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

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

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@chris-wilson.co.uk>
> ---
>  .../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.

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