[PATCH v5 2/8] firmware: arm_scmi: Review virtio free_list handling

[Peter Hilber]

On 17.02.22 14:12, Cristian Marussi wrote:
> Add a new spinlock dedicated to the access of the TX free list and a couple
> of helpers to get and put messages back and forth from the free_list.
> 
> Cc: "Michael S. Tsirkin" <mst at redhat.com>
> Cc: Igor Skalkin <igor.skalkin at opensynergy.com>
> Cc: Peter Hilber <peter.hilber at opensynergy.com>
> Cc: virtualization at lists.linux-foundation.org
> Signed-off-by: Cristian Marussi <cristian.marussi at arm.com>

Acked-by: Peter Hilber <peter.hilber at opensynergy.com>

> ---
>  drivers/firmware/arm_scmi/virtio.c | 88 +++++++++++++++++++-----------
>  1 file changed, 57 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> index ed00b072e981..7ec4085cee7e 100644
> --- a/drivers/firmware/arm_scmi/virtio.c
> +++ b/drivers/firmware/arm_scmi/virtio.c
> @@ -40,20 +40,23 @@
>   *
>   * @vqueue: Associated virtqueue
>   * @cinfo: SCMI Tx or Rx channel
> + * @free_lock: Protects access to the @free_list.
>   * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
>   * @is_rx: Whether channel is an Rx channel
>   * @max_msg: Maximum number of pending messages for this channel.
> - * @lock: Protects access to all members except users.
> + * @lock: Protects access to all members except users, free_list.
>   * @shutdown_done: A reference to a completion used when freeing this channel.
>   * @users: A reference count to currently active users of this channel.
>   */
>  struct scmi_vio_channel {
>  	struct virtqueue *vqueue;
>  	struct scmi_chan_info *cinfo;
> +	/* lock to protect access to the free list. */
> +	spinlock_t free_lock;
>  	struct list_head free_list;
>  	bool is_rx;
>  	unsigned int max_msg;
> -	/* lock to protect access to all members except users. */
> +	/* lock to protect access to all members except users, free_list  */
>  	spinlock_t lock;
>  	struct completion *shutdown_done;
>  	refcount_t users;
> @@ -134,18 +137,49 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
>  	wait_for_completion(vioch->shutdown_done);
>  }
>  
> +/* Assumes to be called with vio channel acquired already */
> +static struct scmi_vio_msg *
> +scmi_virtio_get_free_msg(struct scmi_vio_channel *vioch)
> +{
> +	unsigned long flags;
> +	struct scmi_vio_msg *msg;
> +
> +	spin_lock_irqsave(&vioch->free_lock, flags);
> +	if (list_empty(&vioch->free_list)) {
> +		spin_unlock_irqrestore(&vioch->free_lock, flags);
> +		return NULL;
> +	}
> +
> +	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
> +	list_del_init(&msg->list);
> +	spin_unlock_irqrestore(&vioch->free_lock, flags);
> +
> +	return msg;
> +}
> +
> +/* Assumes to be called with vio channel acquired already */
> +static void scmi_virtio_put_free_msg(struct scmi_vio_channel *vioch,
> +				     struct scmi_vio_msg *msg)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&vioch->free_lock, flags);
> +	list_add_tail(&msg->list, &vioch->free_list);
> +	spin_unlock_irqrestore(&vioch->free_lock, flags);
> +}
> +
>  static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
>  {
>  	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
>  }
>  
>  static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
> -			       struct scmi_vio_msg *msg,
> -			       struct device *dev)
> +			       struct scmi_vio_msg *msg)
>  {
>  	struct scatterlist sg_in;
>  	int rc;
>  	unsigned long flags;
> +	struct device *dev = &vioch->vqueue->vdev->dev;
>  
>  	sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
>  
> @@ -162,17 +196,17 @@ static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
>  	return rc;
>  }
>  
> +/*
> + * Assume to be called with channel already acquired or not ready at all;
> + * vioch->lock MUST NOT have been already acquired.
> + */
>  static void scmi_finalize_message(struct scmi_vio_channel *vioch,
>  				  struct scmi_vio_msg *msg)
>  {
> -	if (vioch->is_rx) {
> -		scmi_vio_feed_vq_rx(vioch, msg, vioch->cinfo->dev);
> -	} else {
> -		/* Here IRQs are assumed to be already disabled by the caller */
> -		spin_lock(&vioch->lock);
> -		list_add(&msg->list, &vioch->free_list);
> -		spin_unlock(&vioch->lock);
> -	}
> +	if (vioch->is_rx)
> +		scmi_vio_feed_vq_rx(vioch, msg);
> +	else
> +		scmi_virtio_put_free_msg(vioch, msg);
>  }
>  
>  static void scmi_vio_complete_cb(struct virtqueue *vqueue)
> @@ -284,7 +318,6 @@ static bool virtio_chan_available(struct device *dev, int idx)
>  static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  			     bool tx)
>  {
> -	unsigned long flags;
>  	struct scmi_vio_channel *vioch;
>  	int index = tx ? VIRTIO_SCMI_VQ_TX : VIRTIO_SCMI_VQ_RX;
>  	int i;
> @@ -314,13 +347,7 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  		if (!msg->input)
>  			return -ENOMEM;
>  
> -		if (tx) {
> -			spin_lock_irqsave(&vioch->lock, flags);
> -			list_add_tail(&msg->list, &vioch->free_list);
> -			spin_unlock_irqrestore(&vioch->lock, flags);
> -		} else {
> -			scmi_vio_feed_vq_rx(vioch, msg, cinfo->dev);
> -		}
> +		scmi_finalize_message(vioch, msg);
>  	}
>  
>  	scmi_vio_channel_ready(vioch, cinfo);
> @@ -354,33 +381,31 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  	if (!scmi_vio_channel_acquire(vioch))
>  		return -EINVAL;
>  
> -	spin_lock_irqsave(&vioch->lock, flags);
> -
> -	if (list_empty(&vioch->free_list)) {
> -		spin_unlock_irqrestore(&vioch->lock, flags);
> +	msg = scmi_virtio_get_free_msg(vioch);
> +	if (!msg) {
>  		scmi_vio_channel_release(vioch);
>  		return -EBUSY;
>  	}
>  
> -	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
> -	list_del(&msg->list);
> -
>  	msg_tx_prepare(msg->request, xfer);
>  
>  	sg_init_one(&sg_out, msg->request, msg_command_size(xfer));
>  	sg_init_one(&sg_in, msg->input, msg_response_size(xfer));
>  
> +	spin_lock_irqsave(&vioch->lock, flags);
> +
>  	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
> -	if (rc) {
> -		list_add(&msg->list, &vioch->free_list);
> +	if (rc)
>  		dev_err(vioch->cinfo->dev,
>  			"failed to add to TX virtqueue (%d)\n", rc);
> -	} else {
> +	else
>  		virtqueue_kick(vioch->vqueue);
> -	}
>  
>  	spin_unlock_irqrestore(&vioch->lock, flags);
>  
> +	if (rc)
> +		scmi_virtio_put_free_msg(vioch, msg);
> +
>  	scmi_vio_channel_release(vioch);
>  
>  	return rc;
> @@ -457,6 +482,7 @@ static int scmi_vio_probe(struct virtio_device *vdev)
>  		unsigned int sz;
>  
>  		spin_lock_init(&channels[i].lock);
> +		spin_lock_init(&channels[i].free_lock);
>  		INIT_LIST_HEAD(&channels[i].free_list);
>  		channels[i].vqueue = vqs[i];
>