On 17/09/2021 14:23, Daniel Vetter wrote:

On Fri, Sep 17, 2021 at 02:34:48PM +0200, Christian König wrote:

quoted

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

Signed-off-by: Christian König <christian.koenig@amd.com>
---
  drivers/dma-buf/dma-resv.c | 61 +++++++++++++++++++++++++++
  include/linux/dma-resv.h   | 84 ++++++++++++++++++++++++++++++++++++++
  2 files changed, 145 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 84fbe60629e3..3e77cad2c9d4 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c

@@ -323,6 +323,67 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
  }
  EXPORT_SYMBOL(dma_resv_add_excl_fence);
  
+/**
+ * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj
+ * @cursor: cursor to record the current position
+ * @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 iterration is started over again.
+ */
+struct dma_fence *dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor,

Bit ocd, but I'd still just call that iter_next.

quoted

+					      bool first)

Hm I'd put all the init code into iter_begin ...

@Christian:

Could you engineer something in here which would, at least in debug 
builds, catch failures to call "iter begin" before using the iterator macro?

quoted

+{
+	struct dma_resv *obj = cursor->obj;

Aren't we missing rcu_read_lock() around the entire thing here?

quoted

+
+	first |= read_seqcount_retry(&obj->seq, cursor->seq);
+	do {
+		/* Drop the reference from the previous round */
+		dma_fence_put(cursor->fence);
+
+		cursor->is_first = first;
+		if (first) {
+			cursor->seq = read_seqcount_begin(&obj->seq);
+			cursor->index = -1;
+			cursor->fences = dma_resv_shared_list(obj);

And then also call iter_begin from here. That way we guarantee that
read_seqcount_begin is always called before _retry(). It's not a problem
with the seqcount implementation (I think at least), but it definitely
looks funny.

Calling iter_begin here also makes it clear that we're essentially
restarting.

quoted

+
+			cursor->fence = dma_resv_excl_fence(obj);
+			if (cursor->fence &&
+			    test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,

Please use the right dma_fence wrapper here for this and don't look at the
bits/flags outside of dma_fence.[hc] code. I just realized that we don't
have the right amount of barriers in there for the fastpath, i.e. if we
have:

x = 0; /* static initializer */

thread a
	x = 1;
	dma_fence_signal(fence);


thread b;
	if (dma_fence_is_signalled(fence))
		printk("%i\n", x);

Then you might actually be able to observe x == 0 in thread b. Which is
not what we want at all.

@Daniel:

What do you mean here - in terms of if 'x' is "external" (not part of 
dma-fence), then are you suggesting dma-fence code should serialise it 
by using barriers?

That would sound incorrect to me, or in other words, I think it's fine 
if x == 0 is observed in your example thread B since that code is mixing 
external data with dma-fence.

Hm also, there is that annoying bit where by using dma_fence_is_signaled 
any code becomes a fence signaling critical path, which I never bought 
into. There should be a way to test the signaled status without actually 
doing the signaling. Or I am misunderstanding something so badly that is 
really really has to be like this?

So no open-coding of dma_fence flag bits code outside of drm_fence.[hc]
please. And yes i915-gem code is unfortunately a disaster.

Don't even miss an opportunity for some good trashing no? :D

But yes, deconstructed dma_fence_signal I thought we were supposed to 
add to core. Or at least propose, don't exactly remember how that went.

quoted

+				     &cursor->fence->flags))
+				cursor->fence = NULL;
+		} else {
+			cursor->fence = NULL;
+		}
+
+		if (cursor->fence) {
+			cursor->fence = dma_fence_get_rcu(cursor->fence);
+		} else if (cursor->all_fences && cursor->fences) {
+			struct dma_resv_list *fences = cursor->fences;
+
+			while (++cursor->index < fences->shared_count) {
+				cursor->fence = rcu_dereference(
+					fences->shared[cursor->index]);
+				if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
+					      &cursor->fence->flags))
+					break;
+			}
+			if (cursor->index < fences->shared_count)
+				cursor->fence =
+					dma_fence_get_rcu(cursor->fence);
+			else
+				cursor->fence = NULL;
+		}

