dma-buf: add reservation_object_fences helper

Add a new helper to get a consistent set of pointers from the reservation
object. While at it group all access helpers together in the header file.

v2: correctly return shared_count as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322378/?series=64837&rev=1

drivers/dma-buf/dma-buf.c

@@ -199,7 +199,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 	struct reservation_object_list *fobj;
 	struct dma_fence *fence_excl;
 	__poll_t events;
-	unsigned shared_count, seq;
+	unsigned shared_count;
 
 	dmabuf = file->private_data;
 	if (!dmabuf || !dmabuf->resv)
@@ -213,21 +213,8 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 	if (!events)
 		return 0;
 
-retry:
-	seq = read_seqcount_begin(&resv->seq);
 	rcu_read_lock();
-
+	reservation_object_fences(resv, &fence_excl, &fobj, &shared_count);
-	fobj = rcu_dereference(resv->fence);
-	if (fobj)
-		shared_count = fobj->shared_count;
-	else
-		shared_count = 0;
-	fence_excl = rcu_dereference(resv->fence_excl);
-	if (read_seqcount_retry(&resv->seq, seq)) {
-		rcu_read_unlock();
-		goto retry;
-	}
-
 	if (fence_excl && (!(events & EPOLLOUT) || shared_count == 0)) {
 		struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_excl;
 		__poll_t pevents = EPOLLIN;
@@ -1157,7 +1144,6 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
 	struct reservation_object *robj;
 	struct reservation_object_list *fobj;
 	struct dma_fence *fence;
-	unsigned seq;
 	int count = 0, attach_count, shared_count, i;
 	size_t size = 0;
 
@@ -1188,16 +1174,9 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
 				buf_obj->name ?: "");
 
 		robj = buf_obj->resv;
-		while (true) {
+		rcu_read_lock();
-			seq = read_seqcount_begin(&robj->seq);
+		reservation_object_fences(robj, &fence, &fobj, &shared_count);
-			rcu_read_lock();
+		rcu_read_unlock();
-			fobj = rcu_dereference(robj->fence);
-			shared_count = fobj ? fobj->shared_count : 0;
-			fence = rcu_dereference(robj->fence_excl);
-			if (!read_seqcount_retry(&robj->seq, seq))
-				break;
-			rcu_read_unlock();
-		}
 
 		if (fence)
 			seq_printf(s, "\tExclusive fence: %s %s %ssignalled\n",

drivers/dma-buf/reservation.c

@@ -317,17 +317,15 @@ int reservation_object_copy_fences(struct reservation_object *dst,
 {
 	struct reservation_object_list *src_list, *dst_list;
 	struct dma_fence *old, *new;
-	unsigned i;
+	unsigned int i, shared_count;
 
 	reservation_object_assert_held(dst);
 
 	rcu_read_lock();
-	src_list = rcu_dereference(src->fence);
 
 retry:
-	if (src_list) {
+	reservation_object_fences(src, &new, &src_list, &shared_count);
-		unsigned shared_count = src_list->shared_count;
+	if (shared_count) {
-
 		rcu_read_unlock();
 
 		dst_list = reservation_object_list_alloc(shared_count);
@@ -335,14 +333,14 @@ retry:
 			return -ENOMEM;
 
 		rcu_read_lock();
-		src_list = rcu_dereference(src->fence);
+		reservation_object_fences(src, &new, &src_list, &shared_count);
-		if (!src_list || src_list->shared_count > shared_count) {
+		if (!src_list || shared_count > dst_list->shared_max) {
 			kfree(dst_list);
 			goto retry;
 		}
 
 		dst_list->shared_count = 0;
-		for (i = 0; i < src_list->shared_count; ++i) {
+		for (i = 0; i < shared_count; ++i) {
 			struct dma_fence *fence;
 
 			fence = rcu_dereference(src_list->shared[i]);
@@ -352,7 +350,6 @@ retry:
 
 			if (!dma_fence_get_rcu(fence)) {
 				reservation_object_list_free(dst_list);
-				src_list = rcu_dereference(src->fence);
 				goto retry;
 			}
 
@@ -367,7 +364,10 @@ retry:
 		dst_list = NULL;
 	}
 
-	new = dma_fence_get_rcu_safe(&src->fence_excl);
+	if (new && !dma_fence_get_rcu(new)) {
+		reservation_object_list_free(dst_list);
+		goto retry;
+	}
 	rcu_read_unlock();
 
 	src_list = reservation_object_get_list(dst);
@@ -413,19 +413,18 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
 
 	do {
 		struct reservation_object_list *fobj;
-		unsigned int i, seq;
+		unsigned int i;
 		size_t sz = 0;
 
