virtio: console: Unlock vqs while freeing buffers

Commit c6017e79 ("virtio: console: add locks around buffer removal
in port unplug path") added locking around the freeing of buffers in the
vq. However, when free_buf() is called with can_sleep = true and rproc
is enabled, it calls dma_free_coherent() directly, requiring interrupts
to be enabled. Currently a WARNING is triggered due to the spin locking
around free_buf, with a call stack like this:

WARNING: CPU: 3 PID: 121 at ./include/linux/dma-mapping.h:433
free_buf+0x1a8/0x288
Call Trace:
[<8040c538>] show_stack+0x74/0xc0
[<80757240>] dump_stack+0xd0/0x110
[<80430d98>] __warn+0xfc/0x130
[<80430ee0>] warn_slowpath_null+0x2c/0x3c
[<807e7c6c>] free_buf+0x1a8/0x288
[<807ea590>] remove_port_data+0x50/0xac
[<807ea6a0>] unplug_port+0xb4/0x1bc
[<807ea858>] virtcons_remove+0xb0/0xfc
[<807b6734>] virtio_dev_remove+0x58/0xc0
[<807f918c>] __device_release_driver+0xac/0x134
[<807f924c>] device_release_driver+0x38/0x50
[<807f7edc>] bus_remove_device+0xfc/0x130
[<807f4b74>] device_del+0x17c/0x21c
[<807f4c38>] device_unregister+0x24/0x38
[<807b6b50>] unregister_virtio_device+0x28/0x44

Fix this by restructuring the loops to allow the locks to only be taken
where it is necessary to protect the vqs, and release it while the
buffer is being freed.

Fixes: c6017e79 ("virtio: console: add locks around buffer removal in port unplug path")
Cc: stable@vger.kernel.org
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/char/virtio_console.c

@@ -1539,19 +1539,29 @@ static void remove_port_data(struct port *port)
 	spin_lock_irq(&port->inbuf_lock);
 	/* Remove unused data this port might have received. */
 	discard_port_data(port);
+	spin_unlock_irq(&port->inbuf_lock);
 
 	/* Remove buffers we queued up for the Host to send us data in. */
-	while ((buf = virtqueue_detach_unused_buf(port->in_vq)))
-		free_buf(buf, true);
+	do {
+		spin_lock_irq(&port->inbuf_lock);
+		buf = virtqueue_detach_unused_buf(port->in_vq);
 		spin_unlock_irq(&port->inbuf_lock);
+		if (buf)
+			free_buf(buf, true);
+	} while (buf);
 
 	spin_lock_irq(&port->outvq_lock);
 	reclaim_consumed_buffers(port);
+	spin_unlock_irq(&port->outvq_lock);
 
 	/* Free pending buffers from the out-queue. */
-	while ((buf = virtqueue_detach_unused_buf(port->out_vq)))
-		free_buf(buf, true);
+	do {
+		spin_lock_irq(&port->outvq_lock);
+		buf = virtqueue_detach_unused_buf(port->out_vq);
 		spin_unlock_irq(&port->outvq_lock);
+		if (buf)
+			free_buf(buf, true);
+	} while (buf);
 }
 
 /*