drm/i915: Remove toplevel struct_mutex locking from debugfs/i915_drop_caches

I have a plan to write a quick test to exercise concurrent usage of
i915_gem_shrink(), the simplest way looks to be to have multiple threads
using debugfs/i915_drop_caches. However, we currently take one lock over
the entire function, serialising the calls into i915_gem_shrink() so
reduce the lock coverage.

Testcase: igt/gem_shrink/reclaim
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524162653.5446-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -4289,27 +4289,28 @@ i915_drop_caches_set(void *data, u64 val)
 {
 	struct drm_i915_private *dev_priv = data;
 	struct drm_device *dev = &dev_priv->drm;
-	int ret;
+	int ret = 0;
 
 	DRM_DEBUG("Dropping caches: 0x%08llx\n", val);
 
 	/* No need to check and wait for gpu resets, only libdrm auto-restarts
 	 * on ioctls on -EAGAIN. */
+	if (val & (DROP_ACTIVE | DROP_RETIRE)) {
 		ret = mutex_lock_interruptible(&dev->struct_mutex);
 		if (ret)
 			return ret;
 
-	if (val & DROP_ACTIVE) {
+		if (val & DROP_ACTIVE)
 			ret = i915_gem_wait_for_idle(dev_priv,
 						     I915_WAIT_INTERRUPTIBLE |
 						     I915_WAIT_LOCKED);
-		if (ret)
-			goto unlock;
-	}
 
 		if (val & DROP_RETIRE)
 			i915_gem_retire_requests(dev_priv);
 
+		mutex_unlock(&dev->struct_mutex);
+	}
+
 	lockdep_set_current_reclaim_state(GFP_KERNEL);
 	if (val & DROP_BOUND)
 		i915_gem_shrink(dev_priv, LONG_MAX, I915_SHRINK_BOUND);
@@ -4321,9 +4322,6 @@ i915_drop_caches_set(void *data, u64 val)
 		i915_gem_shrink_all(dev_priv);
 	lockdep_clear_current_reclaim_state();
 
-unlock:
-	mutex_unlock(&dev->struct_mutex);
-
 	if (val & DROP_FREED) {
 		synchronize_rcu();
 		i915_gem_drain_freed_objects(dev_priv);