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

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Message-ID: <2E1EB2CF9ED1CB4AA966F0EB76EAB4430B47FE1E@SACMVEXC2-PRD.hq.netapp.com>
Date:	Tue, 20 Sep 2011 10:13:31 -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 12:21 PM
> 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 18:38, 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?

You need to ensure that if someone calls rpcb_get_local() and gets a positive result, then they can rely on rpcb_local_clnt/4 being non-zero. Without the write barrier, that is not the case.

Without that guarantee, you can't really ensure that rpcb_put_local() will work as expected either, since it will be possible to trigger the --rpcb_users == 0 case without shutting down rpcb_local_clnt/4 (because clnt/clnt4 == 0).

Cheers
  Trond