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From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Subject: Re: [PATCH v8 7/7] drm: remove drm_file.master_lookup_lock
To:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org,
        christian.koenig@amd.com, jani.nikula@linux.intel.com,
        joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com,
        chris@chris-wilson.co.uk, ville.syrjala@linux.intel.com,
        matthew.auld@intel.com, dan.carpenter@oracle.com,
        tvrtko.ursulin@intel.com, matthew.d.roper@intel.com,
        lucas.demarchi@intel.com, karthik.b.s@intel.com,
        jose.souza@intel.com, manasi.d.navare@intel.com,
        airlied@redhat.com, aditya.swarup@intel.com, andrescj@chromium.org,
        linux-graphics-maintainer@vmware.com, zackr@vmware.com,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Daniel Vetter <daniel@ffwll.ch>
References: <20210826020122.1488002-1-desmondcheongzx@gmail.com>
 <20210826020122.1488002-8-desmondcheongzx@gmail.com>
 <YSeVWivR6p9spRQn@phenom.ffwll.local>
Message-ID: <deadae07-ff1c-538c-9f59-a72ff8367c75@gmail.com>
Date:   Tue, 31 Aug 2021 14:02:39 +0800
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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@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
>>
>