From: Amit Shah <amit.shah@redhat.com>
Date: Wed, 22 Sep 2010 05:55:34 -0400
Subject: [virt] virtio_console: fix poll blocking when data ready
Message-id: <8bf38560a0269afd6eadef20d9b60750e4f5f014.1285073493.git.amit.shah@redhat.com>
Patchwork-id: 28335
O-Subject: [RHEL5.7 / 5.6.z PATCH 3/4] virtio: console: Fix poll blocking even
	though there is data to read
Bugzilla: 636020
RH-Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>

From: Hans de Goede <hdegoede@redhat.com>

I found this while working on a Linux agent for spice, the symptom I was
seeing was select blocking on the spice vdagent virtio serial port even
though there were messages queued up there.

virtio_console's port_fops_poll checks port->inbuf != NULL to determine
if read won't block. However if an application reads enough bytes from
inbuf through port_fops_read, to empty the current port->inbuf,
port->inbuf will be NULL even though there may be buffers left in the
virtqueue.

This causes poll() to block even though there is data to be read,
this patch fixes this by using will_read_block(port) instead of the
port->inbuf != NULL check.

Bugzilla: 636020
Upstream: Posted, sent for 2.6.36 and stable.

Signed-off-By: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index b4a21a1..a39d1c3 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -714,7 +714,7 @@ static unsigned int port_fops_poll(struct file *filp, poll_table *wait)
 		return POLLHUP;
 	}
 	ret = 0;
-	if (port->inbuf)
+	if (!will_read_block(port))
 		ret |= POLLIN | POLLRDNORM;
 	if (!will_write_block(port))
 		ret |= POLLOUT;