Re: [PATCH 10/20] drm/i915: Export a preallocate variant of i915_active_acquire()

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On 09/07/2020 15:36, Maarten Lankhorst wrote:
Op 06-07-2020 om 08:19 schreef Chris Wilson:
Sometimes we have to be very careful not to allocate underneath a mutex
(or spinlock) and yet still want to track activity. Enter
i915_active_acquire_for_context(). This raises the activity counter on
i915_active prior to use and ensures that the fence-tree contains a slot
for the context.

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
  .../gpu/drm/i915/gem/i915_gem_execbuffer.c    |   2 +-
  drivers/gpu/drm/i915/gt/intel_timeline.c      |   4 +-
  drivers/gpu/drm/i915/i915_active.c            | 113 +++++++++++++++---
  drivers/gpu/drm/i915/i915_active.h            |  14 ++-
  4 files changed, 113 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 1015c4fd9f3e..6d20be29ff3c 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -1789,7 +1789,7 @@ __parser_mark_active(struct i915_vma *vma,
  {
  	struct intel_gt_buffer_pool_node *node = vma->private;
- return i915_active_ref(&node->active, tl, fence);
+	return i915_active_ref(&node->active, tl->fence_context, fence);
  }
static int
diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
index 4546284fede1..e4a5326633b8 100644
--- a/drivers/gpu/drm/i915/gt/intel_timeline.c
+++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
@@ -479,7 +479,9 @@ __intel_timeline_get_seqno(struct intel_timeline *tl,
  	 * free it after the current request is retired, which ensures that
  	 * all writes into the cacheline from previous requests are complete.
  	 */
-	err = i915_active_ref(&tl->hwsp_cacheline->active, tl, &rq->fence);
+	err = i915_active_ref(&tl->hwsp_cacheline->active,
+			      tl->fence_context,
+			      &rq->fence);
  	if (err)
  		goto err_cacheline;
diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index d960d0be5bd2..3f595446fd44 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -217,11 +217,10 @@ excl_retire(struct dma_fence *fence, struct dma_fence_cb *cb)
  }
static struct i915_active_fence *
-active_instance(struct i915_active *ref, struct intel_timeline *tl)
+active_instance(struct i915_active *ref, u64 idx)
  {
  	struct active_node *node, *prealloc;
  	struct rb_node **p, *parent;
-	u64 idx = tl->fence_context;
/*
  	 * We track the most recently used timeline to skip a rbtree search
@@ -353,21 +352,17 @@ __active_del_barrier(struct i915_active *ref, struct active_node *node)
  	return ____active_del_barrier(ref, node, barrier_to_engine(node));
  }
-int i915_active_ref(struct i915_active *ref,
-		    struct intel_timeline *tl,
-		    struct dma_fence *fence)
+int i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence)
  {
  	struct i915_active_fence *active;
  	int err;
- lockdep_assert_held(&tl->mutex);
-
  	/* Prevent reaping in case we malloc/wait while building the tree */
  	err = i915_active_acquire(ref);
  	if (err)
  		return err;
- active = active_instance(ref, tl);
+	active = active_instance(ref, idx);
  	if (!active) {
  		err = -ENOMEM;
  		goto out;
@@ -384,32 +379,104 @@ int i915_active_ref(struct i915_active *ref,
  		atomic_dec(&ref->count);
  	}
  	if (!__i915_active_fence_set(active, fence))
-		atomic_inc(&ref->count);
+		__i915_active_acquire(ref);
out:
  	i915_active_release(ref);
  	return err;
  }
-struct dma_fence *
-i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
+static struct dma_fence *
+__i915_active_set_fence(struct i915_active *ref,
+			struct i915_active_fence *active,
+			struct dma_fence *fence)
  {
  	struct dma_fence *prev;
/* We expect the caller to manage the exclusive timeline ordering */
  	GEM_BUG_ON(i915_active_is_idle(ref));
+ if (is_barrier(active)) { /* proto-node used by our idle barrier */
+		/*
+		 * This request is on the kernel_context timeline, and so
+		 * we can use it to substitute for the pending idle-barrer
+		 * request that we want to emit on the kernel_context.
+		 */
+		__active_del_barrier(ref, node_from_active(active));
+		RCU_INIT_POINTER(active->fence, NULL);
+		atomic_dec(&ref->count);
+	}
+
  	rcu_read_lock();
-	prev = __i915_active_fence_set(&ref->excl, f);
+	prev = __i915_active_fence_set(active, fence);
  	if (prev)
  		prev = dma_fence_get_rcu(prev);
  	else
-		atomic_inc(&ref->count);
+		__i915_active_acquire(ref);
  	rcu_read_unlock();
return prev;
  }
