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    Subject[PATCH v8 7/7] drm: remove drm_file.master_lookup_lock
    Date
    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);
    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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