svcrpc: fix svc_xprt_enqueue/svc_recv busy-looping

The rpc server tries to ensure that there will be room to send a reply
before it receives a request.

It does this by tracking, in xpt_reserved, an upper bound on the total
size of the replies that is has already committed to for the socket.

Currently it is adding in the estimate for a new reply *before* it
checks whether there is space available.  If it finds that there is not
space, it then subtracts the estimate back out.

This may lead the subsequent svc_xprt_enqueue to decide that there is
space after all.

The results is a svc_recv() that will repeatedly return -EAGAIN, causing
server threads to loop without doing any actual work.

Cc: stable@vger.kernel.org
Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -316,7 +316,6 @@ static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt)
  */
 void svc_xprt_enqueue(struct svc_xprt *xprt)
 {
-	struct svc_serv	*serv = xprt->xpt_server;
 	struct svc_pool *pool;
 	struct svc_rqst	*rqstp;
 	int cpu;
@@ -362,8 +361,6 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
 				rqstp, rqstp->rq_xprt);
 		rqstp->rq_xprt = xprt;
 		svc_xprt_get(xprt);
-		rqstp->rq_reserved = serv->sv_max_mesg;
-		atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
 		pool->sp_stats.threads_woken++;
 		wake_up(&rqstp->rq_wait);
 	} else {
@@ -640,8 +637,6 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
 	if (xprt) {
 		rqstp->rq_xprt = xprt;
 		svc_xprt_get(xprt);
-		rqstp->rq_reserved = serv->sv_max_mesg;
-		atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
 
 		/* As there is a shortage of threads and this request
 		 * had to be queued, don't allow the thread to wait so
@@ -738,6 +733,8 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
 		else
 			len = xprt->xpt_ops->xpo_recvfrom(rqstp);
 		dprintk("svc: got len=%d\n", len);
+		rqstp->rq_reserved = serv->sv_max_mesg;
+		atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
 	}
 	svc_xprt_received(xprt);