On 09/20/2011 11:38 AM, Stanislav Kinsbursky wrote:
> 20.09.2011 18:58, Bryan Schumaker пишет:
>> On 09/20/2011 10:43 AM, Stanislav Kinsbursky wrote:
>>> 20.09.2011 18:24, Jeff Layton пишет:
>>>> On Tue, 20 Sep 2011 17:49:27 +0400
>>>> Stanislav Kinsbursky<skinsbursky@xxxxxxxxxxxxx> wrote:
>>>>
>>>>> v5: fixed races with rpcb_users in rpcb_get_local()
>>>>>
>>>>> This helpers will be used for dynamical creation and destruction of rpcbind
>>>>> clients.
>>>>> Variable rpcb_users is actually a counter of lauched RPC services. If rpcbind
>>>>> clients has been created already, then we just increase rpcb_users.
>>>>>
>>>>> Signed-off-by: Stanislav Kinsbursky<skinsbursky@xxxxxxxxxxxxx>
>>>>>
>>>>> ---
>>>>> net/sunrpc/rpcb_clnt.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>> 1 files changed, 53 insertions(+), 0 deletions(-)
>>>>>
>>>>> diff --git a/net/sunrpc/rpcb_clnt.c b/net/sunrpc/rpcb_clnt.c
>>>>> index e45d2fb..5f4a406 100644
>>>>> --- a/net/sunrpc/rpcb_clnt.c
>>>>> +++ b/net/sunrpc/rpcb_clnt.c
>>>>> @@ -114,6 +114,9 @@ static struct rpc_program rpcb_program;
>>>>> static struct rpc_clnt * rpcb_local_clnt;
>>>>> static struct rpc_clnt * rpcb_local_clnt4;
>>>>> +DEFINE_SPINLOCK(rpcb_clnt_lock);
>>>>> +unsigned int rpcb_users;
>>>>> +
>>>>> struct rpcbind_args {
>>>>> struct rpc_xprt * r_xprt;
>>>>> @@ -161,6 +164,56 @@ static void rpcb_map_release(void *data)
>>>>> kfree(map);
>>>>> }
>>>>> +static int rpcb_get_local(void)
>>>>> +{
>>>>> + int cnt;
>>>>> +
>>>>> + spin_lock(&rpcb_clnt_lock);
>>>>> + if (rpcb_users)
>>>>> + rpcb_users++;
>>>>> + cnt = rpcb_users;
>>>>> + spin_unlock(&rpcb_clnt_lock);
>>>>> +
>>>>> + return cnt;
>>>>> +}
>>>>> +
>>>>> +void rpcb_put_local(void)
>>>>> +{
>>>>> + struct rpc_clnt *clnt = rpcb_local_clnt;
>>>>> + struct rpc_clnt *clnt4 = rpcb_local_clnt4;
>>>>> + int shutdown;
>>>>> +
>>>>> + spin_lock(&rpcb_clnt_lock);
>>>>> + if (--rpcb_users == 0) {
>>>>> + rpcb_local_clnt = NULL;
>>>>> + rpcb_local_clnt4 = NULL;
>>>>> + }
>>>>
>>>> In the function below, you mention that the above pointers are
>>>> protected by rpcb_create_local_mutex, but it looks like they get reset
>>>> here without that being held?
>>>>
>>>
>>> Assigning of them is protected by rpcb_create_local_mutex.
>>> Dereferencing of them is protected by rpcb_clnt_lock.
>>
>> Shouldn't you be using the same lock for assigning and dereferencing? Otherwise one thread could change these variables while another is using them.
>
> Probably I wasn't clear with my previous explanation.
> Actually, we use only spinlock to make sure, that the pointers are valid when we dereferencing them. Synchronization point here is rpcb_users counter.
> IOW, we use these pointers only from svc code and only after service already started. And with this patch-set we can be sure, that this pointers has been created already to the point, when this service starts.
>
> But when this counter is zero, we can experience races with assigning those pointers. It takes a lot of time, so we use local mutex here instead of spinlock.
>
> Have I answered your question?
I think I understand now. Thanks!
>
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