[Intel-gfx] [PATCH 1/2] drm/i915: Comment userptr recursion on struct_mutex

[Daniel Vetter]

Discussed this a bit with Chris, I think a comment here is warranted
that this will be bad once we have more than one i915 instance. And
lockdep won't catch it.

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
index 74da35611d7c..70dc506a5426 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -135,6 +135,12 @@ userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
 			switch (mutex_trylock_recursive(unlock)) {
 			default:
 			case MUTEX_TRYLOCK_FAILED:
+				/*
+				 * NOTE: This only works because there's only
+				 * ever one i915-style struct_mutex in the
+				 * entire system. If we could have two i915
+				 * instances, this would deadlock.
+				 */
 				if (mutex_lock_killable_nested(unlock, I915_MM_SHRINKER)) {
 					i915_gem_object_put(obj);
 					return -EINTR;
-- 
2.22.0