Re: [PATCH] vduse: Fix msg list race in vduse_dev_read_iter

["Michael S. Tsirkin" <mst@redhat.com>]

Thanks for the patch! yet something to improve:

On Fri, Jan 30, 2026 at 04:15:24PM +0800, Zhang Tianci wrote:
> Move the message to recv_list before dropping msg_lock and copying the
> request to userspace, avoiding a transient unlinked state that can race
> with the msg_sync timeout path. Roll back to send_list on copy failures.

this is not how you write commit messages, though.
describe the problem then how you fix it, please.
something like:

if msg_sync timeout triggers after a message has been removed
from send_list and before it was added to
recv_list, then .... as a result ....

To fix, move the message ...

> 
> Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> Reviewed-by: Xie Yongji <xieyongji@bytedance.com>
> ---
>  drivers/vdpa/vdpa_user/vduse_dev.c | 30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> index ae357d014564c..b6a558341c06c 100644
> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> @@ -325,6 +325,7 @@ static ssize_t vduse_dev_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  	struct file *file = iocb->ki_filp;
>  	struct vduse_dev *dev = file->private_data;
>  	struct vduse_dev_msg *msg;
> +	struct vduse_dev_request req;
>  	int size = sizeof(struct vduse_dev_request);
>  	ssize_t ret;
>  
> @@ -339,7 +340,7 @@ static ssize_t vduse_dev_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  
>  		ret = -EAGAIN;
>  		if (file->f_flags & O_NONBLOCK)
> -			goto unlock;
> +			break;
>  
>  		spin_unlock(&dev->msg_lock);
>  		ret = wait_event_interruptible_exclusive(dev->waitq,
> @@ -349,17 +350,30 @@ static ssize_t vduse_dev_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  
>  		spin_lock(&dev->msg_lock);
>  	}
> +	if (!msg) {
> +		spin_unlock(&dev->msg_lock);
> +		return ret;
> +	}
> +
> +	memcpy(&req, &msg->req, sizeof(req));
> +	/*
> +	 * Move @msg to recv_list before dropping msg_lock.
> +	 * This avoids a window where @msg is detached from any list and
> +	 * vduse_dev_msg_sync() timeout path may operate on an unlinked node.
> +	 */

when standing by itself, not as part of the patch, this
comment confuses more than it clarifies.

> +	vduse_enqueue_msg(&dev->recv_list, msg);
>  	spin_unlock(&dev->msg_lock);
> -	ret = copy_to_iter(&msg->req, size, to);
> -	spin_lock(&dev->msg_lock);
> +
> +	ret = copy_to_iter(&req, size, to);
>  	if (ret != size) {
> +		spin_lock(&dev->msg_lock);
> +		/* Roll back: move msg back to send_list if still pending. */
> +		msg = vduse_find_msg(&dev->recv_list, req.request_id);

Looks like this always scans the whole list.
Make a variant using list_for_each_entry_reverse maybe?

> +		if (msg)
> +			vduse_enqueue_msg(&dev->send_list, msg);

why is it not a concern that it will be at the tail of the send_list now,
reordering the messages?

> +		spin_unlock(&dev->msg_lock);
>  		ret = -EFAULT;
> -		vduse_enqueue_msg(&dev->send_list, msg);
> -		goto unlock;
>  	}
> -	vduse_enqueue_msg(&dev->recv_list, msg);
> -unlock:
> -	spin_unlock(&dev->msg_lock);
>  
>  	return ret;
>  }
> -- 
> 2.39.5