-		shared_count = i = 0;
+		i = 0;
 
 		rcu_read_lock();
-		seq = read_seqcount_begin(&obj->seq);
+		reservation_object_fences(obj, &fence_excl, &fobj,
+					  &shared_count);
 
-		fence_excl = rcu_dereference(obj->fence_excl);
 		if (fence_excl && !dma_fence_get_rcu(fence_excl))
 			goto unlock;
 
-		fobj = rcu_dereference(obj->fence);
 		if (fobj)
 			sz += sizeof(*shared) * fobj->shared_max;
 
@@ -453,7 +452,6 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
 				break;
 			}
 			shared = nshared;
-			shared_count = fobj ? fobj->shared_count : 0;
 			for (i = 0; i < shared_count; ++i) {
 				shared[i] = rcu_dereference(fobj->shared[i]);
 				if (!dma_fence_get_rcu(shared[i]))
@@ -461,7 +459,7 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
 			}
 		}
 
-		if (i != shared_count || read_seqcount_retry(&obj->seq, seq)) {
+		if (i != shared_count) {
 			while (i--)
 				dma_fence_put(shared[i]);
 			dma_fence_put(fence_excl);
@@ -505,18 +503,17 @@ long reservation_object_wait_timeout_rcu(struct reservation_object *obj,
 					 bool wait_all, bool intr,
 					 unsigned long timeout)
 {
+	struct reservation_object_list *fobj;
 	struct dma_fence *fence;
-	unsigned seq, shared_count;
+	unsigned shared_count;
 	long ret = timeout ? timeout : 1;
 	int i;
 
 retry:
-	shared_count = 0;
-	seq = read_seqcount_begin(&obj->seq);
 	rcu_read_lock();
 	i = -1;
 
-	fence = rcu_dereference(obj->fence_excl);
+	reservation_object_fences(obj, &fence, &fobj, &shared_count);
 	if (fence && !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
 		if (!dma_fence_get_rcu(fence))
 			goto unlock_retry;
@@ -531,12 +528,6 @@ retry:
 	}
 
 	if (wait_all) {
-		struct reservation_object_list *fobj =
-						rcu_dereference(obj->fence);
-
-		if (fobj)
-			shared_count = fobj->shared_count;
-
 		for (i = 0; !fence && i < shared_count; ++i) {
 			struct dma_fence *lfence = rcu_dereference(fobj->shared[i]);
 
@@ -559,11 +550,6 @@ retry:
 
 	rcu_read_unlock();
 	if (fence) {
-		if (read_seqcount_retry(&obj->seq, seq)) {
-			dma_fence_put(fence);
-			goto retry;
-		}
-
 		ret = dma_fence_wait_timeout(fence, intr, ret);
 		dma_fence_put(fence);
 		if (ret > 0 && wait_all && (i + 1 < shared_count))
@@ -608,24 +594,19 @@ reservation_object_test_signaled_single(struct dma_fence *passed_fence)
 bool reservation_object_test_signaled_rcu(struct reservation_object *obj,
 					  bool test_all)
 {
-	unsigned seq, shared_count;
+	struct reservation_object_list *fobj;
+	struct dma_fence *fence_excl;
+	unsigned shared_count;
 	int ret;
 
 	rcu_read_lock();
 retry:
 	ret = true;
-	shared_count = 0;
-	seq = read_seqcount_begin(&obj->seq);
 
+	reservation_object_fences(obj, &fence_excl, &fobj, &shared_count);
 	if (test_all) {
 		unsigned i;
 
-		struct reservation_object_list *fobj =
-						rcu_dereference(obj->fence);
-
-		if (fobj)
-			shared_count = fobj->shared_count;
-
 		for (i = 0; i < shared_count; ++i) {
 			struct dma_fence *fence = rcu_dereference(fobj->shared[i]);
 
@@ -635,23 +616,12 @@ retry:
 			else if (!ret)
 				break;
 		}
-
-		if (read_seqcount_retry(&obj->seq, seq))
-			goto retry;
 	}
 
-	if (!shared_count) {
+	if (!shared_count && fence_excl) {
-		struct dma_fence *fence_excl = rcu_dereference(obj->fence_excl);
+		ret = reservation_object_test_signaled_single(fence_excl);
-
+		if (ret < 0)
-		if (fence_excl) {
+			goto retry;
-			ret = reservation_object_test_signaled_single(
-								fence_excl);
-			if (ret < 0)
-				goto retry;
-
-			if (read_seqcount_retry(&obj->seq, seq))
-				goto retry;
-		}
 	}
 
 	rcu_read_unlock();

include/linux/reservation.h

@@ -81,6 +81,25 @@ struct reservation_object {
 #define reservation_object_assert_held(obj) \
 	lockdep_assert_held(&(obj)->lock.base)
 