The control flow here is very hairy, but I'm not sure how to best do this.
With my suggestion to move the read_seqcount_begin into iter_begin maybe
something like this:

iter_next()
{
	do {
		dma_fence_put(cursor->fence)
		cursor->fence = NULL;

		if (cursor->index == -1) { /* reset by iter_begin()
			cursor->fence = get_exclusive();
			cusor->index++;
		} else {
			cursor->fence = shared_fences[++cursor->index]
		}

		if (!dma_fence_is_signalled(cursor->fence))
			continue; /* just grab the next fence. */

		cursor->fence =  dma_fence_get_rcu(cursor->fence);

		if (!cursor->fence || read_seqcount_retry()) {
			/* we lost the race, restart completely */
			iter_begin(); /* ->fence will be cleaned up at beginning of the loop */
			continue;
		}

		return cursor->fence;
	} while (true);
}

Maybe I missed something, but that avoids the duplication of all the
tricky code, i.e. checking for signalling, rcu protected conditional
fence_get, and the retry is also nicely at the end.

quoted

+
+		/* For the eventually next round */
+		first = true;
+	} while (read_seqcount_retry(&obj->seq, cursor->seq));
+
+	return cursor->fence;
+}
+EXPORT_SYMBOL_GPL(dma_resv_iter_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..693d16117153 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h

@@ -149,6 +149,90 @@ struct dma_resv {
  	struct dma_resv_list __rcu *fence;
  };
  
+/**
+ * struct dma_resv_iter - current position into the dma_resv fences
+ *
+ * Don't touch this directly in the driver, use the accessor function instead.
+ */
+struct dma_resv_iter {
+	/** @obj: The dma_resv object we iterate over */
+	struct dma_resv *obj;
+
+	/** @all_fences: If all fences should be returned */
+	bool all_fences;
+
+	/** @fence: the currently handled fence */
+	struct dma_fence *fence;
+
+	/** @seq: sequence number to check for modifications */
+	unsigned int seq;
+
+	/** @index: index into the shared fences */

If you go with my suggestion (assuming it works): Please add "-1 indicates
to pick the exclusive fence instead."

quoted

+	unsigned int index;
+
+	/** @fences: the shared fences */
+	struct dma_resv_list *fences;
+
+	/** @is_first: true if this is the first returned fence */
+	bool is_first;

I think if we just rely on -1 == exclusive fence/is_first we don't need
this one here?

quoted

+};
+
+struct dma_fence *dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor,
+					      bool first);
+
+/**
+ * dma_resv_iter_begin - initialize a dma_resv_iter object
+ * @cursor: The dma_resv_iter object to initialize
+ * @obj: The dma_resv object which we want to iterator over
+ * @all_fences: If all fences should be returned or just the exclusive one

Please add: "Callers must clean up the iterator with dma_resv_iter_end()."

quoted

+ */
+static inline void dma_resv_iter_begin(struct dma_resv_iter *cursor,
+					struct dma_resv *obj,
+					bool all_fences)
+{
+	cursor->obj = obj;
+	cursor->all_fences = all_fences;
+	cursor->fence = NULL;
+}
+
+/**
+ * dma_resv_iter_end - cleanup a dma_resv_iter object
+ * @cursor: the dma_resv_iter object which should be cleaned up
+ *
+ * Make sure that the reference to the fence in the cursor is properly
+ * dropped.

Please add:

"This function must be called every time dma_resv_iter_begin() was called
to clean up any references."

quoted

+ */
+static inline void dma_resv_iter_end(struct dma_resv_iter *cursor)
+{
+	dma_fence_put(cursor->fence);
+}
+
+/**
+ * dma_resv_iter_is_exclusive - test if the current fence is the exclusive one
+ * @cursor: the cursor of the current position
+ *
+ * Returns true if the currently returned fence is the exclusive one.
+ */
+static inline bool dma_resv_iter_is_exclusive(struct dma_resv_iter *cursor)
+{
+	return cursor->index == -1;
+}
+
+/**
+ * dma_resv_for_each_fence_unlocked - unlocked fence iterator
+ * @cursor: a struct dma_resv_iter pointer
+ * @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. The cursor needs to be
+ * initialized with dma_resv_iter_begin_unlocked() and cleaned up with

We don't have an _unlocked version?

@Christian:

I'd also mention that the fence reference is held during the walk so 
someone is less likely to grab extra ones.

quoted

+ * dma_resv_iter_end_unlocked().
+ */
+#define dma_resv_for_each_fence_unlocked(cursor, fence)			\
+	for (fence = dma_resv_iter_walk_unlocked(cursor, true);		\
+	     fence; fence = dma_resv_iter_walk_unlocked(cursor, false))
+
  #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
  #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
  
-- 
2.25.1

Regards,

Tvrtko