[Intel-gfx] [PATCH] drm/i915: Watch out for idling during i915_gem_evict_something

[Chris Wilson]

i915_gem_evict_something() is charged with finding a slot within the GTT
that we may reuse. Since our goal is not to stall, we first look for a
slot that only overlaps idle vma. To this end, on the first pass we move
any active vma to the end of the search list. However, we only stopped
moving active vma after we see the first active vma twice. If during the
search, that first active vma completed, we would not notice and keep on
extending the search list.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1746
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: <stable at vger.kernel.org> # v5.5+
---
 drivers/gpu/drm/i915/i915_gem_evict.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_evict.c b/drivers/gpu/drm/i915/i915_gem_evict.c
index 0ba7b1e881c0..6501939929d5 100644
--- a/drivers/gpu/drm/i915/i915_gem_evict.c
+++ b/drivers/gpu/drm/i915/i915_gem_evict.c
@@ -128,6 +128,13 @@ i915_gem_evict_something(struct i915_address_space *vm,
 	active = NULL;
 	INIT_LIST_HEAD(&eviction_list);
 	list_for_each_entry_safe(vma, next, &vm->bound_list, vm_link) {
+		if (vma == active) { /* now seen this vma twice */
+			if (flags & PIN_NONBLOCK)
+				break;
+
+			active = ERR_PTR(-EAGAIN);
+		}
+
 		/*
 		 * We keep this list in a rough least-recently scanned order
 		 * of active elements (inactive elements are cheap to reap).
@@ -143,21 +150,12 @@ i915_gem_evict_something(struct i915_address_space *vm,
 		 * To notice when we complete one full cycle, we record the
 		 * first active element seen, before moving it to the tail.
 		 */
-		if (i915_vma_is_active(vma)) {
-			if (vma == active) {
-				if (flags & PIN_NONBLOCK)
-					break;
-
-				active = ERR_PTR(-EAGAIN);
-			}
-
-			if (active != ERR_PTR(-EAGAIN)) {
-				if (!active)
-					active = vma;
+		if (active != ERR_PTR(-EAGAIN) && i915_vma_is_active(vma)) {
+			if (!active)
+				active = vma;
 
-				list_move_tail(&vma->vm_link, &vm->bound_list);
-				continue;
-			}
+			list_move_tail(&vma->vm_link, &vm->bound_list);
+			continue;
 		}
 
 		if (mark_free(&scan, vma, flags, &eviction_list))
-- 
2.20.1