[PATCH 2/2] drm/i915/gvt: Fix inconsistent locks holding sequence

[Zhenyu Wang]

On 2017.06.19 17:04:11 +0800, Chuanxiao Dong wrote:
> There are two kinds of locking sequence.
> 
> One is in the thread which is started by vfio ioctl to do
> the iommu unmapping. The locking sequence is:
> 	down_read(&group_lock) ----> mutex_lock(&cached_lock)
> 
> The other is in the vfio release thread which will unpin all
> the cached pages. The lock sequence is:
> 	mutex_lock(&cached_lock) ---> down_read(&group_lock)
> 
> And, the cache_lock is used to protect the rb tree of the cache
> node and doing vfio unpin doesn't require this lock. Move the
> vfio unpin out of the cache_lock protected region.
> 
> Fixes: f30437c5e7bf ("drm/i915/gvt: add KVMGT support")
> Signed-off-by: Chuanxiao Dong <chuanxiao.dong at intel.com>
> ---
>  drivers/gpu/drm/i915/gvt/kvmgt.c | 25 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
> index 88a4d8f..1405c60 100644
> --- a/drivers/gpu/drm/i915/gvt/kvmgt.c
> +++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
> @@ -192,13 +192,6 @@ static void gvt_cache_add(struct intel_vgpu *vgpu, gfn_t gfn,
>  	kfree(new);
>  }
>  
> -static void __gvt_cache_remove_entry(struct intel_vgpu *vgpu,
> -				struct gvt_dma *entry)
> -{
> -	rb_erase(&entry->node, &vgpu->vdev.cache);
> -	kfree(entry);
> -}
> -
>  static void intel_vgpu_unpin_work(struct work_struct *work)
>  {
>  	struct intel_vgpu *vgpu = container_of(work, struct intel_vgpu,
> @@ -260,16 +253,22 @@ static void gvt_cache_destroy(struct intel_vgpu *vgpu)
>  	struct device *dev = mdev_dev(vgpu->vdev.mdev);
>  	unsigned long gfn;
>  
> -	mutex_lock(&vgpu->vdev.cache_lock);
> -	while ((node = rb_first(&vgpu->vdev.cache))) {
> +	do {
> +		mutex_lock(&vgpu->vdev.cache_lock);
> +		node = rb_first(&vgpu->vdev.cache);
> +		if (!node) {
> +			mutex_unlock(&vgpu->vdev.cache_lock);
> +			break;
> +		}
>  		dma = rb_entry(node, struct gvt_dma, node);
>  		gvt_dma_unmap_iova(vgpu, dma->iova);
> -		gfn = dma->gfn;
> +		rb_erase(&dma->node, &vgpu->vdev.cache);
> +		mutex_unlock(&vgpu->vdev.cache_lock);
>  
> +		gfn = dma->gfn;
>  		vfio_unpin_pages(dev, &gfn, 1);
> -		__gvt_cache_remove_entry(vgpu, dma);
> -	}
> -	mutex_unlock(&vgpu->vdev.cache_lock);
> +		kfree(dma);
> +	} while (1);
>  }

For me, I might like to change as

while ((node = rb_first(...))) {
      mutex_lock(&cache_lock);
      dma = rb_entry(node, struct gvt_dma, node);
      gvt_dma_unmap_iova(vgpu, dma->iova);
      gfn = dma->gfn;
      __gvt_cache_remove_entry(vgpu, dma);
      mutex_unlock(&vgpu->vdev.cache_lock);
      vfio_unpin_pages(dev, &gfn, 1);
}


-- 
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