[PATCH v8 7/7] drm: remove drm_file.master_lookup_lock
Desmond Cheong Zhi Xi
desmondcheongzx at gmail.com
Tue Aug 31 06:02:39 UTC 2021
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 at gmail.com>
>> ---
>> 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
>>
>
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