[Intel-gfx] [PATCH] drm/i915/ttm: Fix illegal addition to shrinker list

Thomas Hellström
2021-11-10T08:55:44Z

There's a small window of opportunity during which the adjust_lru()
function can be called with a GEM refcount of zero from the TTM
eviction code. This results in a kernel BUG().

Ensure that we don't attempt to modify the GEM shrinker lists unless
we have a GEM refcount.

Fixes: ebd4a8ec7799 ("drm/i915/ttm: move shrinker management into adjust_lru")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 31 +++++++++++++++++--------
1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
index e98503c0830b..68cfe6e9ceab 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
@@ -771,18 +771,29 @@ void i915_ttm_adjust_lru(struct drm_i915_gem_object *obj)
*
* TODO: consider maybe also bumping the shrinker list here when we have
* already unpinned it, which should give us something more like an LRU.
+ *
+ * TODO: There is a small window of opportunity for this function to
+ * get called from eviction after we've dropped the last GEM refcount,
+ * but before the TTM deleted flag is set on the object. Avoid
+ * adjusting the shrinker list in such cases, since the object is
+ * not available to the shrinker anyway due to its zero refcount.
+ * To fix this properly we should move to a TTM shrinker LRU list for
+ * these objects.
*/
- if (shrinkable != obj->mm.ttm_shrinkable) {
- if (shrinkable) {
- if (obj->mm.madv == I915_MADV_WILLNEED)
- __i915_gem_object_make_shrinkable(obj);
- else
- __i915_gem_object_make_purgeable(obj);
- } else {
- i915_gem_object_make_unshrinkable(obj);
+ if (kref_get_unless_zero(&obj->base.refcount)) {
+ if (shrinkable != obj->mm.ttm_shrinkable) {
+ if (shrinkable) {
+ if (obj->mm.madv == I915_MADV_WILLNEED)
+ __i915_gem_object_make_shrinkable(obj);
+ else
+ __i915_gem_object_make_purgeable(obj);
+ } else {
+ i915_gem_object_make_unshrinkable(obj);
+ }
+
+ obj->mm.ttm_shrinkable = shrinkable;
}
-
- obj->mm.ttm_shrinkable = shrinkable;
+ i915_gem_object_put(obj);
}

/*
--
2.31.1