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

[Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>]

On 26/8/21 9:21 pm, Daniel Vetter wrote:
> 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

Thanks for taking a look at all the patches and for the suggestions, Daniel.

Just my two cents. I think it makes sense to replace the lock with the 
lockdep assertion. This avoids the weirdness with the lock being taken 
both as an outer lock and sometimes as a deeply embedded inner lock.

But we'll probably have to fix some stuff because I don't think we 
always hold the rwsem in the places where the spinlock is grabbed (i.e. 
when drm_is_current_master or drm_file_get_master is called).

I'll split the series as suggested so we can test things up to PATCH 4 
("drm: avoid races with modesetting rights"). For the rest of the series 
to remove the spinlock, I'll take a closer look and probably send out a 
patch later this week.

Best wishes,
Desmond

> >   	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