linux-kernel - [V9fs-developer] [PATCH v3] Fix a hard lockup case in the virtio transport

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Date:   Thu, 19 Jul 2018 15:17:00 +0800
From:   jiangyiwen <jiangyiwen@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Ron Minnich <rminnich@...dia.gov>,
        Latchesar Ionkov <lucho@...kov.net>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        <v9fs-developer@...ts.sourceforge.net>,
        Dominique Martinet <asmadeus@...ewreck.org>
Subject: [V9fs-developer] [PATCH v3] Fix a hard lockup case in the virtio
 transport

When client has multiple threads that issue io requests
all the time, and the server has a very good performance,
it may cause cpu is running in the irq context for a long
time because it can check virtqueue has buf in the *while*
loop.

So we should keep chan->lock in the whole loop.

Signed-off-by: Yiwen Jiang <jiangyiwen@...wei.com>
---
 net/9p/trans_virtio.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 05006cb..3589864 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -144,24 +144,25 @@ static void req_done(struct virtqueue *vq)
 	struct virtio_chan *chan = vq->vdev->priv;
 	unsigned int len;
 	struct p9_req_t *req;
+	bool need_wakeup = false;
 	unsigned long flags;

 	p9_debug(P9_DEBUG_TRANS, ": request done\n");

-	while (1) {
-		spin_lock_irqsave(&chan->lock, flags);
-		req = virtqueue_get_buf(chan->vq, &len);
-		if (req == NULL) {
-			spin_unlock_irqrestore(&chan->lock, flags);
-			break;
+	spin_lock_irqsave(&chan->lock, flags);
+	while ((req = virtqueue_get_buf(chan->vq, &len)) != NULL) {
+		if (!chan->ring_bufs_avail) {
+			chan->ring_bufs_avail = 1;
+			need_wakeup = true;
 		}
-		chan->ring_bufs_avail = 1;
-		spin_unlock_irqrestore(&chan->lock, flags);
-		/* Wakeup if anyone waiting for VirtIO ring space. */
-		wake_up(chan->vc_wq);
+
 		if (len)
 			p9_client_cb(chan->client, req, REQ_STATUS_RCVD);
 	}
+	spin_unlock_irqrestore(&chan->lock, flags);
+	/* Wakeup if anyone waiting for VirtIO ring space. */
+	if (need_wakeup)
+		wake_up(chan->vc_wq);
 }

 /**
-- 
1.8.3.1