[PATCH 1/2] drm/i915/active: Fix potential reuse of non-idle barrier as fence tracker

[Janusz Krzysztofik]

Users reported oopses on list corruptions when using i915 perf with a
number of concurrently running graphics applications.  Root cause analysis
pointed out to an issue in barrier processing code -- a race among perf
open / close replacing active barriers with perf requests on kernel
contexts and concurrent barrier preallocate / acquire operations performed
during user context first pin / last unpin.

When adding a request to a composite tracker, we try to use an existing
tracker already registered with that composite tracker.  The tracker we
obtain can already track another fence, can be an idle barrier, or an
active barrier.  The first two cases seem easy to handle and we seem to
do that correctly.  In the last case, we attempt to replace the active
barrier with our request.  However, when trying to delete that barrier
from a list of barrier tasks it belongs to, we ignore return value from
that operation, which informs us whether the deletion succeeded or not,
and we proceed even if it fails.

On the other side, barriers are now deleted from a barrier tasks list by
temporarily removing the list content, traversing that content with skip
over the node to be deleted, then adding the modified content back to the
list.  Since that complex operation is not serialized with other
concurrent uses of the list, including similar barrier deletions,
functions that depend on the list being either empty or not empty can take
wrong decisions.

A failed barrier deletion can be a side effect of the way we have it
implemented -- another thread can have temporarily emptied the list before
we manage to do that.  If we ignore such failure, that other thread can
then add our fence tracker back to the barrier tasks list.  Since the same
structure field of the tracker is used as a list node with both barrier
tasks lists and fence callback lists, list corruption occurs.  However,
list corruptions were still observed when running the user worload on top
of an experimental patch that serialized all operations on barrier tasks
lists with a spinlock.  Then, other race scenarios leading to list
corruptions must exist.

Based on those observations, respecting the return code from barrier
deletion seems required for effectively fixing the issue.  However, we
need to handle those few above mentioned cases carefully.

Respect results of barrier deletion attempts -- mark the barrier as idle
only after successfully deleted from the list.  Then, before proceeding
with setting our fence as the one currently tracked, make sure that the
tracker we've got is not a non-idle barrier.  If that check fails, don't
use that tracker but go back and try to acquire a usable one.

Extensive testing shows that this fix should be sufficient for barrier
related list corruptions no longer appearing.  However, other issues will
likely pop up from a new subtest we have developed for this case, then we
may still want to get back to this soon and refactor our potentially
fragile way of barrier tasks lists handling.

Fixes: d8af05ff38ae ("drm/i915: Allow sharing the idle-barrier from other kernel requests")
References: https://gitlab.freedesktop.org/drm/intel/-/issues/6333
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
Cc: stable at vger.kernel.org # v5.4
---
 drivers/gpu/drm/i915/i915_active.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index 7412abf166a8c..f9282b8c87c1c 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -422,12 +422,12 @@ replace_barrier(struct i915_active *ref, struct i915_active_fence *active)
 	 * we can use it to substitute for the pending idle-barrer
 	 * request that we want to emit on the kernel_context.
 	 */
-	__active_del_barrier(ref, node_from_active(active));
-	return true;
+	return __active_del_barrier(ref, node_from_active(active));
 }
 
 int i915_active_add_request(struct i915_active *ref, struct i915_request *rq)
 {
+	u64 idx = i915_request_timeline(rq)->fence_context;
 	struct dma_fence *fence = &rq->fence;
 	struct i915_active_fence *active;
 	int err;
@@ -437,16 +437,19 @@ int i915_active_add_request(struct i915_active *ref, struct i915_request *rq)
 	if (err)
 		return err;
 
-	active = active_instance(ref, i915_request_timeline(rq)->fence_context);
-	if (!active) {
-		err = -ENOMEM;
-		goto out;
-	}
+	do {
+		active = active_instance(ref, idx);
+		if (!active) {
+			err = -ENOMEM;
+			goto out;
+		}
+
+		if (replace_barrier(ref, active)) {
+			RCU_INIT_POINTER(active->fence, NULL);
+			atomic_dec(&ref->count);
+		}
+	} while (is_barrier(active));
 
-	if (replace_barrier(ref, active)) {
-		RCU_INIT_POINTER(active->fence, NULL);
-		atomic_dec(&ref->count);
-	}
 	if (!__i915_active_fence_set(active, fence))
 		__i915_active_acquire(ref);
 
-- 
2.25.1