On Wed, Aug 18, 2021 at 03:38:20PM +0800, Desmond Cheong Zhi Xi wrote:
> Currently, drm_device.master_mutex is used to serialize writes to the
> drm_master.magic_map idr and to protect drm_master.unique{_len}.
>
> In preparation for converting drm_device.master_mutex into an outer
> rwsem that might be read locked before entering some of these
> functions, we can instead serialize access to drm_master.magic_map and
> drm_master.unique{_len} using drm_device.master_lookup_lock which is
> an inner lock.
>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheon...@gmail.com>
> ---
> drivers/gpu/drm/drm_auth.c | 12 +++++++-----
> drivers/gpu/drm/drm_ioctl.c | 10 ++++++----
> include/drm/drm_auth.h | 6 +++---
> include/drm/drm_device.h | 7 ++++++-
> 4 files changed, 22 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index b7230604496b..0acb444fbbac 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -98,10 +98,10 @@ int drm_getmagic(struct drm_device *dev, void *data,
> struct drm_file *file_priv)
> struct drm_master *master;
> int ret = 0;
>
> - mutex_lock(&dev->master_mutex);
> + spin_lock(&dev->master_lookup_lock);
> master = file_priv->master;
> if (!master) {
> - mutex_unlock(&dev->master_mutex);
> + spin_unlock(&dev->master_lookup_lock);
> return -EINVAL;
> }
>
> @@ -112,7 +112,7 @@ 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);
> + spin_unlock(&dev->master_lookup_lock);
>
> DRM_DEBUG("%u\n", auth->magic);
>
> @@ -127,13 +127,13 @@ int drm_authmagic(struct drm_device *dev, void *data,
>
> DRM_DEBUG("%u\n", auth->magic);
>
> - mutex_lock(&dev->master_mutex);
> + spin_lock(&dev->master_lookup_lock);
> 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);
> + spin_unlock(&dev->master_lookup_lock);
>
> return file ? 0 : -EINVAL;
> }
> @@ -366,8 +366,10 @@ void drm_master_release(struct drm_file *file_priv)
> if (!master)
> goto unlock;
>
> + spin_lock(&dev->master_lookup_lock);
> if (file_priv->magic)
> idr_remove(&master->magic_map, file_priv->magic);
> + spin_unlock(&dev->master_lookup_lock);
>
> if (!drm_is_current_master_locked(file_priv))
> goto out;
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index 4d029d3061d9..e5c3845b6e62 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -119,21 +119,21 @@ int drm_getunique(struct drm_device *dev, void *data,
> struct drm_unique *u = data;
> struct drm_master *master;
>
> - mutex_lock(&dev->master_mutex);
> + spin_lock(&dev->master_lookup_lock);
> master = file_priv->master;
> if (!master) {
> - mutex_unlock(&dev->master_mutex);
> + spin_unlock(&dev->master_lookup_lock);
> return -EINVAL;
> }
>
> if (u->unique_len >= master->unique_len) {
> if (copy_to_user(u->unique, master->unique,
> master->unique_len)) {
> - mutex_unlock(&dev->master_mutex);
> + spin_unlock(&dev->master_lookup_lock);
copy_to_user while holding a spinlock isn't going to work well, at least
when we take a fault. The might_fault() annotations (if enabled) should
catch that.
Which is really annoying, because it kinda puts a wrench into this neat
plan here :-/
> return -EFAULT;
> }
> }
> u->unique_len = master->unique_len;
> - mutex_unlock(&dev->master_mutex);
> + spin_unlock(&dev->master_lookup_lock);
>
> return 0;
> }
> @@ -405,7 +405,9 @@ static int drm_setversion(struct drm_device *dev, void
> *data, struct drm_file *f
> * Version 1.1 includes tying of DRM to specific device
> * Version 1.4 has proper PCI domain support
> */
> + spin_lock(&dev->master_lookup_lock);
> retcode = drm_set_busid(dev, file_priv);
> + spin_unlock(&dev->master_lookup_lock);
Similar issue with drm_set_busid, calling kmalloc under a spinlock isn't a
good idea. This one here is at least much easier to fix by pushing the
locking down a lot.
I'm wondering a bit whether a better fix for these ioctls wouldn't be to
- drop the DRM_MASTER flag from the ioctl table
- take the rwsem in write mode (which would replace our current
dev->master_lock) and check for master status while holding that lock
I think that would result in simpler locking or am I missing something?
Maybe this could even work as a replacment for the lookup spinlock, since
we're untangling the nesting quite a bit?
Really just tossing ideas around since I feel like we don't have the best
one yet ...
-Daniel
> if (retcode)
> goto done;
> }
> diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h
> index ba248ca8866f..f5be73153798 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_lookup_lock.
> */
> char *unique;
> /**
> * @unique_len: Length of unique field. Protected by
> - * &drm_device.master_mutex.
> + * &drm_device.master_lookup_lock.
> */
> int unique_len;
> /**
> * @magic_map: Map of used authentication tokens. Protected by
> - * &drm_device.master_mutex.
> + * &drm_device.master_lookup_lock.
> */
> struct idr magic_map;
> void *driver_priv;
> diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
> index 506eb2784819..cf5d15aeb25f 100644
> --- a/include/drm/drm_device.h
> +++ b/include/drm/drm_device.h
> @@ -152,7 +152,12 @@ struct drm_device {
> */
> struct mutex master_mutex;
>
> - /** @master_lookup_lock: Serializes &drm_file.master. */
> + /**
> + * @master_lookup_lock:
> + *
> + * Serializes &drm_file.master, &drm_master.magic_map,
> + * &drm_master.unique, and &drm_master.unique_len.
> + */
> spinlock_t master_lookup_lock;
>
> /**
> --
> 2.25.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
