drm/i915/execlists: Use a local lock for dfs_link access

Avoid requiring struct_mutex for exclusive access to the temporary
dfs_link inside the i915_dependency as not all callers may want to touch
struct_mutex. So rather than force them to take a highly contended
lock, introduce a local lock for the execlists schedule operation.

Reported-by: David Weinehall <david.weinehall@linux.intel.com>
Fixes: 9a15198 ("drm/i915: Add execution priority boosting for mmioflips")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161116152721.11053-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/intel_lrc.c

@@ -694,6 +694,7 @@ pt_lock_engine(struct i915_priotree *pt, struct intel_engine_cs *locked)
 
 static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
 {
+	static DEFINE_MUTEX(lock);
 	struct intel_engine_cs *engine = NULL;
 	struct i915_dependency *dep, *p;
 	struct i915_dependency stack;
@@ -702,8 +703,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
 	if (prio <= READ_ONCE(request->priotree.priority))
 		return;
 
-	/* Need BKL in order to use the temporary link inside i915_dependency */
-	lockdep_assert_held(&request->i915->drm.struct_mutex);
+	/* Need global lock to use the temporary link inside i915_dependency */
+	mutex_lock(&lock);
 
 	stack.signaler = &request->priotree;
 	list_add(&stack.dfs_link, &dfs);
@@ -770,6 +771,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
 	if (engine)
 		spin_unlock_irq(&engine->timeline->lock);
 
+	mutex_unlock(&lock);
+
 	/* XXX Do we need to preempt to make room for us and our deps? */
 }