SUNRPC in case of backlog, hand free slots directly to waiting task

If sunrpc.tcp_max_slot_table_entries is small and there are tasks
on the backlog queue, then when a request completes it is freed and the
first task on the queue is woken.  The expectation is that it will wake
and claim that request.  However if it was a sync task and the waiting
process was killed at just that moment, it will wake and NOT claim the
request.

As long as TASK_CONGESTED remains set, requests can only be claimed by
tasks woken from the backlog, and they are woken only as requests are
freed, so when a task doesn't claim a request, no other task can ever
get that request until TASK_CONGESTED is cleared.  Each time this
happens the number of available requests is decreased by one.

With a sufficiently high workload and sufficiently low setting of
max_slot (16 in the case where this was seen), TASK_CONGESTED can remain
set for an extended period, and the above scenario (of a process being
killed just as its task was woken) can repeat until no requests can be
allocated.  Then traffic stops.

This patch addresses the problem by introducing a positive handover of a
request from a completing task to a backlog task - the request is never
freed when there is a backlog.

When a task is woken it might not already have a request attached in
which case it is *not* freed (as with current code) but is initialised
(if needed) and used.  If it isn't used it will eventually be freed by
rpc_exit_task().  xprt_release() is enhanced to be able to correctly
release an uninitialised request.

Fixes: ba60eb25 ("SUNRPC: Fix a livelock problem in the xprt->backlog queue")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/clnt.c

@@ -1677,13 +1677,6 @@ call_reserveresult(struct rpc_task *task)
 		return;
 	}
 
-	/*
-	 * Even though there was an error, we may have acquired
-	 * a request slot somehow.  Make sure not to leak it.
-	 */
-	if (task->tk_rqstp)
-		xprt_release(task);
-
 	switch (status) {
 	case -ENOMEM:
 		rpc_delay(task, HZ >> 2);

net/sunrpc/xprt.c

@@ -70,6 +70,7 @@
 static void	 xprt_init(struct rpc_xprt *xprt, struct net *net);
 static __be32	xprt_alloc_xid(struct rpc_xprt *xprt);
 static void	 xprt_destroy(struct rpc_xprt *xprt);
+static void	 xprt_request_init(struct rpc_task *task);
 
 static DEFINE_SPINLOCK(xprt_list_lock);
 static LIST_HEAD(xprt_list);
@@ -1612,10 +1613,26 @@ static void xprt_add_backlog(struct rpc_xprt *xprt, struct rpc_task *task)
 	rpc_sleep_on(&xprt->backlog, task, NULL);
 }
 
-static void xprt_wake_up_backlog(struct rpc_xprt *xprt)
+static bool __xprt_set_rq(struct rpc_task *task, void *data)
 {
-	if (rpc_wake_up_next(&xprt->backlog) == NULL)
+	struct rpc_rqst *req = data;
+
+	if (task->tk_rqstp == NULL) {
+		memset(req, 0, sizeof(*req));	/* mark unused */
+		task->tk_status = -EAGAIN;
+		task->tk_rqstp = req;
+		return true;
+	}
+	return false;
+}
+
+static bool xprt_wake_up_backlog(struct rpc_xprt *xprt, struct rpc_rqst *req)
+{
+	if (rpc_wake_up_first(&xprt->backlog, __xprt_set_rq, req) == NULL) {
 		clear_bit(XPRT_CONGESTED, &xprt->state);
+		return false;
+	}
+	return true;
 }
 
 static bool xprt_throttle_congested(struct rpc_xprt *xprt, struct rpc_task *task)
@@ -1703,11 +1720,11 @@ EXPORT_SYMBOL_GPL(xprt_alloc_slot);
 void xprt_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *req)
 {
 	spin_lock(&xprt->reserve_lock);
-	if (!xprt_dynamic_free_slot(xprt, req)) {
+	if (!xprt_wake_up_backlog(xprt, req) &&
+	    !xprt_dynamic_free_slot(xprt, req)) {
 		memset(req, 0, sizeof(*req));	/* mark unused */
 		list_add(&req->rq_list, &xprt->free);
 	}
-	xprt_wake_up_backlog(xprt);
 	spin_unlock(&xprt->reserve_lock);
 }
 EXPORT_SYMBOL_GPL(xprt_free_slot);
@@ -1795,6 +1812,10 @@ xprt_request_init(struct rpc_task *task)
 	struct rpc_xprt *xprt = task->tk_xprt;
 	struct rpc_rqst	*req = task->tk_rqstp;
 
+	if (req->rq_task)
+		/* Already initialized */
+		return;
+
 	req->rq_task	= task;
 	req->rq_xprt    = xprt;
 	req->rq_buffer  = NULL;
@@ -1855,8 +1876,10 @@ void xprt_retry_reserve(struct rpc_task *task)
 	struct rpc_xprt *xprt = task->tk_xprt;
 
 	task->tk_status = 0;
-	if (task->tk_rqstp != NULL)
+	if (task->tk_rqstp != NULL) {
+		xprt_request_init(task);
 		return;
+	}
 
 	task->tk_status = -EAGAIN;
 	xprt_do_reserve(xprt, task);
@@ -1881,23 +1904,26 @@ void xprt_release(struct rpc_task *task)
 	}
 
 	xprt = req->rq_xprt;
-	xprt_request_dequeue_xprt(task);
-	spin_lock(&xprt->transport_lock);
-	xprt->ops->release_xprt(xprt, task);
-	if (xprt->ops->release_request)
-		xprt->ops->release_request(task);
-	xprt_schedule_autodisconnect(xprt);
-	spin_unlock(&xprt->transport_lock);
-	if (req->rq_buffer)
-		xprt->ops->buf_free(task);
-	xdr_free_bvec(&req->rq_rcv_buf);
-	xdr_free_bvec(&req->rq_snd_buf);
-	if (req->rq_cred != NULL)
-		put_rpccred(req->rq_cred);
-	task->tk_rqstp = NULL;
-	if (req->rq_release_snd_buf)
-		req->rq_release_snd_buf(req);
+	if (xprt) {
+		xprt_request_dequeue_xprt(task);
+		spin_lock(&xprt->transport_lock);
+		xprt->ops->release_xprt(xprt, task);
+		if (xprt->ops->release_request)
+			xprt->ops->release_request(task);
+		xprt_schedule_autodisconnect(xprt);
+		spin_unlock(&xprt->transport_lock);
+		if (req->rq_buffer)
+			xprt->ops->buf_free(task);
+		xdr_free_bvec(&req->rq_rcv_buf);
+		xdr_free_bvec(&req->rq_snd_buf);
+		if (req->rq_cred != NULL)
+			put_rpccred(req->rq_cred);
+		if (req->rq_release_snd_buf)
+			req->rq_release_snd_buf(req);
+	} else
+		xprt = task->tk_xprt;
 
+	task->tk_rqstp = NULL;
 	if (likely(!bc_prealloc(req)))
 		xprt->ops->free_slot(xprt, req);
 	else