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From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: Stephen Hemminger <stephen@networkplumber.org>
CC: Yipeng Wang <yipeng1.wang@intel.com>, Sameh Gobriel
<sameh.gobriel@intel.com>, Bruce Richardson <bruce.richardson@intel.com>,
"dev@dpdk.org" <dev@dpdk.org>, "pablo.de.lara.guarch@intel.com"
<pablo.de.lara.guarch@intel.com>, nd <nd@arm.com>, Honnappa Nagarahalli
<Honnappa.Nagarahalli@arm.com>, nd <nd@arm.com>
Thread-Topic: [PATCH] hash: document breakage with multi-writer thread
Thread-Index: AQHWOpQbSbTXGH7QCEO1ao+t3FmiRajIuzHAgAADGYCAAAbIYA==
Date: Thu, 4 Jun 2020 18:43:36 +0000
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Subject: Re: [dpdk-dev] [PATCH] hash: document breakage with multi-writer
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> > <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 f=
ix the
> issue instead?
>=20
> Discovered this the hard way...
>=20
> 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 =3D 1;
writer_takes_lock =3D 1;
}
The writer locks are in place already. The code to handle the case when loc=
al cache is taken out is also there.
What we need is another input flag that says 'multi writer + non-eal thread=
s' which would set 'use_local_cache =3D 0' and 'writer_takes_lock =3D 1'.
Not sure, it would be valuable addition. But looks like this is what you we=
re 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.