linux-kernel - Re: [PATCH v7 15/16] openvswitch: use new hashtable implementation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 29 Oct 2012 11:59:57 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	torvalds@...ux-foundation.org, tj@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, paul.gortmaker@...driver.com,
	davem@...emloft.net, rostedt@...dmis.org, mingo@...e.hu,
	ebiederm@...ssion.com, aarcange@...hat.com, ericvh@...il.com,
	netdev@...r.kernel.org, josh@...htriplett.org,
	eric.dumazet@...il.com, axboe@...nel.dk, agk@...hat.com,
	dm-devel@...hat.com, neilb@...e.de, ccaulfie@...hat.com,
	teigland@...hat.com, Trond.Myklebust@...app.com,
	bfields@...ldses.org, fweisbec@...il.com, jesse@...ira.com,
	venkat.x.venkatsubra@...cle.com, ejt@...hat.com,
	snitzer@...hat.com, edumazet@...gle.com, linux-nfs@...r.kernel.org,
	dev@...nvswitch.org, rds-devel@....oracle.com, lw@...fujitsu.com
Subject: Re: [PATCH v7 15/16] openvswitch: use new hashtable implementation

* Sasha Levin (levinsasha928@...il.com) wrote:
> Hi Mathieu,
> 
> On Mon, Oct 29, 2012 at 9:29 AM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> > * Sasha Levin (levinsasha928@...il.com) wrote:
> > [...]
> >> -static struct hlist_head *hash_bucket(struct net *net, const char *name)
> >> -{
> >> -     unsigned int hash = jhash(name, strlen(name), (unsigned long) net);
> >> -     return &dev_table[hash & (VPORT_HASH_BUCKETS - 1)];
> >> -}
> >> -
> >>  /**
> >>   *   ovs_vport_locate - find a port that has already been created
> >>   *
> >> @@ -84,13 +76,12 @@ static struct hlist_head *hash_bucket(struct net *net, const char *name)
> >>   */
> >>  struct vport *ovs_vport_locate(struct net *net, const char *name)
> >>  {
> >> -     struct hlist_head *bucket = hash_bucket(net, name);
> >>       struct vport *vport;
> >>       struct hlist_node *node;
> >> +     int key = full_name_hash(name, strlen(name));
> >>
> >> -     hlist_for_each_entry_rcu(vport, node, bucket, hash_node)
> >> -             if (!strcmp(name, vport->ops->get_name(vport)) &&
> >> -                 net_eq(ovs_dp_get_net(vport->dp), net))
> >> +     hash_for_each_possible_rcu(dev_table, vport, node, hash_node, key)
> >
> > Is applying hash_32() on top of full_name_hash() needed and expected ?
> 
> Since this was pointed out in several of the patches, I'll answer it
> just once here.
> 
> I've intentionally "allowed" double hashing with hash_32 to keep the
> code simple.
> 
> hash_32() is pretty simple and gcc optimizes it to be almost nothing,
> so doing that costs us a multiplication and a shift. On the other
> hand, we benefit from keeping our code simple - how would we avoid
> doing this double hash? adding a different hashtable function for
> strings? or a new function for already hashed keys? I think we benefit
> a lot from having to mul/shr instead of adding extra lines of code
> here.

This could be done, as I pointed out in another email within this
thread, by changing the "key" argument from add/for_each_possible to an
expected "hash" value, and let the caller invoke hash_32() if they want.
I doubt this would add a significant amount of complexity for users of
this API, but would allow much more flexibility to choose hash
functions.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/