[PATCH] dma-buf: Restart reservation_object_get_fences_rcu() after writes

[Chris Wilson]

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 at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Christian König <christian.koenig at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
---
 drivers/dma-buf/reservation.c | 71 +++++++++++++++++++------------------------
 1 file changed, 31 insertions(+), 40 deletions(-)

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index 9566a62ad8e3..f6e483d7cc5a 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -280,18 +280,24 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
 				      unsigned *pshared_count,
 				      struct fence ***pshared)
 {
-	unsigned shared_count = 0;
-	unsigned retry = 1;
-	struct fence **shared = NULL, *fence_excl = NULL;
-	int ret = 0;
+	struct fence **shared = NULL;
+	struct fence *fence_excl;
+	unsigned shared_count;
+	int ret = 1;
 
-	while (retry) {
+	do {
 		struct reservation_object_list *fobj;
 		unsigned seq;
+		unsigned i;
 
-		seq = read_seqcount_begin(&obj->seq);
+		shared_count = i = 0;
 
 		rcu_read_lock();
+		seq = read_seqcount_begin(&obj->seq);
+
+		fence_excl = rcu_dereference(obj->fence_excl);
+		if (fence_excl && !fence_get_rcu(fence_excl))
+			goto unlock;
 
 		fobj = rcu_dereference(obj->fence);
 		if (fobj) {
@@ -309,52 +315,37 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
 				}
 
 				ret = -ENOMEM;
-				shared_count = 0;
 				break;
 			}
 			shared = nshared;
-			memcpy(shared, fobj->shared, sz);
 			shared_count = fobj->shared_count;
-		} else
-			shared_count = 0;
-		fence_excl = rcu_dereference(obj->fence_excl);
-
-		retry = read_seqcount_retry(&obj->seq, seq);
-		if (retry)
-			goto unlock;
-
-		if (!fence_excl || fence_get_rcu(fence_excl)) {
-			unsigned i;
 
 			for (i = 0; i < shared_count; ++i) {
-				if (fence_get_rcu(shared[i]))
-					continue;
-
-				/* uh oh, refcount failed, abort and retry */
-				while (i--)
-					fence_put(shared[i]);
-
-				if (fence_excl) {
-					fence_put(fence_excl);
-					fence_excl = NULL;
-				}
-
-				retry = 1;
-				break;
+				shared[i] = rcu_dereference(fobj->shared[i]);
+				if (!fence_get_rcu(shared[i]))
+					break;
 			}
-		} else
-			retry = 1;
+		}
+
+		if (i != shared_count || read_seqcount_retry(&obj->seq, seq)) {
+			while (i--)
+				fence_put(shared[i]);
+			fence_put(fence_excl);
+			goto unlock;
+		}
 
+		ret = 0;
 unlock:
 		rcu_read_unlock();
-	}
-	*pshared_count = shared_count;
-	if (shared_count)
-		*pshared = shared;
-	else {
-		*pshared = NULL;
+	} while (ret);
+
+	if (!shared_count) {
 		kfree(shared);
+		shared = NULL;
 	}
+
+	*pshared_count = shared_count;
+	*pshared = shared;
 	*pfence_excl = fence_excl;
 
 	return ret;
-- 
2.8.1