drm/i915/execlists: Skip nested spinlock for validating pending

Only along the submission path can we guarantee that the locked request
is indeed from a foreign engine, and so the nesting of engine/rq is
permissible. On the submission tasklet (process_csb()), we may find
ourselves competing with the normal nesting of rq/engine, invalidating
our nesting. As we only use the spinlock for debug purposes, skip the
debug if we cannot acquire the spinlock for safe validation - catching
99% of the bugs is better than causing a hard lockup.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: c95d31c3 ("drm/i915/execlists: Lock the request while validating it during promotion")
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203152631.3107653-2-chris@chris-wilson.co.uk

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

@@ -1315,8 +1315,8 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
 		ce = rq->hw_context;
 
 		/* Hold tightly onto the lock to prevent concurrent retires! */
-		spin_lock_irqsave_nested(&rq->lock, flags,
-					 SINGLE_DEPTH_NESTING);
+		if (!spin_trylock_irqsave(&rq->lock, flags))
+			continue;
 
 		if (i915_request_completed(rq))
 			goto unlock;