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

From: Bryan Schumaker <[email-masked]>
Date: Tue, 20 Sep 2011 10:58:17 -0400

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<[email-masked]>  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<[email-masked]>
>>>
>>> ---
>>>    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.
> 
>> Might it be simpler to just protect rpcb_users with the
>> rpcb_create_local_mutex and ensure that it's held whenever you call one
>> of these routines? None of these are codepaths are particularly hot.
>>
> 
> I just inherited this lock-mutex logic.
> Actually, you right. This codepaths are used rarely.
> But are use sure, that we need to remove this "speed-up" logic if we take into account that it was here already?
> 
>>> +    shutdown = !rpcb_users;
>>> +    spin_unlock(&rpcb_clnt_lock);
>>> +
>>> +    if (shutdown) {
>>> +        /*
>>> +         * cleanup_rpcb_clnt - remove xprtsock's sysctls, unregister
>>> +         */
>>> +        if (clnt4)
>>> +            rpc_shutdown_client(clnt4);
>>> +        if (clnt)
>>> +            rpc_shutdown_client(clnt);
>>> +    }
>>> +    return;
>>> +}
>>> +
>>> +static void rpcb_set_local(struct rpc_clnt *clnt, struct rpc_clnt *clnt4)
>>> +{
>>> +    /* Protected by rpcb_create_local_mutex */
>>> +    rpcb_local_clnt = clnt;
>>> +    rpcb_local_clnt4 = clnt4;
>>> +    rpcb_users++;
>>> +    dprintk("RPC:       created new rpcb local clients (rpcb_local_clnt: "
>>> +            "%p, rpcb_local_clnt4: %p)\n", rpcb_local_clnt,
>>> +            rpcb_local_clnt4);
>>> +}
>>> +
>>>    /*
>>>     * Returns zero on success, otherwise a negative errno value
>>>     * is returned.
>>> -- 
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>>
>>
> 
>