Re: [PATCH v8 7/7] drm: remove drm_file.master_lookup_lock

[Daniel Vetter]

On Thu, Aug 26, 2021 at 10:01:22AM +0800, Desmond Cheong Zhi Xi wrote:
> Previously, master_lookup_lock was introduced in
> commit 0b0860a3cf5e ("drm: serialize drm_file.master with a new
> spinlock") to serialize accesses to drm_file.master. This then allowed
> us to write drm_file_get_master in commit 56f0729a510f ("drm: protect
> drm_master pointers in drm_lease.c").
> 
> The rationale behind introducing a new spinlock at the time was that
> the other lock that could have been used (drm_device.master_mutex) was
> the outermost lock, so embedding calls to drm_file_get_master and
> drm_is_current_master in various functions easily caused us to invert
> the lock hierarchy.
> 
> Following the conversion of master_mutex into a rwsem, and its use to
> plug races with modesetting rights, we've untangled some lock
> hierarchies and removed the need for using drm_file_get_master and the
> unlocked version of drm_is_current_master in multiple places.
> 
> Hence, we can take this opportunity to clean up the locking design by
> replacing master_lookup_lock with drm_device.master_rwsem.
> 
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_auth.c     | 19 +++++++------------
>  drivers/gpu/drm/drm_file.c     |  1 -
>  drivers/gpu/drm/drm_internal.h |  1 +
>  drivers/gpu/drm/drm_ioctl.c    |  4 ++--
>  drivers/gpu/drm/drm_lease.c    | 18 ++++++++----------
>  include/drm/drm_file.h         |  9 +--------
>  6 files changed, 19 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index f2b2f197052a..232416119407 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -61,10 +61,9 @@
>   * trusted clients.
>   */
>  
> -static bool drm_is_current_master_locked(struct drm_file *fpriv)
> +bool drm_is_current_master_locked(struct drm_file *fpriv)
>  {
> -	lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) ||
> -			    lockdep_is_held(&fpriv->minor->dev->master_rwsem));
> +	lockdep_assert_held_once(&fpriv->minor->dev->master_rwsem);
>  
>  	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
>  }
> @@ -83,9 +82,9 @@ bool drm_is_current_master(struct drm_file *fpriv)
>  {
>  	bool ret;
>  
> -	spin_lock(&fpriv->master_lookup_lock);
> +	down_read(&fpriv->minor->dev->master_rwsem);

Looking at the 3 patches and the need to have a locked version of pretty
much everything I'm wondering: Can't we just drop the spinlock completely,
and everywhere we've taking it thus far replace it with a
lockdep_assert_held_once?

The thing is, if there's any path left that doesn't hold the rwsem in at
least read mode we have a bug. And the right way to fix such a bug is to
grab the rwsem sufficiently high up in the callchain. That way I think we
should be able to avoid all these tedious changes to everything, including
touching i915 and vmwgfx drivers.

