linux-kernel - Re: [PATCH v4 1/8] SUNRPC: introduce helpers for reference counted rpcbind clients

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Date:	Tue, 20 Sep 2011 20:20:43 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
CC:	"Schumaker, Bryan" <Bryan.Schumaker@...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 v4 1/8] SUNRPC: introduce helpers for reference counted
 rpcbind clients

20.09.2011 18:38, Myklebust, Trond пишет:
>> -----Original Message-----
>> From: Stanislav Kinsbursky [mailto:skinsbursky@...allels.com]
>> Sent: Tuesday, September 20, 2011 10:35 AM
>> To: Myklebust, Trond
>> Cc: Schumaker, Bryan; linux-nfs@...r.kernel.org; Pavel Emelianov;
>> neilb@...e.de; netdev@...r.kernel.org; linux-kernel@...r.kernel.org;
>> bfields@...ldses.org; davem@...emloft.net
>> Subject: Re: [PATCH v4 1/8] SUNRPC: introduce helpers for reference
>> counted rpcbind clients
>>
>> 20.09.2011 18:14, Myklebust, Trond пишет:
>>
>>>>>
>>>>> Doesn't it  need to be protected by rpcb_clnt_lock too?
>>>>>
>>>>
>>>> Nope from my pow. It's protected by rpcb_create_local_mutex. I.e. no
>>>> one will change rpcb_users since it's zero. If it's non zero - we
>>>> willn't get to rpcb_set_local().
>>>
>>> OK, so you are saying that the rpcb_users++ below could be replaced by
>> rpcb_users=1?
>>>
>>
>> Yes, you right.
>>
>>> In that case, don't you need a smp_wmb() between the setting of
>> rpcb_local_clnt/4 and the setting of rpcb_users? Otherwise, how do you
>> guarantee that rpcb_users != 0 implies rpbc_local_clnt/4 != NULL?
>>>
>>
>> We check rpcb_users under spinlock. Gain spinlock forces memory barrier,
>> doesn't it?
>
> Yes, and so you don't need an smp_rmb() on the reader side. However, you still need to ensure that the processor which _sets_ the rpcb_users and rpcb_local_clnt/4 actually writes them in the correct order.
>

Trond, I've thought again and realized, that even if these writes (rpcb_users 
and rpbc_local_clnt/4) will be done in reversed order, then no barrier required 
here.
Because if we have another process trying to create those clients (it can't use 
them since it's not started yet) on other CPU, than it's waiting on creation 
mutex. When it will gain the mutex, we will have required memory barrier for us.

Or I missed something in your explanation?

-- 
Best regards,
Stanislav Kinsbursky
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