PATCH: virtio_console: Fix poll blocking even though there is data to read (version 2)

Hi All,

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.

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 ready to be read, this patch fixes this by using port_has_data(port) instead of the port->inbuf != NULL check.

Regards,

Hans

Subject: virtio_console: Fix poll blocking even though there is data to read (version 2)
From: Hans de Goede <[email protected]>

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 ready to be read, this patch fixes this by using port_has_data(port) instead of the port->inbuf != NULL check.

diff -up linux-2.6.35.x86_64/drivers/char/virtio_console.c~ linux-2.6.35.x86_64/drivers/char/virtio_console.c
@@ -642,7 +642,7 @@ static unsigned int port_fops_poll(struc
 	poll_wait(filp, &port->waitqueue, wait);
 
 	ret = 0;
-	if (port->inbuf)
+	if (port_has_data(port))
 		ret |= POLLIN | POLLRDNORM;
 	if (!will_write_block(port))
 		ret |= POLLOUT;