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drm/fb-helper: Avoid race with DRM userspace

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drm_fb_helper_is_bound() is used to check if DRM userspace is in control.
This is done by looking at the fb on the primary plane. By the time
fb-helper gets around to committing, it's possible that the facts have
changed.

Avoid this race by holding the drm_device->master_mutex lock while
committing. When DRM userspace does its first open, it will now wait
until fb-helper is done. The helper will stay away if there's a master.

Two igt tests fail with the new 'bail out if master' rule. Work around
this by relaxing this rule for drm_fb_helper_restore_fbdev_mode_unlocked()
until the tests have been fixed. Add todo entry for this.

Locking rule: Always take the fb-helper lock first.

v5: drm_fb_helper_restore_fbdev_mode_unlocked(): Use
    restore_fbdev_mode_force()

v2:
- Remove drm_fb_helper_is_bound() (Daniel Vetter)
- No need to check fb_helper->dev->master in
  drm_fb_helper_single_fb_probe(), restore_fbdev_mode() has the check.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190506180139.6913-3-noralf@tronnes.org
This commit is contained in:
Noralf Trønnes 2019-05-06 20:01:30 +02:00
parent 094aa54f0f
commit 03a9606e7f
4 changed files with 86 additions and 46 deletions
Download patch file Download diff file Expand all files Collapse all files

20
drivers/gpu/drm/drm_auth.c
View file

@ -351,3 +351,23 @@ void drm_master_put(struct drm_master **master)
*master = NULL;
}
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);
if (dev->master) {
mutex_unlock(&dev->master_mutex);
return false;
}
return true;
}
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);
}
EXPORT_SYMBOL(drm_master_internal_release);