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From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>, "Wang, Yipeng1"
<yipeng1.wang@intel.com>, "Van Haaren, Harry" <harry.van.haaren@intel.com>,
"Richardson, Bruce" <bruce.richardson@intel.com>
CC: "De Lara Guarch, Pablo" <pablo.de.lara.guarch@intel.com>, "dev@dpdk.org"
<dev@dpdk.org>, "Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>, Steve
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<nd@arm.com>, "Gobriel, Sameh" <sameh.gobriel@intel.com>
Thread-Topic: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving keys
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Subject: Re: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency
while moving keys
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>=20
> > >-----Original Message-----
> > >From: Van Haaren, Harry
> > >> > > > > /**
> > >> > > > > * Add a key to an existing hash table.
> > >> > > > >@@ -222,7 +222,7 @@ rte_hash_add_key(const struct rte_hash
> > >> > > > >*h, const void
> > >> > > *key);
> > >> > > > > * array of user data. This value is unique for this key=
.
> > >> > > > > */
> > >> > > > > int32_t
> > >> > > > >-rte_hash_add_key_with_hash(const struct rte_hash *h, const
> > >> > > > >void *key,
> > >> > > hash_sig_t sig);
> > >> > > > >+rte_hash_add_key_with_hash(struct rte_hash *h, const void
> > >> > > > >+*key,
> > >> > > hash_sig_t sig);
> > >> > > > >
> > >> > > > > /
> > >> > > >
> > >> > > > I think the above changes will break ABI by changing the
> > >> > > > parameter
> > >> type?
> > >> > > Other people may know better on this.
> > >> > >
> > >> > > Just removing a const should not change the ABI, I believe,
> > >> > > since the const is just advisory hint to the compiler. Actual
> > >> > > parameter size and count remains unchanged so I don't believe th=
ere
> is an issue.
> > >> > > [ABI experts, please correct me if I'm wrong on this]
> > >> >
> > >> >
> > >> > [Certainly no ABI expert, but...]
> > >> >
> > >> > I think this is an API break, not ABI break.
> > >> >
> > >> > Given application code as follows, it will fail to compile - even
> > >> > though
> > >> running
> > >> > the new code as a .so wouldn't cause any issues (AFAIK).
> > >> >
> > >> > void do_hash_stuff(const struct rte_hash *h, ...) {
> > >> > /* parameter passed in is const, but updated function
> > >> > prototype is
> > >> non-
> > >> > const */
> > >> > rte_hash_add_key_with_hash(h, ...); }
> > >> >
> > >> > This means that we can't recompile apps against latest patch
> > >> > without application code changes, if the app was passing a const
> > >> > rte_hash struct
> > >> as
> > >> > the first parameter.
> > >> >
> > >> Agree. Do we need to do anything for this?
> > >
> > >I think we should try to avoid breaking API wherever possible.
> > >If we must, then I suppose we could follow the ABI process of a
> > >deprecation notice.
> > >
> > >From my reading of the versioning docs, it doesn't document this case:
> > >https://doc.dpdk.org/guides/contributing/versioning.html
> > >
> > >I don't recall a similar situation in DPDK previously - so I suggest
> > >you ask Tech board for input here.
> > >
> > >Hope that helps! -Harry
> > [Wang, Yipeng]
> > Honnappa, how about use a pointer to the counter in the rte_hash
> > struct instead of the counter? Will this avoid API change?
> I think it defeats the purpose of 'const' parameter to the API and provid=
es
> incorrect information to the user.
Yipeng, I think I have misunderstood your comment. I believe you meant; we =
could allocate memory to the counter and store the pointer in the structure=
. Please correct me if I am wrong.
This could be a solution, though it will be another cache line access. It m=
ight be ok given that it is a single cache line for entire hash table.
> IMO, DPDK should have guidelines on how to handle the API compatibility
> breaks. I will send an email to tech board on this.
> We can also solve this by having counters on the bucket. I was planning t=
o do
> this little bit later. I will look at the effort involved and may be do i=
t now.