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

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Date:	Tue, 20 Sep 2011 07:41:55 -0700
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"Jeff Layton" <jlayton@...hat.com>,
	"Stanislav Kinsbursky" <skinsbursky@...allels.com>
Cc:	<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 v5 1/8] SUNRPC: introduce helpers for reference counted rpcbind clients

> -----Original Message-----
> From: Jeff Layton [mailto:jlayton@...hat.com]
> Sent: Tuesday, September 20, 2011 10:25 AM
> To: Stanislav Kinsbursky
> Cc: Myklebust, Trond; 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 v5 1/8] SUNRPC: introduce helpers for reference
> counted rpcbind clients
> 
> 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?
> 
> 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.

Alternatively, if you do

	if (rpcb_users == 1) {
		rpcb_local_clnt = NULL;
		rpcb_local_clnt4 = NULL;
		smp_wmb();
		rpcb_users = 0;
	} else
		rpcb_users--;

then the spinlock protection in rpbc_get_local() is still good enough to
guarantee correctness.
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