From mboxrd@z Thu Jan 1 00:00:00 1970 From: Honnappa Nagarahalli Subject: Re: [PATCH 3/4] hash: fix rw concurrency while moving keys Date: Thu, 4 Oct 2018 03:54:24 +0000 Message-ID: References: <1536253938-192391-1-git-send-email-honnappa.nagarahalli@arm.com> <1536253938-192391-4-git-send-email-honnappa.nagarahalli@arm.com> <20180928082610.GA7592@bricha3-MOBL.ger.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Cc: "De Lara Guarch, Pablo" , "dev@dpdk.org" , "Gavin Hu (Arm Technology China)" , Steve Capper , Ola Liljedahl , nd , "Gobriel, Sameh" To: Honnappa Nagarahalli , "Wang, Yipeng1" , "Van Haaren, Harry" , "Richardson, Bruce" Return-path: Received: from EUR02-VE1-obe.outbound.protection.outlook.com (mail-eopbgr20080.outbound.protection.outlook.com [40.107.2.80]) by dpdk.org (Postfix) with ESMTP id 986B04C9D for ; Thu, 4 Oct 2018 05:54:27 +0200 (CEST) In-Reply-To: Content-Language: en-US List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" >=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.