+/**
+ * reservation_object_get_excl - get the reservation object's
+ * exclusive fence, with update-side lock held
+ * @obj: the reservation object
+ *
+ * Returns the exclusive fence (if any).  Does NOT take a
+ * reference. Writers must hold obj->lock, readers may only
+ * hold a RCU read side lock.
+ *
+ * RETURNS
+ * The exclusive fence or NULL
+ */
+static inline struct dma_fence *
+reservation_object_get_excl(struct reservation_object *obj)
+{
+	return rcu_dereference_protected(obj->fence_excl,
+					 reservation_object_held(obj));
+}
+
 /**
  * reservation_object_get_list - get the reservation object's
  * shared fence list, with update-side lock held
@@ -96,6 +115,57 @@ reservation_object_get_list(struct reservation_object *obj)
 					 reservation_object_held(obj));
 }
 
+/**
+ * reservation_object_fences - read consistent fence pointers
+ * @obj: reservation object where we get the fences from
+ * @excl: pointer for the exclusive fence
+ * @list: pointer for the shared fence list
+ *
+ * Make sure we have a consisten exclusive fence and shared fence list.
+ * Must be called with rcu read side lock held.
+ */
+static inline void
+reservation_object_fences(struct reservation_object *obj,
+			  struct dma_fence **excl,
+			  struct reservation_object_list **list,
+			  u32 *shared_count)
+{
+	unsigned int seq;
+
+	do {
+		seq = read_seqcount_begin(&obj->seq);
+		*excl = rcu_dereference(obj->fence_excl);
+		*list = rcu_dereference(obj->fence);
+		*shared_count = *list ? (*list)->shared_count : 0;
+	} while (read_seqcount_retry(&obj->seq, seq));
+}
+
+/**
+ * reservation_object_get_excl_rcu - get the reservation object's
+ * exclusive fence, without lock held.
+ * @obj: the reservation object
+ *
+ * If there is an exclusive fence, this atomically increments it's
+ * reference count and returns it.
+ *
+ * RETURNS
+ * The exclusive fence or NULL if none
+ */
+static inline struct dma_fence *
+reservation_object_get_excl_rcu(struct reservation_object *obj)
+{
+	struct dma_fence *fence;
+
+	if (!rcu_access_pointer(obj->fence_excl))
+		return NULL;
+
+	rcu_read_lock();
+	fence = dma_fence_get_rcu_safe(&obj->fence_excl);
+	rcu_read_unlock();
+
+	return fence;
+}
+
 /**
  * reservation_object_lock - lock the reservation object
  * @obj: the reservation object
@@ -239,51 +309,6 @@ reservation_object_unlock(struct reservation_object *obj)
 	ww_mutex_unlock(&obj->lock);
 }
 
-/**
- * reservation_object_get_excl - get the reservation object's
- * exclusive fence, with update-side lock held
- * @obj: the reservation object
- *
- * Returns the exclusive fence (if any).  Does NOT take a
- * reference. Writers must hold obj->lock, readers may only
- * hold a RCU read side lock.
- *
- * RETURNS
- * The exclusive fence or NULL
- */
-static inline struct dma_fence *
-reservation_object_get_excl(struct reservation_object *obj)
-{
-	return rcu_dereference_protected(obj->fence_excl,
-					 reservation_object_held(obj));
-}
-
-/**
- * reservation_object_get_excl_rcu - get the reservation object's
- * exclusive fence, without lock held.
- * @obj: the reservation object
- *
- * If there is an exclusive fence, this atomically increments it's
- * reference count and returns it.
- *
- * RETURNS
- * The exclusive fence or NULL if none
- */
-static inline struct dma_fence *
-reservation_object_get_excl_rcu(struct reservation_object *obj)
-{
-	struct dma_fence *fence;
-
-	if (!rcu_access_pointer(obj->fence_excl))
-		return NULL;
-
-	rcu_read_lock();
-	fence = dma_fence_get_rcu_safe(&obj->fence_excl);
-	rcu_read_unlock();
-
-	return fence;
-}
-
 void reservation_object_init(struct reservation_object *obj);
 void reservation_object_fini(struct reservation_object *obj);
 int reservation_object_reserve_shared(struct reservation_object *obj,