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

-----Original Message-----
From: Jeff Layton [mailto:jlayton@redhat.com]
Sent: Tuesday, September 20, 2011 10:25 AM
To: Stanislav Kinsbursky
Cc: Myklebust, Trond; linux-nfs@vger.kernel.org; Pavel Emelianov;
neilb@suse.de; netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
bfields@fieldses.org; davem@davemloft.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 [off-list ref] wrote:

quoted

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

quoted

rpcbind clients has been created already, then we just increase

rpcb_users.

quoted

Signed-off-by: Stanislav Kinsbursky <redacted>

---
  net/sunrpc/rpcb_clnt.c |   53

++++++++++++++++++++++++++++++++++++++++++++++++

quoted

  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.