Re: backchannel transport

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Dec 10, 2010 at 07:20:13PM -0500, J. Bruce Fields wrote:
> On Wed, Dec 01, 2010 at 12:53:49PM -0500, bfields wrote:
> > This is just a small question about the nfsd-side backchannel code that
> > touches the rpc client code:
> > 
> > There's nothing preventing multiple backchannels from sharing the same
> > tcp connection, either simultaneously or over time; from rfc 5661
> > section 2.10.3.1:
> > 
> > 	A connection's association with a session is not exclusive.  A
> > 	connection associated with the channel(s) of one session may be
> > 	simultaneously associated with the channel(s) of other sessions
> > 	including sessions associated with other client IDs.
> > 
> > The current code creates a new rpc_xprt every time an rpc client is
> > created for a backchannel.
> > 
> > But that's wrong: if two sessions share a backchannel, then they don't
> > necessarily want to share the same rpc_client, but they *do* need to
> > share the same rpc_xprt, to prevent xid reuse at least.
> > 
> > So I think we should create one rpc_xprt the first time we use a
> > connection for a backchannel, and then keep it around as long as the
> > connection is open (in case the client reassociates it with a
> > backchannel again).
> > 
> > rpc_clone_client() allows clients to share an rpc_xprt.  I could use
> > that for example by keeping an nfsd-level data structure allowing me to
> > look up rpc_client's by connection.
> > 
> > However, the server- and client- side xprt's already reference each
> > other--the client's pointer to the server is used to send requests, and
> > the server's pointer to the client is used to match incoming replies
> > with requests.
> > 
> > So it would seem simplest to use the already existing pointer from the
> > svc_xprt back to the rpc_xprt.
> > 
> > So, that would mean:
> > 
> > 	- Allowing nfsd to create a client with the already-existing
> > 	  rpc_xprt--maybe export rpc_new_client()?  Or allow the
> > 	  xprt->setup method to find an already-existing transport?
> > 	- Keeping any backchannel rpc_xprt around somehow as long as the
> > 	  connection is open.  Maybe let svc_xprt keep a reference on
> > 	  the client xprt until svc_delete_xprt()??
> 
> So that would be something like the following.

And I merged this with Chuck's git tree (including the xdr patches), and
ran some tests.  No merge conflicts, and the tests passed.

--b.

> 
> ?
> 
> --b.
> 
> commit 1eb4f46889940b861805742d67ba114e5de7ddf0
> Author: J. Bruce Fields <bfields@redhat.com>
> Date:   Tue Nov 30 19:15:01 2010 -0500
> 
>     rpc: move sk_bc_xprt to svc_xprt
>     
>     This seems obviously transport-level information even if it's currently
>     used only by the server socket code.
>     
>     Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> 
> diff --git a/include/linux/sunrpc/svc_xprt.h b/include/linux/sunrpc/svc_xprt.h
> index aea0d43..92ac407 100644
> --- a/include/linux/sunrpc/svc_xprt.h
> +++ b/include/linux/sunrpc/svc_xprt.h
> @@ -80,6 +80,7 @@ struct svc_xprt {
>  	struct list_head	xpt_users;	/* callbacks on free */
>  
>  	struct net		*xpt_net;
> +	struct rpc_xprt		*xpt_bc_xprt;	/* NFSv4.1 backchannel */
>  };
>  
>  static inline void unregister_xpt_user(struct svc_xprt *xpt, struct svc_xpt_user *u)
> diff --git a/include/linux/sunrpc/svcsock.h b/include/linux/sunrpc/svcsock.h
> index 1b353a7..04dba23 100644
> --- a/include/linux/sunrpc/svcsock.h
> +++ b/include/linux/sunrpc/svcsock.h
> @@ -28,7 +28,6 @@ struct svc_sock {
>  	/* private TCP part */
>  	u32			sk_reclen;	/* length of record */
>  	u32			sk_tcplen;	/* current read length */
> -	struct rpc_xprt		*sk_bc_xprt;	/* NFSv4.1 backchannel xprt */
>  };
>  
>  /*
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 07919e1..5a27d09 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -985,15 +985,17 @@ static int svc_process_calldir(struct svc_sock *svsk, struct svc_rqst *rqstp,
>  		vec[0] = rqstp->rq_arg.head[0];
>  	} else {
>  		/* REPLY */
> -		if (svsk->sk_bc_xprt)
> -			req = xprt_lookup_rqst(svsk->sk_bc_xprt, xid);
> +		struct rpc_xprt *bc_xprt = svsk->sk_xprt.xpt_bc_xprt;
> +
> +		if (bc_xprt)
> +			req = xprt_lookup_rqst(bc_xprt, xid);
>  
>  		if (!req) {
>  			printk(KERN_NOTICE
>  				"%s: Got unrecognized reply: "
> -				"calldir 0x%x sk_bc_xprt %p xid %08x\n",
> +				"calldir 0x%x xpt_bc_xprt %p xid %08x\n",
>  				__func__, ntohl(calldir),
> -				svsk->sk_bc_xprt, xid);
> +				bc_xprt, xid);
>  			vec[0] = rqstp->rq_arg.head[0];
>  			goto out;
