCH] hash: document breakage with multi-writer thread
> > >
> > > The code in rte_cuckoo_hash multi-writer support is broken if write
> > > operations are called from a non-EAL thread.
> > >
> > > rte_lcore_id() wil return LCORE_ID_ANY (UINT32_MAX) for non EAL
> > > thread and that leads to using wrong local cache.
> > >
> > > Add error checks and document the restriction.
> > Having multiple non-EAL writer threads is a valid use case. Should we fix the
> issue instead?
> 
> Discovered this the hard way...
> 
> Fixing is non-trivial. Basically, the local cache has to be take out and that
> leads to having to do real locking or atomic operations.
Looking at rte_hash_create function:

        if (params->extra_flag & RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD) {
                use_local_cache = 1;
                writer_takes_lock = 1;
        }

The writer locks are in place already. The code to handle the case when local cache is taken out is also there.
What we need is another input flag that says 'multi writer + non-eal threads' which would set 'use_local_cache = 0' and 'writer_takes_lock = 1'.
Not sure, it would be valuable addition. But looks like this is what you were expecting when you had enabled 'RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD'. Many other APIs in DPDK do not provide this kind of MT safety.