Hans de Goede
2010-Sep-15 13:04 UTC
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 -------------- next part -------------- An embedded and charset-unspecified text was scrubbed... Name: linux-2.6.35.4-virtio_console-fix-poll.patch Url: http://lists.linux-foundation.org/pipermail/virtualization/attachments/20100915/00617423/attachment.txt
Amit Shah
2010-Sep-15 13:25 UTC
PATCH: virtio_console: Fix poll blocking even though there is data to read
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
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