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.
> 
> [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>
> 	}
> }
The other thing I wanted to do was saving on the memory allocated for the local cache when the writers are non-eal threads. Without knowing the kind of threads upfront, we might have to create the local cache when a writer adds the entry first time.