From 4017ad7bbc3296b8ccd4b35df5aa5f512c309531 Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Tue, 4 Feb 2020 16:01:46 +0100 Subject: [PATCH] drm: Nerf drm_global_mutex BKL for good drivers 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 Date: Fri Jan 24 13:01:07 2020 +0000 drm: Avoid drm_global_mutex for simple inc/dec of dev->open_count v2: - Rebase and fix locking in drm_open() (Chris) - Indentation fix in drm_release - Typo fix in the commit message (Sam) Cc: Chris Wilson Cc: Sam Ravnborg Reviewed-by: Chris Wilson Tested-by: Thomas Zimmermann Acked-by: Thomas Zimmermann Signed-off-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20200204150146.2006481-6-daniel.vetter@ffwll.ch --- drivers/gpu/drm/drm_drv.c | 6 +++-- drivers/gpu/drm/drm_file.c | 48 +++++++++++++++++++++++++++++----- drivers/gpu/drm/drm_internal.h | 1 + 3 files changed, 47 insertions(+), 8 deletions(-) diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c index b8f716e61037..7b1a628d1f6e 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); ret = drm_minor_register(dev, DRM_MINOR_RENDER); if (ret) @@ -986,7 +987,8 @@ err_minors: 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 80d556402ab4..c4c704e01961 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_lock(&drm_global_mutex); - dev = minor->dev; + if (drm_dev_needs_global_mutex(dev)) + mutex_lock(&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; } @@ -444,7 +478,8 @@ int drm_release(struct inode *inode, struct file *filp) struct drm_minor *minor = file_priv->minor; struct drm_device *dev = minor->dev; - mutex_lock(&drm_global_mutex); + 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);