Re: [PATCH v4 06/30] xprtrdma: Don't wake pending tasks until disconnect is done

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2018-12-17 at 11:39 -0500, Chuck Lever wrote:
> Transport disconnect processing does a "wake pending tasks" at
> various points.
> 
> Suppose an RPC Reply is being processed. The RPC task that Reply
> goes with is waiting on the pending queue. If a disconnect wake-up
> happens before reply processing is done, that reply, even if it is
> good, is thrown away, and the RPC has to be sent again.
> 
> This window apparently does not exist for socket transports because
> there is a lock held while a reply is being received which prevents
> the wake-up call until after reply processing is done.
> 
> To resolve this, all RPC replies being processed on an RPC-over-RDMA
> transport have to complete before pending tasks are awoken due to a
> transport disconnect.
> 
> Callers that already hold the transport write lock may invoke
> ->ops->close directly. Others use a generic helper that schedules
> a close when the write lock can be taken safely.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  include/linux/sunrpc/xprt.h                |    1 +
>  net/sunrpc/xprt.c                          |   19
> +++++++++++++++++++
>  net/sunrpc/xprtrdma/backchannel.c          |   13 +++++++------
>  net/sunrpc/xprtrdma/svc_rdma_backchannel.c |    8 +++++---
>  net/sunrpc/xprtrdma/transport.c            |   16 ++++++++++------
>  net/sunrpc/xprtrdma/verbs.c                |    5 ++---
>  6 files changed, 44 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/sunrpc/xprt.h
> b/include/linux/sunrpc/xprt.h
> index a4ab4f8..ee94ed0 100644
> --- a/include/linux/sunrpc/xprt.h
> +++ b/include/linux/sunrpc/xprt.h
> @@ -401,6 +401,7 @@ static inline __be32
> *xprt_skip_transport_header(struct rpc_xprt *xprt, __be32 *
>  bool			xprt_request_get_cong(struct rpc_xprt *xprt,
> struct rpc_rqst *req);
>  void			xprt_disconnect_done(struct rpc_xprt *xprt);
>  void			xprt_force_disconnect(struct rpc_xprt *xprt);
> +void			xprt_disconnect_nowake(struct rpc_xprt *xprt);
>  void			xprt_conditional_disconnect(struct rpc_xprt
> *xprt, unsigned int cookie);
>  
>  bool			xprt_lock_connect(struct rpc_xprt *, struct
> rpc_task *, void *);
> diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> index ce92700..afe412e 100644
> --- a/net/sunrpc/xprt.c
> +++ b/net/sunrpc/xprt.c
> @@ -685,6 +685,25 @@ void xprt_force_disconnect(struct rpc_xprt
> *xprt)
>  }
>  EXPORT_SYMBOL_GPL(xprt_force_disconnect);
>  
> +/**
> + * xprt_disconnect_nowake - force a call to xprt->ops->close
> + * @xprt: transport to disconnect
> + *
> + * The caller must ensure that xprt_wake_pending_tasks() is
> + * called later.
> + */
> +void xprt_disconnect_nowake(struct rpc_xprt *xprt)
> +{
> +       /* Don't race with the test_bit() in xprt_clear_locked() */
> +       spin_lock_bh(&xprt->transport_lock);
> +       set_bit(XPRT_CLOSE_WAIT, &xprt->state);
> +       /* Try to schedule an autoclose RPC call */
> +       if (test_and_set_bit(XPRT_LOCKED, &xprt->state) == 0)
> +               queue_work(xprtiod_workqueue, &xprt->task_cleanup);
> +       spin_unlock_bh(&xprt->transport_lock);
> +}
> +EXPORT_SYMBOL_GPL(xprt_disconnect_nowake);
> +

We shouldn't need both xprt_disconnect_nowake() and
xprt_force_disconnect() to be exported given that you can build the
latter from the former + xprt_wake_pending_tasks() (which is also
already exported).

>  static unsigned int
>  xprt_connect_cookie(struct rpc_xprt *xprt)
>  {
> diff --git a/net/sunrpc/xprtrdma/backchannel.c
> b/net/sunrpc/xprtrdma/backchannel.c
> index 2cb07a3..5d462e8 100644
> --- a/net/sunrpc/xprtrdma/backchannel.c
> +++ b/net/sunrpc/xprtrdma/backchannel.c
> @@ -193,14 +193,15 @@ static int rpcrdma_bc_marshal_reply(struct
> rpc_rqst *rqst)
>   */
>  int xprt_rdma_bc_send_reply(struct rpc_rqst *rqst)
>  {
> -	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(rqst->rq_xprt);
> +	struct rpc_xprt *xprt = rqst->rq_xprt;
> +	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
>  	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);
>  	int rc;
>  
> -	if (!xprt_connected(rqst->rq_xprt))
> -		goto drop_connection;
> +	if (!xprt_connected(xprt))
> +		return -ENOTCONN;
>  
> -	if (!xprt_request_get_cong(rqst->rq_xprt, rqst))
> +	if (!xprt_request_get_cong(xprt, rqst))
>  		return -EBADSLT;
>  
>  	rc = rpcrdma_bc_marshal_reply(rqst);
> @@ -215,7 +216,7 @@ int xprt_rdma_bc_send_reply(struct rpc_rqst
> *rqst)
>  	if (rc != -ENOTCONN)
>  		return rc;
>  drop_connection:
> -	xprt_disconnect_done(rqst->rq_xprt);
> +	xprt->ops->close(xprt);

Why use an indirect call here? Is this ever going to be different to
xprt_rdma_close()?

