drm/i915/execlists: Fixup cancel_port_requests()

I rushed a last minute correction to cancel_port_requests() to prevent
the snooping of *execlists->active as the inflight array was being
updated, without noticing we iterated the inflight array starting from
active! Oops.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112387
Fixes: 331bf905 ("drm/i915/gt: Mark the execlists->active as the primary volatile access")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191125112520.1760492-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -2007,19 +2007,17 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 static void
 cancel_port_requests(struct intel_engine_execlists * const execlists)
 {
-	struct i915_request * const *port, *rq;
+	struct i915_request * const *port;
 
-	for (port = execlists->pending; (rq = *port); port++)
-		execlists_schedule_out(rq);
+	for (port = execlists->pending; *port; port++)
+		execlists_schedule_out(*port);
 	memset(execlists->pending, 0, sizeof(execlists->pending));
 
 	/* Mark the end of active before we overwrite *active */
-	WRITE_ONCE(execlists->active, execlists->pending);
-
-	for (port = execlists->active; (rq = *port); port++)
-		execlists_schedule_out(rq);
-	execlists->active =
-		memset(execlists->inflight, 0, sizeof(execlists->inflight));
+	for (port = xchg(&execlists->active, execlists->pending); *port; port++)
+		execlists_schedule_out(*port);
+	WRITE_ONCE(execlists->active,
+		   memset(execlists->inflight, 0, sizeof(execlists->inflight)));
 }
 
 static inline void