Or am I missing something big time?
-Daniel

>  	ret = drm_is_current_master_locked(fpriv);
> -	spin_unlock(&fpriv->master_lookup_lock);
> +	up_read(&fpriv->minor->dev->master_rwsem);
>  
>  	return ret;
>  }
> @@ -120,7 +119,7 @@ int drm_authmagic(struct drm_device *dev, void *data,
>  	DRM_DEBUG("%u\n", auth->magic);
>  
>  	down_write(&dev->master_rwsem);
> -	if (unlikely(!drm_is_current_master(file_priv))) {
> +	if (unlikely(!drm_is_current_master_locked(file_priv))) {
>  		up_write(&dev->master_rwsem);
>  		return -EACCES;
>  	}
> @@ -178,9 +177,7 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
>  	new_master = drm_master_create(dev);
>  	if (!new_master)
>  		return -ENOMEM;
> -	spin_lock(&fpriv->master_lookup_lock);
>  	fpriv->master = new_master;
> -	spin_unlock(&fpriv->master_lookup_lock);
>  
>  	fpriv->is_master = 1;
>  	fpriv->authenticated = 1;
> @@ -343,9 +340,7 @@ int drm_master_open(struct drm_file *file_priv)
>  	if (!dev->master) {
>  		ret = drm_new_set_master(dev, file_priv);
>  	} else {
> -		spin_lock(&file_priv->master_lookup_lock);
>  		file_priv->master = drm_master_get(dev->master);
> -		spin_unlock(&file_priv->master_lookup_lock);
>  	}
>  	up_write(&dev->master_rwsem);
>  
> @@ -413,13 +408,13 @@ struct drm_master *drm_file_get_master(struct drm_file *file_priv)
>  	if (!file_priv)
>  		return NULL;
>  
> -	spin_lock(&file_priv->master_lookup_lock);
> +	down_read(&file_priv->minor->dev->master_rwsem);
>  	if (!file_priv->master)
>  		goto unlock;
>  	master = drm_master_get(file_priv->master);
>  
>  unlock:
> -	spin_unlock(&file_priv->master_lookup_lock);
> +	up_read(&file_priv->minor->dev->master_rwsem);
>  	return master;
>  }
>  EXPORT_SYMBOL(drm_file_get_master);
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index 90b62f360da1..8c846e0179d7 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -176,7 +176,6 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
>  	init_waitqueue_head(&file->event_wait);
>  	file->event_space = 4096; /* set aside 4k for event buffer */
>  
> -	spin_lock_init(&file->master_lookup_lock);
>  	mutex_init(&file->event_read_lock);
>  
>  	if (drm_core_check_feature(dev, DRIVER_GEM))
> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> index 17f3548c8ed2..5d421f749a17 100644
> --- a/drivers/gpu/drm/drm_internal.h
> +++ b/drivers/gpu/drm/drm_internal.h
> @@ -132,6 +132,7 @@ int drm_crtc_queue_sequence_ioctl(struct drm_device *dev, void *data,
>  				  struct drm_file *filp);
>  
>  /* drm_auth.c */
> +bool drm_is_current_master_locked(struct drm_file *fpriv);
>  int drm_getmagic(struct drm_device *dev, void *data,
>  		 struct drm_file *file_priv);
>  int drm_authmagic(struct drm_device *dev, void *data,
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index 8bea39ffc5c0..c728437466c3 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -386,7 +386,7 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
>  	int if_version, retcode = 0;
>  
>  	down_write(&dev->master_rwsem);
> -	if (unlikely(!drm_is_current_master(file_priv))) {
> +	if (unlikely(!drm_is_current_master_locked(file_priv))) {
>  		retcode = -EACCES;
>  		goto unlock;
>  	}
> @@ -540,7 +540,7 @@ static int drm_ioctl_permit(u32 flags, struct drm_file *file_priv)
>  
>  	/* MASTER is only for master or control clients */
>  	if (unlikely((flags & DRM_MASTER) &&
> -		     !drm_is_current_master(file_priv)))
> +		     !drm_is_current_master_locked(file_priv)))
>  		return -EACCES;
>  
>  	/* Render clients must be explicitly allowed */
> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> index 15bf3a3c76d1..0eecf320b1ab 100644
> --- a/drivers/gpu/drm/drm_lease.c
> +++ b/drivers/gpu/drm/drm_lease.c
> @@ -498,12 +498,12 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  		return PTR_ERR(lessee_file);
>  
>  	down_read(&dev->master_rwsem);
> -	if (unlikely(!drm_is_current_master(lessor_priv))) {
> +	if (unlikely(!drm_is_current_master_locked(lessor_priv))) {
>  		ret = -EACCES;
>  		goto out_file;
>  	}
>  
> -	lessor = drm_file_get_master(lessor_priv);
> +	lessor = lessor_priv->master;
>  	/* Do not allow sub-leases */
>  	if (lessor->lessor) {
>  		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
> @@ -565,7 +565,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	/* Hook up the fd */
>  	fd_install(fd, lessee_file);
>  
> -	drm_master_put(&lessor);
>  	up_read(&dev->master_rwsem);
>  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
>  	return 0;
> @@ -600,7 +599,8 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> -	lessor = drm_file_get_master(lessor_priv);
> +	lockdep_assert_held_once(&dev->master_rwsem);
> +	lessor = lessor_priv->master;
>  	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
> @@ -624,7 +624,6 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  		arg->count_lessees = count;
>  
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> -	drm_master_put(&lessor);
>  
>  	return ret;
>  }
> @@ -650,7 +649,8 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> -	lessee = drm_file_get_master(lessee_priv);
> +	lockdep_assert_held_once(&dev->master_rwsem);
> +	lessee = lessee_priv->master;
>  	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
> @@ -678,7 +678,6 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  		arg->count_objects = count;
>  
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> -	drm_master_put(&lessee);
>  
>  	return ret;
>  }
> @@ -703,11 +702,11 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  		return -EOPNOTSUPP;
>  
>  	down_write(&dev->master_rwsem);
> -	if (unlikely(!drm_is_current_master(lessor_priv))) {
> +	if (unlikely(!drm_is_current_master_locked(lessor_priv))) {
>  		ret = -EACCES;
>  		goto unlock;
>  	}
> -	lessor = drm_file_get_master(lessor_priv);
> +	lessor = lessor_priv->master;
>  	mutex_lock(&dev->mode_config.idr_mutex);
>  
>  	lessee = _drm_find_lessee(lessor, arg->lessee_id);
> @@ -728,7 +727,6 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  
>  fail:
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> -	drm_master_put(&lessor);
>  
>  unlock:
>  	up_write(&dev->master_rwsem);
> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
> index d12bb2ba7814..e2d49fe3e32d 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -227,16 +227,12 @@ struct drm_file {
>  	 * @master:
>  	 *
>  	 * Master this node is currently associated with. Protected by struct
> -	 * &drm_device.master_rwsem, and serialized by @master_lookup_lock.
> +	 * &drm_device.master_rwsem.
>  	 *
>  	 * Only relevant if drm_is_primary_client() returns true. Note that
>  	 * this only matches &drm_device.master if the master is the currently
>  	 * active one.
>  	 *
> -	 * To update @master, both &drm_device.master_rwsem and
> -	 * @master_lookup_lock need to be held, therefore holding either of
> -	 * them is safe and enough for the read side.
> -	 *
>  	 * When dereferencing this pointer, either hold struct
>  	 * &drm_device.master_rwsem for the duration of the pointer's use, or
>  	 * use drm_file_get_master() if struct &drm_device.master_rwsem is not
> @@ -248,9 +244,6 @@ struct drm_file {
>  	 */
>  	struct drm_master *master;
>  
> -	/** @master_lock: Serializes @master. */
> -	spinlock_t master_lookup_lock;
> -
>  	/** @pid: Process that opened this file. */
>  	struct pid *pid;
>  
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch