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

From: "Van Haaren, Harry" <harry.van.haaren@intel.com>
To: "Richardson, Bruce" <bruce.richardson@intel.com>,
	"Wang, Yipeng1" <yipeng1.wang@intel.com>
Cc: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>,
	"De Lara Guarch, Pablo" <pablo.de.lara.guarch@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	"gavin.hu@arm.com" <gavin.hu@arm.com>,
	"steve.capper@arm.com" <steve.capper@arm.com>,
	"ola.liljedahl@arm.com" <ola.liljedahl@arm.com>,
	"nd@arm.com" <nd@arm.com>
Subject: Re: [PATCH 3/4] hash: fix rw concurrency while moving	keys
Date: Fri, 28 Sep 2018 08:55:25 +0000	[thread overview]
Message-ID: <E923DB57A917B54B9182A2E928D00FA65E2E0E47@IRSMSX102.ger.corp.intel.com> (raw)
In-Reply-To: <20180928082610.GA7592@bricha3-MOBL.ger.corp.intel.com>

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> Sent: Friday, September 28, 2018 9:26 AM
> To: Wang, Yipeng1 <yipeng1.wang@intel.com>
> Cc: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>; De Lara Guarch,
> Pablo <pablo.de.lara.guarch@intel.com>; dev@dpdk.org; gavin.hu@arm.com;
> steve.capper@arm.com; ola.liljedahl@arm.com; nd@arm.com
> Subject: Re: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving
> keys
> 
> On Fri, Sep 28, 2018 at 02:00:00AM +0100, Wang, Yipeng1 wrote:
> > Reply inlined:
> >
> > >-----Original Message-----
> > >From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Honnappa Nagarahalli
> > >Sent: Thursday, September 6, 2018 10:12 AM
> > >To: Richardson, Bruce <bruce.richardson@intel.com>; De Lara Guarch, Pablo
> <pablo.de.lara.guarch@intel.com>
> > >Cc: dev@dpdk.org; honnappa.nagarahalli@dpdk.org; gavin.hu@arm.com;
> steve.capper@arm.com; ola.liljedahl@arm.com;
> > >nd@arm.com; Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > >Subject: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving
> keys
> > >
> > >Reader-writer concurrency issue, caused by moving the keys
> > >to their alternative locations during key insert, is solved
> > >by introducing a global counter(tbl_chng_cnt) indicating a
> > >change in table.

<snip>

> > > /**
> > >@@ -200,7 +200,7 @@ rte_hash_add_key_with_hash_data(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(const struct rte_hash *h, const void *key);
> > >+rte_hash_add_key(struct rte_hash *h, const void *key);
> > >
> > > /**
> > >  * 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.

-Harry