Re: [PATCH 01/14] dma-buf: add dma_resv_for_each_fence_unlocked

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Am 14.09.21 um 19:04 schrieb Daniel Vetter:
On Fri, Sep 10, 2021 at 10:26:42AM +0200, Christian König wrote:
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.

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
  drivers/dma-buf/dma-resv.c | 63 ++++++++++++++++++++++++++++++++++++++
  include/linux/dma-resv.h   | 36 ++++++++++++++++++++++
  2 files changed, 99 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 84fbe60629e3..213a9b7251ca 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -323,6 +323,69 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
  }
  EXPORT_SYMBOL(dma_resv_add_excl_fence);
+/**
+ * dma_resv_walk_unlocked - walk over fences in a dma_resv obj
+ * @obj: the dma_resv object
+ * @cursor: cursor to record the current position
+ * @all_fences: true returns also the shared fences
+ * @first: if we should start over
+ *
+ * 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 iterator is started over again.
+ */
+struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj,
+					 struct dma_resv_cursor *cursor,
+					 bool all_fences, bool first)
+{
+	struct dma_fence *fence = NULL;
+
+	do {
+		/* Drop the reference from the previous round */
+		dma_fence_put(fence);
+
+		cursor->is_first = first;
+		if (first) {
+			cursor->seq = read_seqcount_begin(&obj->seq);
+			cursor->index = -1;
+			cursor->fences = dma_resv_shared_list(obj);
+			cursor->is_exclusive = true;
+
+			fence = dma_resv_excl_fence(obj);
+			if (fence && test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
+					      &fence->flags))
+				fence = NULL;
+		} else {
+			fence = NULL;
+		}
+
+		if (fence) {
+			fence = dma_fence_get_rcu(fence);
+		} else if (all_fences && cursor->fences) {
+			struct dma_resv_list *fences = cursor->fences;
+
+			cursor->is_exclusive = false;
+			while (++cursor->index < fences->shared_count) {
+				fence = rcu_dereference(fences->shared[
+							cursor->index]);
+				if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
+					      &fence->flags))
+					break;
+			}
+			if (cursor->index < fences->shared_count)
+				fence = dma_fence_get_rcu(fence);
+			else
+				fence = NULL;
+		}
+
+		/* For the eventually next round */
+		first = true;
+	} while (read_seqcount_retry(&obj->seq, cursor->seq));
+
+	return fence;
+}
+EXPORT_SYMBOL_GPL(dma_resv_walk_unlocked);
+
  /**
   * dma_resv_copy_fences - Copy all fences from src to dst.
   * @dst: the destination reservation object
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 9100dd3dc21f..f5b91c292ee0 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -149,6 +149,39 @@ struct dma_resv {
  	struct dma_resv_list __rcu *fence;
  };
+/**
+ * struct dma_resv_cursor - current position into the dma_resv fences
+ * @seq: sequence number to check
+ * @index: index into the shared fences
+ * @shared: the shared fences
+ * @is_first: true if this is the first returned fence
+ * @is_exclusive: if the current fence is the exclusive one
+ */
+struct dma_resv_cursor {
+	unsigned int seq;
+	unsigned int index;
+	struct dma_resv_list *fences;
+	bool is_first;
+	bool is_exclusive;
+};
A bit a bikeshed, but I think I'd be nice to align this with the other
iterators we have, e.g. for the drm_connector list.

So struct dma_resv_fence_iter, dma_resv_fence_iter_begin/next/end().

I've renamed the structure to dma_resv_iter.

Also I think the for_each macro must not include begin/end calls. If we
include that then it saves 2 lines of code at the cost of a pile of
awkward bugs because people break; out of the loop or return early  (only
continue is safe) and we leak a fence. Or worse.

Explicit begin/end is much more robust at a very marginal cost imo.

The key point is that this makes it quite a bunch more complicated to implement. See those functions are easiest when you centralize them and try to not spread the functionality into begin/end.

The only thing I could see in the end function would be to drop the reference for the dma_fence and that is not really something I would like to do because we actually need to keep that reference in a bunch of cases.

Regards,
Christian.


Otherwise I think this fence iterator is a solid concept that yeah we
should roll out everywhere.
-Daniel

+
+/**
+ * dma_resv_for_each_fence_unlocked - fence iterator
+ * @obj: a dma_resv object pointer
+ * @cursor: a struct dma_resv_cursor pointer
+ * @all_fences: true if all fences should be returned
+ * @fence: the current fence
+ *
+ * Iterate over the fences in a struct dma_resv object without holding the
+ * dma_resv::lock. The RCU read side lock must be hold when using this, but can
+ * be dropped and re-taken as necessary inside the loop. @all_fences controls
+ * if the shared fences are returned as well.
+ */
+#define dma_resv_for_each_fence_unlocked(obj, cursor, all_fences, fence)    \
+	for (fence = dma_resv_walk_unlocked(obj, cursor, all_fences, true); \
+	     fence; dma_fence_put(fence),				    \
+	     fence = dma_resv_walk_unlocked(obj, cursor, all_fences, false))
+
  #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
  #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
@@ -366,6 +399,9 @@ void dma_resv_fini(struct dma_resv *obj);
  int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences);
  void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence);
  void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence);
+struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj,
+					 struct dma_resv_cursor *cursor,
+					 bool first, bool all_fences);
  int dma_resv_get_fences(struct dma_resv *obj, struct dma_fence **pfence_excl,
  			unsigned *pshared_count, struct dma_fence ***pshared);
  int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src);
--
2.25.1





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