xprtrdma: Fix trace point use-after-free race

It's not safe to use resources pointed to by the @send_wr of
ib_post_send() _after_ that function returns. Those resources are
typically freed by the Send completion handler, which can run before
ib_post_send() returns.

Thus the trace points currently around ib_post_send() in the
client's RPC/RDMA transport are a hazard, even when they are
disabled. Rearrange them so that they touch the Work Request only
_before_ ib_post_send() is invoked.

Fixes: ab03eff5 ("xprtrdma: Add trace points in RPC Call transmit paths")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/trace/events/rpcrdma.h

@@ -692,11 +692,10 @@ TRACE_EVENT(xprtrdma_prepsend_failed,
 
 TRACE_EVENT(xprtrdma_post_send,
 	TP_PROTO(
-		const struct rpcrdma_req *req,
-		int status
+		const struct rpcrdma_req *req
 	),
 
-	TP_ARGS(req, status),
+	TP_ARGS(req),
 
 	TP_STRUCT__entry(
 		__field(const void *, req)
@@ -705,7 +704,6 @@ TRACE_EVENT(xprtrdma_post_send,
 		__field(unsigned int, client_id)
 		__field(int, num_sge)
 		__field(int, signaled)
-		__field(int, status)
 	),
 
 	TP_fast_assign(
@@ -718,15 +716,13 @@ TRACE_EVENT(xprtrdma_post_send,
 		__entry->sc = req->rl_sendctx;
 		__entry->num_sge = req->rl_wr.num_sge;
 		__entry->signaled = req->rl_wr.send_flags & IB_SEND_SIGNALED;
-		__entry->status = status;
 	),
 
-	TP_printk("task:%u@%u req=%p sc=%p (%d SGE%s) %sstatus=%d",
+	TP_printk("task:%u@%u req=%p sc=%p (%d SGE%s) %s",
 		__entry->task_id, __entry->client_id,
 		__entry->req, __entry->sc, __entry->num_sge,
 		(__entry->num_sge == 1 ? "" : "s"),
-		(__entry->signaled ? "signaled " : ""),
-		__entry->status
+		(__entry->signaled ? "signaled" : "")
 	)
 );
 

net/sunrpc/xprtrdma/verbs.c

@@ -1356,8 +1356,8 @@ int rpcrdma_post_sends(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req)
 		--ep->re_send_count;
 	}
 
+	trace_xprtrdma_post_send(req);
 	rc = frwr_send(r_xprt, req);
-	trace_xprtrdma_post_send(req, rc);
 	if (rc)
 		return -ENOTCONN;
 	return 0;