netdev - Re: [PATCH v5 1/8] SUNRPC: introduce helpers for reference counted rpcbind clients

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Date:	Tue, 20 Sep 2011 12:06:21 -0400
From:	Bryan Schumaker <bjschuma@...app.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
CC:	Jeff Layton <jlayton@...hat.com>,
	"Trond.Myklebust@...app.com" <Trond.Myklebust@...app.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Pavel Emelianov <xemul@...allels.com>,
	"neilb@...e.de" <neilb@...e.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"bfields@...ldses.org" <bfields@...ldses.org>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH v5 1/8] SUNRPC: introduce helpers for reference counted
 rpcbind clients

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@...allels.com>   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@...allels.com>
>>>>>
>>>>> ---
>>>>>     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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