On 07/19/2013 01:45 PM, Amit Shah wrote: > On (Fri) 19 Jul 2013 [13:07:49], Jason Wang wrote: >> On 07/19/2013 04:16 AM, Amit Shah wrote: >>> If a port gets unplugged while a user is blocked on read(), -ENODEV is >>> returned. However, subsequent read()s returned 0, indicating there's no >>> host-side connection (but not indicating the device went away). >>> >>> This also happened when a port was unplugged and the user didn't have >>> any blocking operation pending. If the user didn't monitor the SIGIO >>> signal, they won't have a chance to find out if the port went away. >>> >>> Fix by returning -ENODEV on all read()s after the port gets unplugged. >>> write() already behaves this way. >>> >>> CC: <sta...@vger.kernel.org> >>> Signed-off-by: Amit Shah <amit.s...@redhat.com> >>> --- >>> drivers/char/virtio_console.c | 6 +++++- >>> 1 file changed, 5 insertions(+), 1 deletion(-) >>> >>> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c >>> index 6bf0df3..a39702a 100644 >>> --- a/drivers/char/virtio_console.c >>> +++ b/drivers/char/virtio_console.c >>> @@ -749,6 +749,10 @@ static ssize_t port_fops_read(struct file *filp, char >>> __user *ubuf, >>> >>> port = filp->private_data; >>> >>> + /* Port is hot-unplugged. */ >>> + if (!port->guest_connected) >>> + return -ENODEV; >>> + >> What if the port is hot-unplugged after this check? Should we serialize >> the hot plug with inbuf_lock here? > If that happens, port_has_data() returns false, and the later > functions return appropriately, as unplug_port() clears out all the > data. > > However, I spotted a couple of things that need fixing: > > 1. In the condition you describe above, port_has_data will return > false, and host_connected will be false as well, and fops_read() will > return 0 instead of -ENODEV once. Subsequent reads will return > -ENODEV.
Then the check is not needed here. > > 2. get_inbuf(), which is called by port_has_data() accesses the vqs > even if the port is unplugged. The vqs are still available, and won't > have data in them (since the port is unplugged), but it's best to not > rely on such behaviour. For the next merge window, I'll add extra > state, port_(un)plugged to track this instead of abusing > guest_connected. > > That also simplifies the path for later if we get vq hot-plug as well. > > I think the current code will behave satisfactorily for now; what do > you think? Yes, sounds good. > > Amit _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
