Linux Virtualization - Sep 2010 - [PATCH 1/2] virtio: console: Fix poll blocking even though there is data to read

From: Hans de Goede <hdegoede at 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.

Signed-off-By: Hans de Goede <hdegoede at redhat.com>
Signed-off-by: Amit Shah <amit.shah at redhat.com>
---
 drivers/char/virtio_console.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index be67d6b..2dbcd41 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -710,7 +710,7 @@ 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;
-- 
1.7.2.3

The ports are char devices; do not have seeking capabilities.  Calling
nonseekable_open() from the fops_open() call and setting the llseek fops
pointer to no_llseek ensures an lseek() call from userspace returns
-ESPIPE.

Signed-off-by: Amit Shah <amit.shah at redhat.com>
CC: Arnd Bergmann <arnd at arndb.de>
---
 drivers/char/virtio_console.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 2dbcd41..28718a9 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -798,6 +798,8 @@ static int port_fops_open(struct inode *inode, struct file
*filp)
 	reclaim_consumed_buffers(port);
 	spin_unlock_irq(&port->outvq_lock);
 
+	nonseekable_open(inode, filp);
+
 	/* Notify host of port being opened */
 	send_control_msg(filp->private_data, VIRTIO_CONSOLE_PORT_OPEN, 1);
 
@@ -829,6 +831,7 @@ static const struct file_operations port_fops = {
 	.poll  = port_fops_poll,
 	.release = port_fops_release,
 	.fasync = port_fops_fasync,
+	.llseek = no_llseek,
 };
 
 /*
-- 
1.7.2.3

On Thu, 16 Sep 2010 06:43:08 pm Amit Shah wrote:
> From: Hans de Goede <hdegoede at 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.
> 
> Signed-off-By: Hans de Goede <hdegoede at redhat.com>
> Signed-off-by: Amit Shah <amit.shah at redhat.com>
Thanks.  I assume this one should go into stable too?

Cheers,
Rusty.