linux-kernel - [PATCH 5.15 916/917] SUNRPC: Partial revert of commit 6f9f17287e78

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Date:   Mon, 15 Nov 2021 18:06:51 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Trond Myklebust <trond.myklebust@...merspace.com>
Subject: [PATCH 5.15 916/917] SUNRPC: Partial revert of commit 6f9f17287e78

From: Trond Myklebust <trond.myklebust@...merspace.com>

commit ea7a1019d8baf8503ecd6e3ec8436dec283569e6 upstream.

The premise of commit 6f9f17287e78 ("SUNRPC: Mitigate cond_resched() in
xprt_transmit()") was that cond_resched() is expensive and unnecessary
when there has been just a single send.
The point of cond_resched() is to ensure that tasks that should pre-empt
this one get a chance to do so when it is safe to do so. The code prior
to commit 6f9f17287e78 failed to take into account that it was keeping a
rpc_task pinned for longer than it needed to, and so rather than doing a
full revert, let's just move the cond_resched.

Signed-off-by: Trond Myklebust <trond.myklebust@...merspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/sunrpc/xprt.c |   28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -1603,15 +1603,14 @@ xprt_transmit(struct rpc_task *task)
 {
 	struct rpc_rqst *next, *req = task->tk_rqstp;
 	struct rpc_xprt	*xprt = req->rq_xprt;
-	int counter, status;
+	int status;
 
 	spin_lock(&xprt->queue_lock);
-	counter = 0;
-	while (!list_empty(&xprt->xmit_queue)) {
-		if (++counter == 20)
+	for (;;) {
+		next = list_first_entry_or_null(&xprt->xmit_queue,
+						struct rpc_rqst, rq_xmit);
+		if (!next)
 			break;
-		next = list_first_entry(&xprt->xmit_queue,
-				struct rpc_rqst, rq_xmit);
 		xprt_pin_rqst(next);
 		spin_unlock(&xprt->queue_lock);
 		status = xprt_request_transmit(next, task);
@@ -1619,13 +1618,16 @@ xprt_transmit(struct rpc_task *task)
 			status = 0;
 		spin_lock(&xprt->queue_lock);
 		xprt_unpin_rqst(next);
-		if (status == 0) {
-			if (!xprt_request_data_received(task) ||
-			    test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
-				continue;
-		} else if (test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
-			task->tk_status = status;
-		break;
+		if (status < 0) {
+			if (test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+				task->tk_status = status;
+			break;
+		}
+		/* Was @task transmitted, and has it received a reply? */
+		if (xprt_request_data_received(task) &&
+		    !test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+			break;
+		cond_resched_lock(&xprt->queue_lock);
 	}
 	spin_unlock(&xprt->queue_lock);
 }