netdev - RE: [PATCH v4 1/8] SUNRPC: introduce helpers for reference counted rpcbind clients

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Date:	Tue, 20 Sep 2011 07:38:53 -0700
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"Stanislav Kinsbursky" <skinsbursky@...allels.com>
Cc:	"Schumaker, Bryan" <Bryan.Schumaker@...app.com>,
	<linux-nfs@...r.kernel.org>,
	"Pavel Emelianov" <xemul@...allels.com>, <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

> -----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.

Cheers
  Trond