Re: [PATCH] vfio: Remove unnecessary uses of vfio_container.group_lock

[Kirti Wankhede]

Sounds reasonable to me.

Thanks,
Kirti

On 7/8/2017 3:45 AM, Alex Williamson wrote:
> The original intent of vfio_container.group_lock is to protect
> vfio_container.group_list, however over time it's become a crutch to
> prevent changes in container composition any time we call into the
> iommu driver backend.  This introduces problems when we start to have
> more complex interactions, for example when a user's DMA unmap request
> triggers a notification to an mdev vendor driver, who responds by
> attempting to unpin mappings within that request, re-entering the
> iommu backend.  We incorrectly assume that the use of read-locks here
> allow for this nested locking behavior, but a poorly timed write-lock
> could in fact trigger a deadlock.
> 
> The current use of group_lock seems to fall into the trap of locking
> code, not data.  Correct that by removing uses of group_lock that are
> not directly related to group_list.  Note that the vfio type1 iommu
> backend has its own mutex, vfio_iommu.lock, which it uses to protect
> itself for each of these interfaces anyway.  The group_lock appears to
> be a redundancy for these interfaces and type1 even goes so far as to
> release its mutex to allow for exactly the re-entrant code path above.
> 
> Reported-by: Chuanxiao Dong <chuanxiao.dong@xxxxxxxxx>
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
> 
> Alexey, does the SPAPR/TCE iommu backend have any dependencies on this
> lock?  If so, let's create a lock in the spapr_tce backend like we
> have in type1 to handle it.  I believe the ioctl passthrough is the
> only interface that can reach spapr_tce.  Thanks,
> 
> Alex
> 
>  drivers/vfio/vfio.c |   38 --------------------------------------
>  1 file changed, 38 deletions(-)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 7597a377eb4e..330d50582f40 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -1175,15 +1175,11 @@ static long vfio_fops_unl_ioctl(struct file *filep,
>  		ret = vfio_ioctl_set_iommu(container, arg);
>  		break;
>  	default:
> -		down_read(&container->group_lock);
> -
>  		driver = container->iommu_driver;
>  		data = container->iommu_data;
>  
>  		if (driver) /* passthrough all unrecognized ioctls */
>  			ret = driver->ops->ioctl(data, cmd, arg);
> -
> -		up_read(&container->group_lock);
>  	}
>  
>  	return ret;
> @@ -1237,15 +1233,11 @@ static ssize_t vfio_fops_read(struct file *filep, char __user *buf,
>  	struct vfio_iommu_driver *driver;
>  	ssize_t ret = -EINVAL;
>  
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->read))
>  		ret = driver->ops->read(container->iommu_data,
>  					buf, count, ppos);
>  
> -	up_read(&container->group_lock);
> -
>  	return ret;
>  }
>  
> @@ -1256,15 +1248,11 @@ static ssize_t vfio_fops_write(struct file *filep, const char __user *buf,
>  	struct vfio_iommu_driver *driver;
>  	ssize_t ret = -EINVAL;
>  
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->write))
>  		ret = driver->ops->write(container->iommu_data,
>  					 buf, count, ppos);
>  
> -	up_read(&container->group_lock);
> -
>  	return ret;
>  }
>  
> @@ -1274,14 +1262,10 @@ static int vfio_fops_mmap(struct file *filep, struct vm_area_struct *vma)
>  	struct vfio_iommu_driver *driver;
>  	int ret = -EINVAL;
>  
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->mmap))
>  		ret = driver->ops->mmap(container->iommu_data, vma);
>  
> -	up_read(&container->group_lock);
> -
>  	return ret;
>  }
>  
> @@ -1993,8 +1977,6 @@ int vfio_pin_pages(struct device *dev, unsigned long *user_pfn, int npage,
>  		goto err_pin_pages;
>  
>  	container = group->container;
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->pin_pages))
>  		ret = driver->ops->pin_pages(container->iommu_data, user_pfn,
> @@ -2002,7 +1984,6 @@ int vfio_pin_pages(struct device *dev, unsigned long *user_pfn, int npage,
>  	else
>  		ret = -ENOTTY;
>  
> -	up_read(&container->group_lock);
>  	vfio_group_try_dissolve_container(group);
>  
>  err_pin_pages:
> @@ -2042,8 +2023,6 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
>  		goto err_unpin_pages;
>  
>  	container = group->container;
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->unpin_pages))
>  		ret = driver->ops->unpin_pages(container->iommu_data, user_pfn,
> @@ -2051,7 +2030,6 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
>  	else
>  		ret = -ENOTTY;
>  
> -	up_read(&container->group_lock);
>  	vfio_group_try_dissolve_container(group);
>  
>  err_unpin_pages:
> @@ -2073,8 +2051,6 @@ static int vfio_register_iommu_notifier(struct vfio_group *group,
>  		return -EINVAL;
>  
>  	container = group->container;
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->register_notifier))
>  		ret = driver->ops->register_notifier(container->iommu_data,
> @@ -2082,7 +2058,6 @@ static int vfio_register_iommu_notifier(struct vfio_group *group,
>  	else
>  		ret = -ENOTTY;
>  
> -	up_read(&container->group_lock);
>  	vfio_group_try_dissolve_container(group);
>  
>  	return ret;
> @@ -2100,8 +2075,6 @@ static int vfio_unregister_iommu_notifier(struct vfio_group *group,
>  		return -EINVAL;
>  
>  	container = group->container;
> -	down_read(&container->group_lock);
> -
>  	driver = container->iommu_driver;
>  	if (likely(driver && driver->ops->unregister_notifier))
>  		ret = driver->ops->unregister_notifier(container->iommu_data,
> @@ -2109,7 +2082,6 @@ static int vfio_unregister_iommu_notifier(struct vfio_group *group,
>  	else
>  		ret = -ENOTTY;
>  
> -	up_read(&container->group_lock);
>  	vfio_group_try_dissolve_container(group);
>  
>  	return ret;
> @@ -2127,7 +2099,6 @@ static int vfio_register_group_notifier(struct vfio_group *group,
>  					unsigned long *events,
>  					struct notifier_block *nb)
>  {
> -	struct vfio_container *container;
>  	int ret;
>  	bool set_kvm = false;
>  
> @@ -2145,9 +2116,6 @@ static int vfio_register_group_notifier(struct vfio_group *group,
>  	if (ret)
>  		return -EINVAL;
>  
> -	container = group->container;
> -	down_read(&container->group_lock);
> -
>  	ret = blocking_notifier_chain_register(&group->notifier, nb);
>  
>  	/*
> @@ -2158,7 +2126,6 @@ static int vfio_register_group_notifier(struct vfio_group *group,
>  		blocking_notifier_call_chain(&group->notifier,
>  					VFIO_GROUP_NOTIFY_SET_KVM, group->kvm);
>  
> -	up_read(&container->group_lock);
>  	vfio_group_try_dissolve_container(group);
>  
>  	return ret;
> @@ -2167,19 +2134,14 @@ static int vfio_register_group_notifier(struct vfio_group *group,
>  static int vfio_unregister_group_notifier(struct vfio_group *group,
>  					 struct notifier_block *nb)
>  {
> -	struct vfio_container *container;
>  	int ret;
>  
>  	ret = vfio_group_add_container_user(group);
>  	if (ret)
>  		return -EINVAL;
>  
> -	container = group->container;
> -	down_read(&container->group_lock);
> -
>  	ret = blocking_notifier_chain_unregister(&group->notifier, nb);
>  
> -	up_read(&container->group_lock);
>  	vfio_group_try_dissolve_container(group);
>  
>  	return ret;
>