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: 290271de34f6 ("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>
(cherry picked from commit 6cb0c6ad9e07f2c7971c4e8e0d9b7ceba151a925)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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();