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

["Wang, Yipeng1" <yipeng1.wang@intel.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.

[Wang, Yipeng]
If possible, we can try to not add new flags, because there are already a lot of flag options.
How about in the code, we check if the writer is a non-eal or not by checking the rte_lcore_id, and operate on the global queue?
Could this work?
If(h->use_local_cache) {
	lcore_id = rte_lcore_id();
	if(lcore_id == LCORE_ID_ANY) {   // this is non-eal threads
		<call rte_ring_mp/mc_* to directly operate on global queue>
	}
	Else {
		<original path>
	}
}