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

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

[Hans de Goede]

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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.

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

Regards,

Hans


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Subject: virtio_console: Fix poll blocking even though there is data to read (version 2)
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 ready to be read, this
patch fixes this by using port_has_data(port) instead of the
port->inbuf != NULL check.

Signed-off-By: Hans de Goede <hdegoede@redhat.com>
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 (port_has_data(port))
 		ret |= POLLIN | POLLRDNORM;
 	if (!will_write_block(port))
 		ret |= POLLOUT;

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

[Amit Shah]

On (Wed) Sep 15 2010 [17:18:23], Hans de Goede wrote:
> 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 (port_has_data(port))
>  		ret |= POLLIN | POLLRDNORM;
>  	if (!will_write_block(port))
>  		ret |= POLLOUT;

Thanks, this works.

I'll send a git-formatted patch to Rusty.

		Amit