[ovs-dev] [PATCH 2/2] datapath: Directly use methods for protecting RCU/RTNL data.

[Jesse Gross]

On Mon, Nov 21, 2011 at 10:00 AM, Ben Pfaff <blp at nicira.com> wrote:
> On Sat, Nov 19, 2011 at 02:11:48PM -0800, Jesse Gross wrote:
>> We currently have a wrapper to protect the datapath ports array.
>> However, this can lead to confusion over exactly what lock is
>> protecting the access (either RTNL or RCU).  This removes the
>> wrapper in favor of directly accessing the data, which also has
>> the benefit of being less permissive about what lock we allow so
>> it can be restricted to the one that we expect.
>>
>> Signed-off-by: Jesse Gross <jesse at nicira.com>
>
>> @@ -132,7 +126,7 @@ static int get_dpifindex(struct datapath *dp)
>>
>>       rcu_read_lock();
>>
>> -     local = get_vport_protected(dp, OVSP_LOCAL);
>> +     local = rcu_dereference(dp->ports[OVSP_LOCAL]);
>>       if (local)
>>               ifindex = local->ops->get_ifindex(local);
>>       else
>
> get_dpifindex() is called from dp_fill_ifinfo(), which holds RTNL but
> I don't see that it holds RCU.

It's called from other places as well that don't hold RTNL, such as
dp_upcall().  We do hold RCU read lock inside of this function (see
the snippet above) and that's all that is really needed to protect
access to the vport since we are returning an integer and not a
pointer to the protected data.