+static struct i915_active_fence *
+__active_lookup(struct i915_active *ref, u64 idx)
+{
+	struct active_node *node;
+	struct rb_node *p;
+
+	/* Like active_instance() but with no malloc */
+
+	node = READ_ONCE(ref->cache);
+	if (node && node->timeline == idx)
+		return &node->base;
+
+	spin_lock_irq(&ref->tree_lock);
+	GEM_BUG_ON(i915_active_is_idle(ref));
+
+	p = ref->tree.rb_node;
+	while (p) {
+		node = rb_entry(p, struct active_node, node);
+		if (node->timeline == idx) {
+			ref->cache = node;
+			spin_unlock_irq(&ref->tree_lock);
+			return &node->base;
+		}
+
+		if (node->timeline < idx)
+			p = p->rb_right;
+		else
+			p = p->rb_left;
+	}
+
+	spin_unlock_irq(&ref->tree_lock);
+
+	return NULL;
+}
+
+struct dma_fence *
+__i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence)
+{
+	struct dma_fence *prev = ERR_PTR(-ENOENT);
+	struct i915_active_fence *active;
+
+	if (!i915_active_acquire_if_busy(ref))
+		return ERR_PTR(-EINVAL);
+
+	active = __active_lookup(ref, idx);
+	if (active)
+		prev = __i915_active_set_fence(ref, active, fence);
+
+	i915_active_release(ref);
+	return prev;
+}
+
+struct dma_fence *
+i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
+{
+	/* We expect the caller to manage the exclusive timeline ordering */
+	return __i915_active_set_fence(ref, &ref->excl, f);
+}
+
  bool i915_active_acquire_if_busy(struct i915_active *ref)
  {
  	debug_active_assert(ref);
@@ -443,6 +510,24 @@ int i915_active_acquire(struct i915_active *ref)
  	return err;
  }
+int i915_active_acquire_for_context(struct i915_active *ref, u64 idx)
+{
+	struct i915_active_fence *active;
+	int err;
+
+	err = i915_active_acquire(ref);
+	if (err)
+		return err;
+
+	active = active_instance(ref, idx);
+	if (!active) {
+		i915_active_release(ref);
+		return -ENOMEM;
+	}
+
+	return 0; /* return with active ref */
+}
+
  void i915_active_release(struct i915_active *ref)
  {
  	debug_active_assert(ref);
@@ -804,7 +889,7 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
  			 */
  			RCU_INIT_POINTER(node->base.fence, ERR_PTR(-EAGAIN));
  			node->base.cb.node.prev = (void *)engine;
-			atomic_inc(&ref->count);
+			__i915_active_acquire(ref);
  		}
  		GEM_BUG_ON(rcu_access_pointer(node->base.fence) != ERR_PTR(-EAGAIN));
diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h
index cf4058150966..2e0bcb3289ec 100644
--- a/drivers/gpu/drm/i915/i915_active.h
+++ b/drivers/gpu/drm/i915/i915_active.h
@@ -163,14 +163,18 @@ void __i915_active_init(struct i915_active *ref,
  	__i915_active_init(ref, active, retire, &__mkey, &__wkey);	\
  } while (0)
-int i915_active_ref(struct i915_active *ref,
-		    struct intel_timeline *tl,
-		    struct dma_fence *fence);
+struct dma_fence *
+__i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence);
+int i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence);
static inline int
  i915_active_add_request(struct i915_active *ref, struct i915_request *rq)
  {
-	return i915_active_ref(ref, i915_request_timeline(rq), &rq->fence);
+	struct intel_timeline *tl = i915_request_timeline(rq);
+
+	lockdep_assert_held(&tl->mutex);
+
+	return i915_active_ref(ref, tl->fence_context, &rq->fence);
  }
struct dma_fence *
@@ -198,7 +202,9 @@ int i915_request_await_active(struct i915_request *rq,
  #define I915_ACTIVE_AWAIT_BARRIER BIT(2)
int i915_active_acquire(struct i915_active *ref);
+int i915_active_acquire_for_context(struct i915_active *ref, u64 idx);
  bool i915_active_acquire_if_busy(struct i915_active *ref);
+
  void i915_active_release(struct i915_active *ref);
static inline void __i915_active_acquire(struct i915_active *ref)

Somewhere this is hiding a locking inversion in the later part of this series.

tl->mutex needs to be an inner lock of ww objects, see my patch series.

Since this is not the case, you're somehow hiding a locking inversion.

Which locks you think and from where? I want to make sure I am not missing something while reading the series..

There used to be (still is in upstream) a possible deadlock with async get pages if batch buffer was an userptr object, but this series solves that by pre-allocating page directories in the reservation phase. That's not a plain locking inversion, but maybe that's what you had in mind?

Does it take tl->mutex undeer dma_fence_wait? I can't see that it does.

Can't think of any other options at the moment.

Regards,

Tvrtko
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