>  	return -ENOTCONN;
>  }
>  
> @@ -338,7 +339,7 @@ void rpcrdma_bc_receive_call(struct rpcrdma_xprt
> *r_xprt,
>  
>  out_overflow:
>  	pr_warn("RPC/RDMA backchannel overflow\n");
> -	xprt_disconnect_done(xprt);
> +	xprt_disconnect_nowake(xprt);
>  	/* This receive buffer gets reposted automatically
>  	 * when the connection is re-established.
>  	 */
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
> b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
> index f3c147d..b908f2c 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
> @@ -200,11 +200,10 @@ static int svc_rdma_bc_sendto(struct
> svcxprt_rdma *rdma,
>  		svc_rdma_send_ctxt_put(rdma, ctxt);
>  		goto drop_connection;
>  	}
> -	return rc;
> +	return 0;
>  
>  drop_connection:
>  	dprintk("svcrdma: failed to send bc call\n");
> -	xprt_disconnect_done(xprt);
>  	return -ENOTCONN;
>  }
>  
> @@ -225,8 +224,11 @@ static int svc_rdma_bc_sendto(struct
> svcxprt_rdma *rdma,
>  
>  	ret = -ENOTCONN;
>  	rdma = container_of(sxprt, struct svcxprt_rdma, sc_xprt);
> -	if (!test_bit(XPT_DEAD, &sxprt->xpt_flags))
> +	if (!test_bit(XPT_DEAD, &sxprt->xpt_flags)) {
>  		ret = rpcrdma_bc_send_request(rdma, rqst);
> +		if (ret == -ENOTCONN)
> +			svc_close_xprt(sxprt);
> +	}
>  
>  	mutex_unlock(&sxprt->xpt_mutex);
>  
> diff --git a/net/sunrpc/xprtrdma/transport.c
> b/net/sunrpc/xprtrdma/transport.c
> index 91c476a..a16296b 100644
> --- a/net/sunrpc/xprtrdma/transport.c
> +++ b/net/sunrpc/xprtrdma/transport.c
> @@ -453,13 +453,13 @@
>  
>  	if (test_and_clear_bit(RPCRDMA_IAF_REMOVING, &ia->ri_flags)) {
>  		rpcrdma_ia_remove(ia);
> -		return;
> +		goto out;
>  	}
> +
>  	if (ep->rep_connected == -ENODEV)
>  		return;
>  	if (ep->rep_connected > 0)
>  		xprt->reestablish_timeout = 0;
> -	xprt_disconnect_done(xprt);
>  	rpcrdma_ep_disconnect(ep, ia);
>  
>  	/* Prepare @xprt for the next connection by reinitializing
> @@ -467,6 +467,10 @@
>  	 */
>  	r_xprt->rx_buf.rb_credits = 1;
>  	xprt->cwnd = RPC_CWNDSHIFT;
> +
> +out:
> +	++xprt->connect_cookie;
> +	xprt_disconnect_done(xprt);
>  }
>  
>  /**
> @@ -515,7 +519,7 @@
>  static void
>  xprt_rdma_timer(struct rpc_xprt *xprt, struct rpc_task *task)
>  {
> -	xprt_force_disconnect(xprt);
> +	xprt_disconnect_nowake(xprt);
>  }
>  
>  /**
> @@ -717,7 +721,7 @@
>  #endif	/* CONFIG_SUNRPC_BACKCHANNEL */
>  
>  	if (!xprt_connected(xprt))
> -		goto drop_connection;
> +		return -ENOTCONN;
>  
>  	if (!xprt_request_get_cong(xprt, rqst))
>  		return -EBADSLT;
> @@ -749,8 +753,8 @@
>  	if (rc != -ENOTCONN)
>  		return rc;
>  drop_connection:
> -	xprt_disconnect_done(xprt);
> -	return -ENOTCONN;	/* implies disconnect */
> +	xprt_rdma_close(xprt);
> +	return -ENOTCONN;
>  }
>  
>  void xprt_rdma_print_stats(struct rpc_xprt *xprt, struct seq_file
> *seq)
> diff --git a/net/sunrpc/xprtrdma/verbs.c
> b/net/sunrpc/xprtrdma/verbs.c
> index 9a0a765..38a757c 100644
> --- a/net/sunrpc/xprtrdma/verbs.c
> +++ b/net/sunrpc/xprtrdma/verbs.c
> @@ -252,7 +252,7 @@ static void rpcrdma_xprt_drain(struct
> rpcrdma_xprt *r_xprt)
>  #endif
>  		set_bit(RPCRDMA_IAF_REMOVING, &ia->ri_flags);
>  		ep->rep_connected = -ENODEV;
> -		xprt_force_disconnect(xprt);
> +		xprt_disconnect_nowake(xprt);
>  		wait_for_completion(&ia->ri_remove_done);
>  
>  		ia->ri_id = NULL;
> @@ -280,10 +280,9 @@ static void rpcrdma_xprt_drain(struct
> rpcrdma_xprt *r_xprt)
>  			ep->rep_connected = -EAGAIN;
>  		goto disconnected;
>  	case RDMA_CM_EVENT_DISCONNECTED:
> -		++xprt->connect_cookie;
>  		ep->rep_connected = -ECONNABORTED;
>  disconnected:
> -		xprt_force_disconnect(xprt);
> +		xprt_disconnect_nowake(xprt);
>  		wake_up_all(&ep->rep_connect_wait);
>  		break;
>  	default:
> 

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com