dma-buf: Restart reservation_object_test_signaled_rcu() after writes

In order to be completely generic, we have to double check the read
seqlock after acquiring a reference to the fence. If the driver is
allocating fences from a SLAB_DESTROY_BY_RCU, or similar freelist, then
within an RCU grace period a fence may be freed and reallocated. The RCU
read side critical section does not prevent this reallocation, instead
we have to inspect the reservation's seqlock to double check if the
fences have been reassigned as we were acquiring our reference.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829070834.22296-9-chris@chris-wilson.co.uk

drivers/dma-buf/reservation.c

@@ -474,12 +474,13 @@ bool reservation_object_test_signaled_rcu(struct reservation_object *obj,
 					  bool test_all)
 {
 	unsigned seq, shared_count;
-	int ret = true;
+	int ret;
 
+	rcu_read_lock();
 retry:
+	ret = true;
 	shared_count = 0;
 	seq = read_seqcount_begin(&obj->seq);
-	rcu_read_lock();
 
 	if (test_all) {
 		unsigned i;
@@ -490,46 +491,35 @@ retry:
 		if (fobj)
 			shared_count = fobj->shared_count;
 
-		if (read_seqcount_retry(&obj->seq, seq))
-			goto unlock_retry;
-
 		for (i = 0; i < shared_count; ++i) {
 			struct fence *fence = rcu_dereference(fobj->shared[i]);
 
 			ret = reservation_object_test_signaled_single(fence);
 			if (ret < 0)
-				goto unlock_retry;
+				goto retry;
 			else if (!ret)
 				break;
 		}
 
-		/*
-		 * There could be a read_seqcount_retry here, but nothing cares
-		 * about whether it's the old or newer fence pointers that are
-		 * signaled. That race could still have happened after checking
-		 * read_seqcount_retry. If you care, use ww_mutex_lock.
-		 */
+		if (read_seqcount_retry(&obj->seq, seq))
+			goto retry;
 	}
 
 	if (!shared_count) {
 		struct fence *fence_excl = rcu_dereference(obj->fence_excl);
 
-		if (read_seqcount_retry(&obj->seq, seq))
-			goto unlock_retry;
-
 		if (fence_excl) {
 			ret = reservation_object_test_signaled_single(
 								fence_excl);
 			if (ret < 0)
-				goto unlock_retry;
+				goto retry;
+
+			if (read_seqcount_retry(&obj->seq, seq))
+				goto retry;
 		}
 	}
 
 	rcu_read_unlock();
 	return ret;
-
-unlock_retry:
-	rcu_read_unlock();
-	goto retry;
 }
 EXPORT_SYMBOL_GPL(reservation_object_test_signaled_rcu);