[PATCH 3.16 190/306] virtio: console: Unlock vqs while freeing buffers

[Ben Hutchings]

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Matt Redfearn <matt.redfearn@xxxxxxxxxx>

commit 34563769e438d2881f62cf4d9badc4e589ac0ec0 upstream.

Commit c6017e793b93 ("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: c6017e793b93 ("virtio: console: add locks around buffer removal in port unplug path")
Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/char/virtio_console.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1532,19 +1532,29 @@ static void remove_port_data(struct 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);
-	spin_unlock_irq(&port->inbuf_lock);
+	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);
-	spin_unlock_irq(&port->outvq_lock);
+	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);
 }
 
 /*