SUNRPC: Destroy the back channel when we destroy the host transport

When we're destroying the host transport mechanism, we should ensure
that we do not leak memory by failing to release any back channel
slots that might still exist.

Reported-by: Neil Brown <[e-mail]>
Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

include/linux/sunrpc/bc_xprt.h

···
 	return 0;
 }
 
+static inline void xprt_destroy_backchannel(struct rpc_xprt *xprt,
+					    unsigned int max_reqs)
+{
+}
+
 static inline bool svc_is_backchannel(const struct svc_rqst *rqstp)
 {
 	return false;

net/sunrpc/backchannel_rqst.c

···
 		goto out;
 
 	spin_lock_bh(&xprt->bc_pa_lock);
-	xprt->bc_alloc_max -= max_reqs;
+	xprt->bc_alloc_max -= min(max_reqs, xprt->bc_alloc_max);
 	list_for_each_entry_safe(req, tmp, &xprt->bc_pa_list, rq_bc_pa_list) {
 		dprintk("RPC:        req=%p\n", req);
 		list_del(&req->rq_bc_pa_list);

net/sunrpc/xprt.c

···
 	rpc_destroy_wait_queue(&xprt->backlog);
 	kfree(xprt->servername);
 	/*
+	 * Destroy any existing back channel
+	 */
+	xprt_destroy_backchannel(xprt, UINT_MAX);
+
+	/*
 	 * Tear down transport state and free the rpc_xprt
 	 */
 	xprt->ops->destroy(xprt);