sunrpc: Fix misplaced barrier in call_decode

Fix a misplaced barrier in call_decode. The struct rpc_rqst is modified
as follows by xprt_complete_rqst:

req->rq_private_buf.len = copied;
/* Ensure all writes are done before we update */
/* req->rq_reply_bytes_recvd */
smp_wmb();
req->rq_reply_bytes_recvd = copied;

And currently read as follows by call_decode:

smp_rmb(); // misplaced
if (!req->rq_reply_bytes_recvd)
   goto out;
req->rq_rcv_buf.len = req->rq_private_buf.len;

This patch places the smp_rmb after the if to ensure that
rq_reply_bytes_recvd and rq_private_buf.len are read in order.

Fixes: 9ba8288 ("SUNRPC: Don't try to parse incomplete RPC messages")
Signed-off-by: Baptiste Lepers <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>

Author: BLepers

net/sunrpc/clnt.c

@@ -2456,12 +2456,6 @@ call_decode(struct rpc_task *task)
 		task->tk_flags &= ~RPC_CALL_MAJORSEEN;
 	}
 
-	/*
-	 * Ensure that we see all writes made by xprt_complete_rqst()
-	 * before it changed req->rq_reply_bytes_recvd.
-	 */
-	smp_rmb();
-
 	/*
 	 * Did we ever call xprt_complete_rqst()? If not, we should assume
 	 * the message is incomplete.
@@ -2470,6 +2464,11 @@ call_decode(struct rpc_task *task)
 	if (!req->rq_reply_bytes_recvd)
 		goto out;
 
+	/* Ensure that we see all writes made by xprt_complete_rqst()
+	 * before it changed req->rq_reply_bytes_recvd.
+	 */
+	smp_rmb();
+
 	req->rq_rcv_buf.len = req->rq_private_buf.len;
 	trace_rpc_xdr_recvfrom(task, &req->rq_rcv_buf);