Re: [Intel-gfx] [PATCH 01/17] drm/i915/selftests: Apply a heavy handed flush to i915_active

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Due to the ordering of cmpxchg()/dma_fence_signal() inside node_retire(),
> we must also use the xchg() as our primary memory barrier to flush the
> outstanding callbacks after expected completion of the i915_active.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/selftests/i915_active.c | 29 ++++++++++++++------
>  1 file changed, 21 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/selftests/i915_active.c b/drivers/gpu/drm/i915/selftests/i915_active.c
> index 3a37c67ab6c4..68bbb1580162 100644
> --- a/drivers/gpu/drm/i915/selftests/i915_active.c
> +++ b/drivers/gpu/drm/i915/selftests/i915_active.c
> @@ -311,20 +311,33 @@ static void spin_unlock_wait(spinlock_t *lock)
>  	spin_unlock_irq(lock);
>  }
>  
> +static void active_flush(struct i915_active *ref,
> +			 struct i915_active_fence *active)
> +{
> +	struct dma_fence *fence;
> +
> +	fence = xchg(__active_fence_slot(active), NULL);
> +	if (!fence)
> +		return;
> +
> +	spin_lock_irq(fence->lock);
> +	__list_del_entry(&active->cb.node);
> +	spin_unlock_irq(fence->lock); /* serialise with fence->cb_list */
> +	atomic_dec(&ref->count);
> +
> +	GEM_BUG_ON(!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags));
> +}
> +
>  void i915_active_unlock_wait(struct i915_active *ref)
>  {
>  	if (i915_active_acquire_if_busy(ref)) {
>  		struct active_node *it, *n;
>  
> +		/* Wait for all active callbacks */
>  		rcu_read_lock();
> -		rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
> -			struct dma_fence *f;
> -
> -			/* Wait for all active callbacks */
> -			f = rcu_dereference(it->base.fence);
> -			if (f)
> -				spin_unlock_wait(f->lock);
> -		}
> +		active_flush(ref, &ref->excl);
> +		rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node)
> +			active_flush(ref, &it->base);
>  		rcu_read_unlock();
>  
>  		i915_active_release(ref);
> -- 
> 2.25.1
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