Re: [PATCH v8 2/7] drm: convert drm_device.master_mutex into a rwsem

On Thu, Aug 26, 2021 at 10:01:17AM +0800, Desmond Cheong Zhi Xi wrote:

drm_device.master_mutex currently protects the following:

• drm_device.master
• drm_file.master
• drm_file.was_master
• drm_file.is_master
• drm_master.unique
• drm_master.unique_len
• drm_master.magic_map

There is a clear separation between functions that read or change these attributes. Hence, convert master_mutex into a rwsem to enable concurrent readers.

Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com

Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch

---

drivers/gpu/drm/drm_auth.c | 35 ++++++++++++++++++----------------- drivers/gpu/drm/drm_debugfs.c | 4 ++-- drivers/gpu/drm/drm_drv.c | 3 +-- drivers/gpu/drm/drm_ioctl.c | 10 +++++----- include/drm/drm_auth.h | 6 +++--- include/drm/drm_device.h | 10 ++++++---- include/drm/drm_file.h | 12 ++++++------ 7 files changed, 41 insertions(+), 39 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index 60a6b21474b1..73ade0513ccb 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -64,7 +64,7 @@ static 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_mutex));
  

• 	    lockdep_is_held(&fpriv->minor->dev->master_rwsem));
  
  

  return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;

} @@ -96,7 +96,7 @@ int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv) struct drm_auth *auth = data; int ret = 0;

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem); if (!file_priv->magic) { ret = idr_alloc(&file_priv->master->magic_map, file_priv, 1, 0, GFP_KERNEL);

@@ -104,7 +104,7 @@ int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv) file_priv->magic = ret; } auth->magic = file_priv->magic;

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem);

  DRM_DEBUG("%u\n", auth->magic);

@@ -119,13 +119,13 @@ int drm_authmagic(struct drm_device *dev, void *data,

DRM_DEBUG("%u\n", auth->magic);

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem); file = idr_find(&file_priv->master->magic_map, auth->magic); if (file) { file->authenticated = 1; idr_replace(&file_priv->master->magic_map, NULL, auth->magic); }

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem);

  return file ? 0 : -EINVAL;

} @@ -167,7 +167,7 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv) struct drm_master *old_master; struct drm_master *new_master;

• lockdep_assert_held_once(&dev->master_mutex);

• lockdep_assert_held_once(&dev->master_rwsem);

  WARN_ON(fpriv->is_master); old_master = fpriv->master;

@@ -249,7 +249,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data, { int ret;

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem);

  ret = drm_master_check_perm(dev, file_priv); if (ret)

@@ -281,7 +281,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,

drm_set_master(dev, file_priv, false); out_unlock:

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem); return ret;

}

@@ -298,7 +298,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data, { int ret;

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem);

  ret = drm_master_check_perm(dev, file_priv); if (ret)

@@ -321,8 +321,9 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data, }

drm_drop_master(dev, file_priv);

out_unlock:

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem); return ret;

}

@@ -334,7 +335,7 @@ int drm_master_open(struct drm_file *file_priv) /* if there is no current master make this fd it, but do not create * any master object for render clients */

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem); if (!dev->master) { ret = drm_new_set_master(dev, file_priv); } else {

@@ -342,7 +343,7 @@ int drm_master_open(struct drm_file *file_priv) file_priv->master = drm_master_get(dev->master); spin_unlock(&file_priv->master_lookup_lock); }

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem);

  return ret;

} @@ -352,7 +353,7 @@ void drm_master_release(struct drm_file *file_priv) struct drm_device *dev = file_priv->minor->dev; struct drm_master *master;

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem); master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);

@@ -375,7 +376,7 @@ void drm_master_release(struct drm_file *file_priv) /* drop the master reference held by the file priv */ if (file_priv->master) drm_master_put(&file_priv->master);

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem);

}

/** @@ -450,9 +451,9 @@ EXPORT_SYMBOL(drm_master_put); /* Used by drm_client and drm_fb_helper */ bool drm_master_internal_acquire(struct drm_device *dev) {

• mutex_lock(&dev->master_mutex);

• down_read(&dev->master_rwsem); if (dev->master) {

• mutex_unlock(&dev->master_mutex);
  

• up_read(&dev->master_rwsem);
  

  return false; }

@@ -463,6 +464,6 @@ EXPORT_SYMBOL(drm_master_internal_acquire); /* Used by drm_client and drm_fb_helper */ void drm_master_internal_release(struct drm_device *dev) {

• mutex_unlock(&dev->master_mutex);

• up_read(&dev->master_rwsem);

} EXPORT_SYMBOL(drm_master_internal_release); diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c index b0a826489488..b34c9c263188 100644 --- a/drivers/gpu/drm/drm_debugfs.c +++ b/drivers/gpu/drm/drm_debugfs.c @@ -55,7 +55,7 @@ static int drm_name_info(struct seq_file *m, void *data) struct drm_device *dev = minor->dev; struct drm_master *master;

• mutex_lock(&dev->master_mutex);

• down_read(&dev->master_rwsem); master = dev->master; seq_printf(m, "%s", dev->driver->name); if (dev->dev)

@@ -65,7 +65,7 @@ static int drm_name_info(struct seq_file *m, void *data) if (dev->unique) seq_printf(m, " unique=%s", dev->unique); seq_printf(m, "\n");

• mutex_unlock(&dev->master_mutex);

• up_read(&dev->master_rwsem);

  return 0;

} diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c index 7a5097467ba5..4556bf42954c 100644 --- a/drivers/gpu/drm/drm_drv.c +++ b/drivers/gpu/drm/drm_drv.c @@ -570,7 +570,6 @@ static void drm_dev_init_release(struct drm_device *dev, void *res) /* Prevent use-after-free in drm_managed_release when debugging is * enabled. Slightly awkward, but can't really be helped. */ dev->dev = NULL;

• mutex_destroy(&dev->master_mutex); mutex_destroy(&dev->clientlist_mutex); mutex_destroy(&dev->filelist_mutex); mutex_destroy(&dev->struct_mutex);

@@ -611,7 +610,7 @@ static int drm_dev_init(struct drm_device *dev, mutex_init(&dev->struct_mutex); mutex_init(&dev->filelist_mutex); mutex_init(&dev->clientlist_mutex);

• mutex_init(&dev->master_mutex);

• init_rwsem(&dev->master_rwsem);

  ret = drmm_add_action(dev, drm_dev_init_release, NULL); if (ret)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 26f3a9ede8fe..d25713b09b80 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -119,16 +119,16 @@ int drm_getunique(struct drm_device *dev, void *data, struct drm_unique *u = data; struct drm_master *master;

• mutex_lock(&dev->master_mutex);

• down_read(&dev->master_rwsem); master = file_priv->master; if (u->unique_len >= master->unique_len) { if (copy_to_user(u->unique, master->unique, master->unique_len)) {

• 	mutex_unlock(&dev->master_mutex);
  

• 	up_read(&dev->master_rwsem);
  return -EFAULT;
  

  } } u->unique_len = master->unique_len;

• mutex_unlock(&dev->master_mutex);

• up_read(&dev->master_rwsem);

  return 0;

} @@ -385,7 +385,7 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f struct drm_set_version *sv = data; int if_version, retcode = 0;

• mutex_lock(&dev->master_mutex);

• down_write(&dev->master_rwsem); if (sv->drm_di_major != -1) { if (sv->drm_di_major != DRM_IF_MAJOR || sv->drm_di_minor < 0 || sv->drm_di_minor > DRM_IF_MINOR) {

@@ -420,7 +420,7 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f sv->drm_di_minor = DRM_IF_MINOR; sv->drm_dd_major = dev->driver->major; sv->drm_dd_minor = dev->driver->minor;

• mutex_unlock(&dev->master_mutex);

• up_write(&dev->master_rwsem);

  return retcode;

} diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h index ba248ca8866f..f0a89e5fcaad 100644 --- a/include/drm/drm_auth.h +++ b/include/drm/drm_auth.h @@ -67,17 +67,17 @@ struct drm_master { struct drm_device *dev; /** * @unique: Unique identifier: e.g. busid. Protected by

• * &drm_device.master_mutex.
  

• * &drm_device.master_rwsem.
  

  */ char *unique; /**
  • @unique_len: Length of unique field. Protected by

• * &drm_device.master_mutex.
  

• * &drm_device.master_rwsem.
  

  */ int unique_len; /**
  • @magic_map: Map of used authentication tokens. Protected by

• * &drm_device.master_mutex.
  

• * &drm_device.master_rwsem.
  

  */ struct idr magic_map; void *driver_priv;

diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h index 604b1d1b2d72..142fb2f6e74d 100644 --- a/include/drm/drm_device.h +++ b/include/drm/drm_device.h @@ -107,7 +107,7 @@ struct drm_device { * @master: * * Currently active master for this device.

• * Protected by &master_mutex
  

• * Protected by &master_rwsem
  

  */ struct drm_master *master;

@@ -146,11 +146,13 @@ struct drm_device { struct mutex struct_mutex;

/**

• * @master_mutex:
  

• * @master_rwsem:
  

• * Lock for &drm_minor.master and &drm_file.is_master
  

• * Lock for &drm_device.master, &drm_file.was_master,
  

• * &drm_file.is_master, &drm_file.master, &drm_master.unique,
  

• * &drm_master.unique_len, and &drm_master.magic_map.
  

  */

• struct mutex master_mutex;

• struct rw_semaphore master_rwsem;

  /**

  • @open_count:

diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h index a3acb7ac3550..d12bb2ba7814 100644 --- a/include/drm/drm_file.h +++ b/include/drm/drm_file.h @@ -205,7 +205,7 @@ struct drm_file { * @was_master: * * This client has or had, master capability. Protected by struct

• * &drm_device.master_mutex.
  

• * &drm_device.master_rwsem.
  

  • This is used to ensure that CAP_SYS_ADMIN is not enforced, if the
  • client is or was master in the past.

@@ -216,7 +216,7 @@ struct drm_file { * @is_master: * * This client is the creator of @master. Protected by struct

• * &drm_device.master_mutex.
  

• * &drm_device.master_rwsem.
  

  • See also the :ref:`section on primary nodes and authentication
  • <drm_primary_node>`.

@@ -227,19 +227,19 @@ struct drm_file { * @master: * * Master this node is currently associated with. Protected by struct

• * &drm_device.master_mutex, and serialized by @master_lookup_lock.
  

• * &drm_device.master_rwsem, and serialized by @master_lookup_lock.
  

  • 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_mutex and
  

• * 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_mutex for the duration of the pointer's use, or
  

• * use drm_file_get_master() if struct &drm_device.master_mutex is not
  

• * &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
  

  • currently held and there is no other need to hold it. This prevents
  • @master from being freed during use.

-- 2.25.1