Linux Virtualization - Sep 2010 - PATCH: virtio_console: Fix poll blocking even though there is data to read

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 to be read, this patch
fixes this by using the alredy defined will_read_block utility function
instead of the port->inbuf != NULL check.

Signed-off-By: Hans de Goede <hdegoede at redhat.com>

Regards,

Hans
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On (Wed) Sep 15 2010 [15:04:53], Hans de Goede wrote:
> 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 to be read, this
patch
> fixes this by using the alredy defined will_read_block utility function
> instead of the port->inbuf != NULL check.
> 
> Signed-off-By: Hans de Goede <hdegoede at redhat.com>
> 
> Regards,
> 
> Hans
> diff -up linux-2.6.35.x86_64/drivers/char/virtio_console.c~
linux-2.6.35.x86_64/drivers/char/virtio_console.c
> --- linux-2.6.35.x86_64/drivers/char/virtio_console.c~	2010-08-02
00:11:14.000000000 +0200
> +++ linux-2.6.35.x86_64/drivers/char/virtio_console.c	2010-09-15
13:39:29.043505000 +0200
> @@ -642,7 +642,7 @@ static unsigned int port_fops_poll(struc
>  	poll_wait(filp, &port->waitqueue, wait);
>  
>  	ret = 0;
> -	if (port->inbuf)
> +	if (!will_read_block(port))
Looks correct, but this should be

	if (port_has_data(port))

instead.

will_read_block() also tests if the host is connected, which is not what
we want for POLLIN to be set.

		Amit