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From: Jeffrey Vander Stoep <jeffv@google.com>
To: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Will Deacon <will@kernel.org>,
	SElinux list <selinux@vger.kernel.org>,
	Paul Moore <paul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Jovana Knezevic <jovanak@google.com>
Subject: Re: [PATCH v5] selinux: sidtab: reverse lookup hash table
Date: Thu, 7 Nov 2019 21:06:33 +0100
Message-ID: <CABXk95AMYEayjt_1v_hv-bft2V=9q4xMSWuMbAOmTOvFj+SC0A@mail.gmail.com> (raw)
In-Reply-To: <CAFqZXNuCugt05+4nejHYvkAGrW+WMmwsRQv8pjKEhKxdBMDVew@mail.gmail.com>

> > If you use the existing _rcu accessors you will get correctly enforced
> > dependency ordering on the reader side and correctly placed release
> > ordering on the updater side. I don't think that's a big overkill, and
> > you can use the RCU accessors to achieve the lockless traversal.
>
> OK, but you don't need the read_lock()/unlock(), rcu_head field in the
> entries, and kfree_rcu(), right?

Hmm... I can get rid of rcu_head/kfree_rcu in any case because we never
remove items during normal use so we don't require the "rcu grace period".
>
> >
> > hlist_add() is not safe against a concurrent traversal because the
> > WRITE_ONCE() provides no memory ordering guarantees, allowing the readers
> > to see an uninitialised node.
>
> Right, so we would need a new function that does smp_store_release() instead.
>
> >
> > How exactly would list_for_each_entry_lockless() and hlist_add_something()
> > differ from the RCU variants, assuming they're implemented correctly?
>
> Looking at the implementation of rcu_dereference() and
> rcu_assign_pointer(), they would probably be practically the same,
> except the rcu_read_lock_held() check in rcu_dereference(). That and
> they would clearly communicate that they are not doing actual RCU, but
> just allow lockless traversal of an add-only hlist.

>
> --
> Ondrej Mosnacek <omosnace at redhat dot com>
> Software Engineer, Security Technologies
> Red Hat, Inc.
>

      reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-07 10:17 Jeff Vander Stoep
2019-11-07 12:01 ` Jeffrey Vander Stoep
2019-11-07 14:54 ` Stephen Smalley
2019-11-07 15:42   ` Ondrej Mosnacek
2019-11-07 15:59     ` Jeffrey Vander Stoep
2019-11-07 15:49   ` Jeffrey Vander Stoep
2019-11-07 16:12 ` Ondrej Mosnacek
2019-11-07 16:44   ` Will Deacon
2019-11-07 18:41     ` Ondrej Mosnacek
2019-11-07 20:06       ` Jeffrey Vander Stoep [this message]

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