Re: [PATCH 3/4] hash: fix rw concurrency while moving keys

From: "Wang, Yipeng1" <yipeng1.wang@intel.com>
To: "Van Haaren, Harry" <harry.van.haaren@intel.com>,
	Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.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 Capper" <Steve.Capper@arm.com>,
	Ola Liljedahl <Ola.Liljedahl@arm.com>, nd <nd@arm.com>,
	"Gobriel, Sameh" <sameh.gobriel@intel.com>
Subject: Re: [PATCH 3/4] hash: fix rw concurrency while moving	keys
Date: Tue, 2 Oct 2018 23:58:21 +0000	[thread overview]
Message-ID: <D2C4A16CA39F7F4E8E384D204491D7A6614EB071@FMSMSX151.amr.corp.intel.com> (raw)
In-Reply-To: <E923DB57A917B54B9182A2E928D00FA65E2E4D28@IRSMSX102.ger.corp.intel.com>


>-----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 there 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?