dma-buf: add dma_resv_for_each_fence_unlocked v8

Abstract the complexity of iterating over all the fences
in a dma_resv object.

The new loop handles the whole RCU and retry dance and
returns only fences where we can be sure we grabbed the
right one.

v2: fix accessing the shared fences while they might be freed,
    improve kerneldoc, rename _cursor to _iter, add
    dma_resv_iter_is_exclusive, add dma_resv_iter_begin/end

v3: restructor the code, move rcu_read_lock()/unlock() into the
    iterator, add dma_resv_iter_is_restarted()

v4: fix NULL deref when no explicit fence exists, drop superflous
    rcu_read_lock()/unlock() calls.

v5: fix typos in the documentation

v6: fix coding error when excl fence is NULL

v7: one more logic fix

v8: fix index check in dma_resv_iter_is_exclusive()

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v7)
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-2-christian.koenig@amd.com

1 files changed, 100 insertions, 0 deletions

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 84fbe60629e3..3cbcf66a137e 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -324,6 +324,106 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 EXPORT_SYMBOL(dma_resv_add_excl_fence);
 
 /**
+ * dma_resv_iter_restart_unlocked - restart the unlocked iterator
+ * @cursor: The dma_resv_iter object to restart
+ *
+ * Restart the unlocked iteration by initializing the cursor object.
+ */
+static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
+{
+	cursor->seq = read_seqcount_begin(&cursor->obj->seq);
+	cursor->index = -1;
+	if (cursor->all_fences)
+		cursor->fences = dma_resv_shared_list(cursor->obj);
+	else
+		cursor->fences = NULL;
+	cursor->is_restarted = true;
+}
+
+/**
+ * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj
+ * @cursor: cursor to record the current position
+ *
+ * Return all the fences in the dma_resv object which are not yet signaled.
+ * The returned fence has an extra local reference so will stay alive.
+ * If a concurrent modify is detected the whole iteration is started over again.
+ */
+static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
+{
+	struct dma_resv *obj = cursor->obj;
+
+	do {
+		/* Drop the reference from the previous round */
+		dma_fence_put(cursor->fence);
+
+		if (cursor->index == -1) {
+			cursor->fence = dma_resv_excl_fence(obj);
+			cursor->index++;
+			if (!cursor->fence)
+				continue;
+
+		} else if (!cursor->fences ||
+			   cursor->index >= cursor->fences->shared_count) {
+			cursor->fence = NULL;
+			break;
+
+		} else {
+			struct dma_resv_list *fences = cursor->fences;
+			unsigned int idx = cursor->index++;
+
+			cursor->fence = rcu_dereference(fences->shared[idx]);
+		}
+		cursor->fence = dma_fence_get_rcu(cursor->fence);
+		if (!cursor->fence || !dma_fence_is_signaled(cursor->fence))
+			break;
+	} while (true);
+}
+
+/**
+ * dma_resv_iter_first_unlocked - first fence in an unlocked dma_resv obj.
+ * @cursor: the cursor with the current position
+ *
+ * Returns the first fence from an unlocked dma_resv obj.
+ */
+struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor)
+{
+	rcu_read_lock();
+	do {
+		dma_resv_iter_restart_unlocked(cursor);
+		dma_resv_iter_walk_unlocked(cursor);
+	} while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
+	rcu_read_unlock();
+
+	return cursor->fence;
+}
+EXPORT_SYMBOL(dma_resv_iter_first_unlocked);
+
+/**
+ * dma_resv_iter_next_unlocked - next fence in an unlocked dma_resv obj.
+ * @cursor: the cursor with the current position
+ *
+ * Returns the next fence from an unlocked dma_resv obj.
+ */
+struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor)
+{
+	bool restart;
+
+	rcu_read_lock();
+	cursor->is_restarted = false;
+	restart = read_seqcount_retry(&cursor->obj->seq, cursor->seq);
+	do {
+		if (restart)
+			dma_resv_iter_restart_unlocked(cursor);
+		dma_resv_iter_walk_unlocked(cursor);
+		restart = true;
+	} while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
+	rcu_read_unlock();
+
+	return cursor->fence;
+}
+EXPORT_SYMBOL(dma_resv_iter_next_unlocked);
+
+/**
  * dma_resv_copy_fences - Copy all fences from src to dst.
  * @dst: the destination reservation object
  * @src: the source reservation object