From 290271de34f6c22ec2337e3293224575459747d6 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Fri, 9 Jun 2017 12:03:49 +0100
Subject: [PATCH] drm/i915: Spin for struct_mutex inside shrinker
Git-commit: 290271de34f6c22ec2337e3293224575459747d6
Patch-mainline: v4.13-rc1
References: FATE#322643 bsc#1055900

Having resolved whether or not we would deadlock upon a call to
mutex_lock(&dev->struct_mutex), we can then spin for the contended
struct_mutex if we are not the owner. We cannot afford to simply block
and wait for the mutex, as the owner may itself be waiting for the
allocator -- i.e. a cyclic deadlock. This should significantly improve
the chance of running the shrinker for other processes whilst the GPU is
busy.

A more balanced approach would be to optimistically spin whilst the
mutex owner was on the cpu and there was an opportunity to acquire the
mutex for ourselves quickly. However, that requires support from
kernel/locking/ and a new mutex_spin_trylock() primitive.

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>
Link: http://patchwork.freedesktop.org/patch/msgid/20170609110350.1767-4-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/gpu/drm/i915/i915_gem_shrinker.c |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

--- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
+++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
@@ -38,16 +38,21 @@
 static bool shrinker_lock(struct drm_i915_private *dev_priv, bool *unlock)
 {
 	switch (mutex_trylock_recursive(&dev_priv->drm.struct_mutex)) {
-	case MUTEX_TRYLOCK_FAILED:
-		return false;
-
-	case MUTEX_TRYLOCK_SUCCESS:
-		*unlock = true;
-		return true;
-
 	case MUTEX_TRYLOCK_RECURSIVE:
 		*unlock = false;
 		return true;
+
+	case MUTEX_TRYLOCK_FAILED:
+		do {
+			cpu_relax();
+			if (mutex_trylock(&dev_priv->drm.struct_mutex)) {
+	case MUTEX_TRYLOCK_SUCCESS:
+				*unlock = true;
+				return true;
+			}
+		} while (!need_resched());
+
+		return false;
 	}
 
 	BUG();