A process issuing blocking writes to a virtio console may get stuck
indefinitely if another thread polls the device. Here is how to trigger
the bug:

- Thread A writes to the port until the virtqueue is full.
- Thread A calls wait_port_writable() and goes to sleep, waiting on
  port->waitqueue.
- The host processes some of the write, marks buffers as used and raises
  an interrupt.
- Before the interrupt is serviced, thread B executes port_fops_poll().
  This calls reclaim_consumed_buffers() via will_write_block() and
  consumes all used buffers.
- The interrupt is serviced. vring_interrupt() finds no used buffers
  via more_used() and returns without waking port->waitqueue.
- Thread A is still in wait_event(port->waitqueue), waiting for a
  wakeup that never arrives.

The crux is that invoking reclaim_consumed_buffers() may cause
vring_interrupt() to omit wakeups.

Fix this by making reclaim_consumed_buffers() issue an additional wake
up if it consumed any buffers.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
---
As far as I can tell all currently maintained stable series kernels need
this commit. I've tested that it applies cleanly to 5.10.247, however
wasn't able to build the kernel due to an unrelated link error. Instead
I applied it to 5.15.197, which compiled and verified to be fixed.
---
 drivers/char/virtio_console.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 088182e54debd6029ea2c2a5542d7a28500e67b8..7cd3ad9da9b53a7a570410f12501acc7fd7e3b9b 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -581,6 +581,7 @@ static ssize_t send_control_msg(struct port *port, unsigned int event,
 static void reclaim_consumed_buffers(struct port *port)
 {
 	struct port_buffer *buf;
+	bool freed = false;
 	unsigned int len;
 
 	if (!port->portdev) {
@@ -589,7 +590,15 @@ static void reclaim_consumed_buffers(struct port *port)
 	}
 	while ((buf = virtqueue_get_buf(port->out_vq, &len))) {
 		free_buf(buf, false);
+		freed = true;
+	}
+	if (freed) {
+		/* We freed all used buffers. Issue a wake up so that other pending
+		 * tasks do not get stuck. This is necessary because vring_interrupt()
+		 * will drop wakeups from the host if there are no used buffers.
+		 */
 		port->outvq_full = false;
+		wake_up_interruptible(&port->waitqueue);
 	}
 }
 

---
base-commit: d358e5254674b70f34c847715ca509e46eb81e6f
change-id: 20251215-virtio-console-lost-wakeup-0f566c5cd35f

Best regards,
-- 
Lorenz Bauer <lmb@isovalent.com>