linux-kernel - Re: suspicious RCU usage in security/selinux/netnode.c

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Date:	Tue, 15 May 2012 10:24:23 -0400
From:	Eric Paris <eparis@...isplace.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Dave Jones <davej@...hat.com>, sds@...ho.nsa.gov,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	paul@...l-moore.com
Subject: Re: suspicious RCU usage in security/selinux/netnode.c

On Tue, May 15, 2012 at 1:16 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Tue, May 15, 2012 at 12:41:45AM -0400, Dave Jones wrote:
>> I just triggered this on Linus' current tree.
>
> This is a bare:
>
>        rcu_dereference(sel_netnode_hash[idx].list.prev)
>
> which needs to be in an RCU read-side critical section.  Alternatively,
> the above should instead be something like:
>
>        rcu_dereference_check(sel_netnode_hash[idx].list.prev,
>                              lockdep_is_held(&sel_netnode_lock));

Right, but that 'bare' dereference comes from
list_for_each_entry_rcu(), [from sel_netnode_sid_slow()] which I don't
see how to easily annotate with the lock.  Nor do I think it's within
my brain power (or my willingness to maintain such in the future) to
want to open code that logic.

Should we just take the rcu_read_lock() where we take the spinlock?
Is that a perf hit and figuring out how to do the annotation correctly
is the better idea?

-Eric
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