drm/i915: Add an rcu_barrier option to i915_drop_caches

Sometimes a test has to wait for RCU to complete a grace period and
perform its callbacks, for example waiting for a close(fd) to actually
perform the fput(filp) and so trigger all the callbacks such as closing
GEM contexts. There is no trivial means of triggering an RCU barrier
from userspace, so add one for our convenience in
debugfs/i915_drop_caches

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011173823.20432-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -3599,6 +3599,7 @@ DEFINE_SIMPLE_ATTRIBUTE(i915_wedged_fops,
 #define DROP_IDLE	BIT(6)
 #define DROP_RESET_ACTIVE	BIT(7)
 #define DROP_RESET_SEQNO	BIT(8)
+#define DROP_RCU	BIT(9)
 #define DROP_ALL (DROP_UNBOUND	| \
 		  DROP_BOUND	| \
 		  DROP_RETIRE	| \
@@ -3607,7 +3608,8 @@ DEFINE_SIMPLE_ATTRIBUTE(i915_wedged_fops,
 		  DROP_SHRINK_ALL |\
 		  DROP_IDLE	| \
 		  DROP_RESET_ACTIVE | \
-		  DROP_RESET_SEQNO)
+		  DROP_RESET_SEQNO | \
+		  DROP_RCU)
 static int
 i915_drop_caches_get(void *data, u64 *val)
 {
@@ -3659,6 +3661,9 @@ i915_drop_caches_set(void *data, u64 val)
 		i915_gem_shrink_all(i915);
 	fs_reclaim_release(GFP_KERNEL);
 
+	if (val & DROP_RCU)
+		rcu_barrier();
+
 	if (val & DROP_FREED)
 		i915_gem_drain_freed_objects(i915);