Subject: Re: [PATCH net-next v6 1/4] net: vhost: lock the vqs one by one

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 22 Jul 2018 18:26:26 +0300

On Sat, Jul 21, 2018 at 11:03:59AM -0700, [email-masked] wrote:
> From: Tonghao Zhang <[email-masked]>
> 
> This patch changes the way that lock all vqs
> at the same, to lock them one by one. It will
> be used for next patch to avoid the deadlock.
> 
> Signed-off-by: Tonghao Zhang <[email-masked]>
> Acked-by: Jason Wang <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/vhost.c | 24 +++++++-----------------
>  1 file changed, 7 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a502f1a..a1c06e7 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -294,8 +294,11 @@ static void vhost_vq_meta_reset(struct vhost_dev *d)
>  {
>  	int i;
>  
> -	for (i = 0; i < d->nvqs; ++i)
> +	for (i = 0; i < d->nvqs; ++i) {
> +		mutex_lock(&d->vqs[i]->mutex);
>  		__vhost_vq_meta_reset(d->vqs[i]);
> +		mutex_unlock(&d->vqs[i]->mutex);
> +	}
>  }
>  
>  static void vhost_vq_reset(struct vhost_dev *dev,
> @@ -890,20 +893,6 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq,
>  #define vhost_get_used(vq, x, ptr) \
>  	vhost_get_user(vq, x, ptr, VHOST_ADDR_USED)
>  
> -static void vhost_dev_lock_vqs(struct vhost_dev *d)
> -{
> -	int i = 0;
> -	for (i = 0; i < d->nvqs; ++i)
> -		mutex_lock_nested(&d->vqs[i]->mutex, i);
> -}
> -
> -static void vhost_dev_unlock_vqs(struct vhost_dev *d)
> -{
> -	int i = 0;
> -	for (i = 0; i < d->nvqs; ++i)
> -		mutex_unlock(&d->vqs[i]->mutex);
> -}
> -
>  static int vhost_new_umem_range(struct vhost_umem *umem,
>  				u64 start, u64 size, u64 end,
>  				u64 userspace_addr, int perm)
> @@ -953,7 +942,10 @@ static void vhost_iotlb_notify_vq(struct vhost_dev *d,
>  		if (msg->iova <= vq_msg->iova &&
>  		    msg->iova + msg->size - 1 > vq_msg->iova &&
>  		    vq_msg->type == VHOST_IOTLB_MISS) {
> +			mutex_lock(&node->vq->mutex);
>  			vhost_poll_queue(&node->vq->poll);
> +			mutex_unlock(&node->vq->mutex);
> +
>  			list_del(&node->node);
>  			kfree(node);
>  		}
> @@ -985,7 +977,6 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
>  	int ret = 0;
>  
>  	mutex_lock(&dev->mutex);
> -	vhost_dev_lock_vqs(dev);
>  	switch (msg->type) {
>  	case VHOST_IOTLB_UPDATE:
>  		if (!dev->iotlb) {
> @@ -1019,7 +1010,6 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
>  		break;
>  	}
>  
> -	vhost_dev_unlock_vqs(dev);
>  	mutex_unlock(&dev->mutex);
>  
>  	return ret;

I do prefer the finer-grained locking but I remember we
discussed something like this in the past and Jason saw issues
with such a locking.

Jason?

> -- 
> 1.8.3.1