net/9p/virtio: Fix hard lockup in req_done

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.

[ Dominique: reworded subject line ]
Link: http://lkml.kernel.org/r/5B503AEC.5080404@huawei.com
Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Eric Van Hensbergen <ericvh@gmail.com>
To: Ron Minnich <rminnich@sandia.gov>
To: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>

1 files changed, 11 insertions, 10 deletions

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index c5464ec311fe..d422bfc81eca 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);
 }
 
 /**