>  		}
> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index dfcab5a..18dc42e 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -2379,9 +2379,9 @@ static struct rpc_xprt *xs_setup_bc_tcp(struct xprt_create *args)
>  	 * The backchannel uses the same socket connection as the
>  	 * forechannel
>  	 */
> +	args->bc_xprt->xpt_bc_xprt = xprt;
>  	xprt->bc_xprt = args->bc_xprt;
>  	bc_sock = container_of(args->bc_xprt, struct svc_sock, sk_xprt);
> -	bc_sock->sk_bc_xprt = xprt;
>  	transport->sock = bc_sock->sk_sock;
>  	transport->inet = bc_sock->sk_sk;
>  
> 
> commit 218a5c9b8fb3e9f33a3ef761dfaff2f9387686c4
> Author: J. Bruce Fields <bfields@redhat.com>
> Date:   Wed Dec 8 12:45:44 2010 -0500
> 
>     rpc: keep backchannel xprt as long as server connection
>     
>     Multiple backchannels can share the same tcp connection; from rfc 5661
>     section 2.10.3.1:
>     
>     	A connection's association with a session is not exclusive.  A
>     	connection associated with the channel(s) of one session may be
>     	simultaneously associated with the channel(s) of other sessions
>     	including sessions associated with other client IDs.
>     
>     However, multiple backchannels share a connection, they must all share
>     the same xid stream (hence the same rpc_xprt); the only way we have to
>     match replies with calls at the rpc layer is using the xid.
>     
>     So, keep the rpc_xprt around as long as the connection lasts, in case
>     we're asked to use the connection as a backchannel again.
>     
>     Requests to create new backchannel clients over a given server
>     connection should results in creating new clients that reuse the
>     existing rpc_xprt.
>     
>     But to start, just reject attempts to associate multiple rpc_xprt's with
>     the same underlying bc_xprt.
>     
>     Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> 
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index ea2ff78..597c79c 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -13,6 +13,7 @@
>  #include <linux/sunrpc/stats.h>
>  #include <linux/sunrpc/svc_xprt.h>
>  #include <linux/sunrpc/svcsock.h>
> +#include <linux/sunrpc/xprt.h>
>  
>  #define RPCDBG_FACILITY	RPCDBG_SVCXPRT
>  
> @@ -128,6 +129,9 @@ static void svc_xprt_free(struct kref *kref)
>  	if (test_bit(XPT_CACHE_AUTH, &xprt->xpt_flags))
>  		svcauth_unix_info_release(xprt);
>  	put_net(xprt->xpt_net);
> +	/* See comment on corresponding get in xs_setup_bc_tcp(): */
> +	if (xprt->xpt_bc_xprt)
> +		xprt_put(xprt->xpt_bc_xprt);
>  	xprt->xpt_ops->xpo_free(xprt);
>  	module_put(owner);
>  }
> diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> index 4c8f18a..749ad15 100644
> --- a/net/sunrpc/xprt.c
> +++ b/net/sunrpc/xprt.c
> @@ -965,6 +965,7 @@ struct rpc_xprt *xprt_alloc(struct net *net, int size, int max_req)
>  	xprt = kzalloc(size, GFP_KERNEL);
>  	if (xprt == NULL)
>  		goto out;
> +	kref_init(&xprt->kref);
>  
>  	xprt->max_reqs = max_req;
>  	xprt->slot = kcalloc(max_req, sizeof(struct rpc_rqst), GFP_KERNEL);
> @@ -1102,7 +1103,6 @@ found:
>  		return xprt;
>  	}
>  
> -	kref_init(&xprt->kref);
>  	spin_lock_init(&xprt->transport_lock);
>  	spin_lock_init(&xprt->reserve_lock);
>  
> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index 18dc42e..0ef4dd4 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -2375,16 +2375,6 @@ static struct rpc_xprt *xs_setup_bc_tcp(struct xprt_create *args)
>  	xprt->reestablish_timeout = 0;
>  	xprt->idle_timeout = 0;
>  
> -	/*
> -	 * The backchannel uses the same socket connection as the
> -	 * forechannel
> -	 */
> -	args->bc_xprt->xpt_bc_xprt = xprt;
> -	xprt->bc_xprt = args->bc_xprt;
> -	bc_sock = container_of(args->bc_xprt, struct svc_sock, sk_xprt);
> -	transport->sock = bc_sock->sk_sock;
> -	transport->inet = bc_sock->sk_sk;
> -
>  	xprt->ops = &bc_tcp_ops;
>  
>  	switch (addr->sa_family) {
> @@ -2407,6 +2397,29 @@ static struct rpc_xprt *xs_setup_bc_tcp(struct xprt_create *args)
>  			xprt->address_strings[RPC_DISPLAY_PROTO]);
>  
>  	/*
> +	 * The backchannel uses the same socket connection as the
> +	 * forechannel
> +	 */
> +	if (args->bc_xprt->xpt_bc_xprt) {
> +		/* XXX: actually, want to catch this case... */
> +		ret = ERR_PTR(-EINVAL);
> +		goto out_err;
> +	}
> +	/*
> +	 * Once we've associated a backchannel xprt with a connection,
> +	 * we want to keep it around as long as long as the connection
> +	 * lasts, in case we need to start using it for a backchannel
> +	 * again; this reference won't be dropped until bc_xprt is
> +	 * destroyed.
