drm/i915/shrinker: Wrap need_resched() inside preempt-disable

In order for us to successfully detect the end of a timeslice,
preemption must be disabled. Otherwise, inside the loop we may be
preempted many times without our noticing, and each time our timeslice
will be reset, invalidating need_resched()

Reported-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reported-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Fixes: 290271de34 ("drm/i915: Spin for struct_mutex inside shrinker")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org> # v4.13-rc1+
Link: https://patchwork.freedesktop.org/patch/msgid/20170804104135.26805-1-chris@chris-wilson.co.uk
Tested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_shrinker.c

@@ -43,16 +43,21 @@ static bool shrinker_lock(struct drm_i915_private *dev_priv, bool *unlock)
 		return true;
 
 	case MUTEX_TRYLOCK_FAILED:
+		*unlock = false;
+		preempt_disable();
 		do {
 			cpu_relax();
 			if (mutex_trylock(&dev_priv->drm.struct_mutex)) {
-	case MUTEX_TRYLOCK_SUCCESS:
 				*unlock = true;
-				return true;
+				break;
 			}
 		} while (!need_resched());
+		preempt_enable();
+		return *unlock;
 
-		return false;
+	case MUTEX_TRYLOCK_SUCCESS:
+		*unlock = true;
+		return true;
 	}
 
 	BUG();