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 <[email protected]>
Reported-by: Tomi Sarvela <[email protected]>
Fixes: 290271de34f6 ("drm/i915: Spin for struct_mutex inside shrinker")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: <[email protected]> # v4.13-rc1+
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Tested-by: Joonas Lahtinen <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
(cherry picked from commit 6cb0c6ad9e07f2c7971c4e8e0d9b7ceba151a925)
Signed-off-by: Jani Nikula <[email protected]>

1 files changed, 8 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
index 1032f98add11..77fb39808131 100644
--- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
+++ b/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();