[PATCH] drm/i915: drop the coditional mutex

[Sebastian Andrzej Siewior]

mutex_is_locked_by() checks the owner of the lock against current. This
is done by accessing a private member of struct mutex which works on
mainline but does not on RT.
I did not figure out, why this "lock-owner-check" and the "lock stealing
flag" is required. If the lock can not be acquire the lock (because it
would deadlock) then it can return -1.
The lock stealing makes actually no sense to me. If
shrinker_no_lock_stealing is true then the same functions
(i915_gem_purge(), i915_gem_shrink_all()) are called from
i915_gem_object_create_mmap_offset() as from i915_gem_inactive_shrink().
I haven't found a path in which i915_gem_inactive_shrink() is invoked
from i915_gem_object_create_mmap_offset() that means there is no way
shrinker_no_lock_stealing is true _and_ the lock is owned by the current
process.
Since I don't see why the i915 needs this hack while all other user do
not I recommend to remove this hack and return -1 in case of a dead
lock. Here is the patch.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
---
 drivers/gpu/drm/i915/i915_drv.h |    1 -
 drivers/gpu/drm/i915/i915_gem.c |   32 +++-----------------------------
 2 files changed, 3 insertions(+), 30 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 01769e2..47f28ee 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -724,7 +724,6 @@ struct i915_gem_mm {
 	struct i915_hw_ppgtt *aliasing_ppgtt;
 
 	struct shrinker inactive_shrinker;
-	bool shrinker_no_lock_stealing;
 
 	/**
 	 * List of objects currently involved in rendering.
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 0e207e6..7949517 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1500,8 +1500,6 @@ static int i915_gem_object_create_mmap_offset(struct drm_i915_gem_object *obj)
 	if (obj->base.map_list.map)
 		return 0;
 
-	dev_priv->mm.shrinker_no_lock_stealing = true;
-
 	ret = drm_gem_create_mmap_offset(&obj->base);
 	if (ret != -ENOSPC)
 		goto out;
@@ -1521,8 +1519,6 @@ static int i915_gem_object_create_mmap_offset(struct drm_i915_gem_object *obj)
 	i915_gem_shrink_all(dev_priv);
 	ret = drm_gem_create_mmap_offset(&obj->base);
 out:
-	dev_priv->mm.shrinker_no_lock_stealing = false;
-
 	return ret;
 }
 
@@ -4368,19 +4364,6 @@ void i915_gem_release(struct drm_device *dev, struct drm_file *file)
 	spin_unlock(&file_priv->mm.lock);
 }
 
-static bool mutex_is_locked_by(struct mutex *mutex, struct task_struct *task)
-{
-	if (!mutex_is_locked(mutex))
-		return false;
-
-#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_MUTEXES)
-	return mutex->owner == task;
-#else
-	/* Since UP may be pre-empted, we cannot assume that we own the lock */
-	return false;
-#endif
-}
-
 static int
 i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
 {
@@ -4391,18 +4374,10 @@ i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
 	struct drm_device *dev = dev_priv->dev;
 	struct drm_i915_gem_object *obj;
 	int nr_to_scan = sc->nr_to_scan;
-	bool unlock = true;
 	int cnt;
 
-	if (!mutex_trylock(&dev->struct_mutex)) {
-		if (!mutex_is_locked_by(&dev->struct_mutex, current))
-			return 0;
-
-		if (dev_priv->mm.shrinker_no_lock_stealing)
-			return 0;
-
-		unlock = false;
-	}
+	if (!mutex_trylock(&dev->struct_mutex))
+		return -1;
 
 	if (nr_to_scan) {
 		nr_to_scan -= i915_gem_purge(dev_priv, nr_to_scan);
@@ -4421,7 +4396,6 @@ i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
 		if (obj->pin_count == 0 && obj->pages_pin_count == 0)
 			cnt += obj->base.size >> PAGE_SHIFT;
 
-	if (unlock)
-		mutex_unlock(&dev->struct_mutex);
+	mutex_unlock(&dev->struct_mutex);
 	return cnt;
 }
-- 
1.7.6.5