Hi Daniel.
In the nit-pick department today - sorry.
Subject: [PATCH 5/5] drm: Nerv drm_global_mutex BKL for good drivers
I did not understand this subject... - what is "Nerv"?
Sam
On Wed, Jan 29, 2020 at 09:24:10AM +0100, Daniel Vetter wrote:
> This catches the majority of drivers (unfortunately not if we take
> users into account, because all the big drivers have at least a
> lastclose hook).
>
> With the prep patches out of the way all drm state is fully protected
> and either prevents or can deal with the races from dropping the BKL
> around open/close. The only thing left to audit are the various driver
> hooks - by keeping the BKL around if any of them are set we have a
> very simple cop-out!
>
> Note that one of the biggest prep pieces to get here was making
> dev->open_count atomic, which was done in
>
> commit 7e13ad896484a0165a68197a2e64091ea28c9602
> Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Date: Fri Jan 24 13:01:07 2020 +0000
>
> drm: Avoid drm_global_mutex for simple inc/dec of dev->open_count
>
> Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
> drivers/gpu/drm/drm_drv.c | 6 +++--
> drivers/gpu/drm/drm_file.c | 46 ++++++++++++++++++++++++++++++----
> drivers/gpu/drm/drm_internal.h | 1 +
> 3 files changed, 46 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 05bdf0b9d2b3..9fcd6ab3c154 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -946,7 +946,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
> struct drm_driver *driver = dev->driver;
> int ret;
>
> - mutex_lock(&drm_global_mutex);
> + if (drm_dev_needs_global_mutex(dev))
> + mutex_lock(&drm_global_mutex);
>
> if (dev->driver->load) {
> if (!drm_core_check_feature(dev, DRIVER_LEGACY))
> @@ -992,7 +993,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
> drm_minor_unregister(dev, DRM_MINOR_PRIMARY);
> drm_minor_unregister(dev, DRM_MINOR_RENDER);
> out_unlock:
> - mutex_unlock(&drm_global_mutex);
> + if (drm_dev_needs_global_mutex(dev))
> + mutex_unlock(&drm_global_mutex);
> return ret;
> }
> EXPORT_SYMBOL(drm_dev_register);
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index d36cb74ebe0c..efd6fe0b6b4f 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -51,6 +51,37 @@
> /* from BKL pushdown */
> DEFINE_MUTEX(drm_global_mutex);
>
> +bool drm_dev_needs_global_mutex(struct drm_device *dev)
> +{
> + /*
> + * Legacy drivers rely on all kinds of BKL locking semantics, don't
> + * bother. They also still need BKL locking for their ioctls, so better
> + * safe than sorry.
> + */
> + if (drm_core_check_feature(dev, DRIVER_LEGACY))
> + return true;
> +
> + /*
> + * The deprecated ->load callback must be called after the driver is
> + * already registered. This means such drivers rely on the BKL to make
> + * sure an open can't proceed until the driver is actually fully set up.
> + * Similar hilarity holds for the unload callback.
> + */
> + if (dev->driver->load || dev->driver->unload)
> + return true;
> +
> + /*
> + * Drivers with the lastclose callback assume that it's synchronized
> + * against concurrent opens, which again needs the BKL. The proper fix
> + * is to use the drm_client infrastructure with proper locking for each
> + * client.
> + */
> + if (dev->driver->lastclose)
> + return true;
> +
> + return false;
> +}
> +
> /**
> * DOC: file operations
> *
> @@ -378,9 +409,10 @@ int drm_open(struct inode *inode, struct file *filp)
> if (IS_ERR(minor))
> return PTR_ERR(minor);
>
> - mutex_unlock(&drm_global_mutex);
> -
> dev = minor->dev;
> + if (drm_dev_needs_global_mutex(dev))
> + mutex_unlock(&drm_global_mutex);
> +
> if (!atomic_fetch_inc(&dev->open_count))
> need_setup = 1;
>
> @@ -398,13 +430,15 @@ int drm_open(struct inode *inode, struct file *filp)
> }
> }
>
> - mutex_unlock(&drm_global_mutex);
> + if (drm_dev_needs_global_mutex(dev))
> + mutex_unlock(&drm_global_mutex);
>
> return 0;
>
> err_undo:
> atomic_dec(&dev->open_count);
> - mutex_unlock(&drm_global_mutex);
> + if (drm_dev_needs_global_mutex(dev))
> + mutex_unlock(&drm_global_mutex);
> drm_minor_release(minor);
> return retcode;
> }
> @@ -444,6 +478,7 @@ int drm_release(struct inode *inode, struct file *filp)
> struct drm_minor *minor = file_priv->minor;
> struct drm_device *dev = minor->dev;
>
> + if (drm_dev_needs_global_mutex(dev))
> mutex_lock(&drm_global_mutex);
>
> DRM_DEBUG("open_count = %d\n", atomic_read(&dev->open_count));
> @@ -453,7 +488,8 @@ int drm_release(struct inode *inode, struct file *filp)
> if (atomic_dec_and_test(&dev->open_count))
> drm_lastclose(dev);
>
> - mutex_unlock(&drm_global_mutex);
> + if (drm_dev_needs_global_mutex(dev))
> + mutex_unlock(&drm_global_mutex);
>
> drm_minor_release(minor);
>
> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> index 6937bf923f05..aeec2e68d772 100644
> --- a/drivers/gpu/drm/drm_internal.h
> +++ b/drivers/gpu/drm/drm_internal.h
> @@ -41,6 +41,7 @@ struct drm_printer;
>
> /* drm_file.c */
> extern struct mutex drm_global_mutex;
> +bool drm_dev_needs_global_mutex(struct drm_device *dev);
> struct drm_file *drm_file_alloc(struct drm_minor *minor);
> void drm_file_free(struct drm_file *file);
> void drm_lastclose(struct drm_device *dev);
> --
> 2.24.1
>
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