> +	 */
> +	xprt_get(xprt);
> +	args->bc_xprt->xpt_bc_xprt = xprt;
> +	xprt->bc_xprt = args->bc_xprt;
> +	bc_sock = container_of(args->bc_xprt, struct svc_sock, sk_xprt);
> +	transport->sock = bc_sock->sk_sock;
> +	transport->inet = bc_sock->sk_sk;
> +
> +	/*
>  	 * Since we don't want connections for the backchannel, we set
>  	 * the xprt status to connected
>  	 */
> @@ -2415,6 +2428,7 @@ static struct rpc_xprt *xs_setup_bc_tcp(struct xprt_create *args)
>  
>  	if (try_module_get(THIS_MODULE))
>  		return xprt;
> +	xprt_put(xprt);
>  	ret = ERR_PTR(-EINVAL);
>  out_err:
>  	xprt_free(xprt);
> 
> commit ba7febf8e1ff482a6dc535e08e550053d9e34a74
> Author: J. Bruce Fields <bfields@redhat.com>
> Date:   Wed Dec 8 13:48:19 2010 -0500
> 
>     rpc: allow xprt_class->setup to return a preexisting xprt
>     
>     This allows us to reuse the xprt associated with a server connection if
>     one has already been set up.
>     
>     Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> 
> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
> index 89d10d2..bef0f53 100644
> --- a/include/linux/sunrpc/xprt.h
> +++ b/include/linux/sunrpc/xprt.h
> @@ -321,6 +321,7 @@ void			xprt_conditional_disconnect(struct rpc_xprt *xprt, unsigned int cookie);
>  #define XPRT_CLOSING		(6)
>  #define XPRT_CONNECTION_ABORT	(7)
>  #define XPRT_CONNECTION_CLOSE	(8)
> +#define XPRT_INITIALIZED	(9)
>  
>  static inline void xprt_set_connected(struct rpc_xprt *xprt)
>  {
> diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> index 749ad15..856274d 100644
> --- a/net/sunrpc/xprt.c
> +++ b/net/sunrpc/xprt.c
> @@ -1102,6 +1102,9 @@ found:
>  				-PTR_ERR(xprt));
>  		return xprt;
>  	}
> +	if (test_and_set_bit(XPRT_INITIALIZED, &xprt->state))
> +		/* ->setup returned a pre-initialized xprt: */
> +		return xprt;
>  
>  	spin_lock_init(&xprt->transport_lock);
>  	spin_lock_init(&xprt->reserve_lock);
> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index 0ef4dd4..4aced17 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -2359,6 +2359,15 @@ static struct rpc_xprt *xs_setup_bc_tcp(struct xprt_create *args)
>  	struct svc_sock *bc_sock;
>  	struct rpc_xprt *ret;
>  
> +	if (args->bc_xprt->xpt_bc_xprt) {
> +		/*
> +		 * This server connection already has a backchannel
> +		 * export; we can't create a new one, as we wouldn't be
> +		 * able to match replies based on xid any more.  So,
> +		 * reuse the already-existing one:
> +		 */
> +		 return args->bc_xprt->xpt_bc_xprt;
> +	}
>  	xprt = xs_setup_xprt(args, xprt_tcp_slot_table_entries);
>  	if (IS_ERR(xprt))
>  		return xprt;