[PATCH] dma-buf/sw_sync: hold a fence reference when check if it signaled

[Chris Wilson]

Quoting Gustavo Padovan (2017-07-27 20:03:53)
> From: Gustavo Padovan <gustavo.padovan at collabora.com>
> 
> If userspace already dropped its own reference by closing the sw_sync
> fence fd we might end up in a deadlock where
> dma_fence_is_signaled_locked() will trigger the release of the fence a
> thus try to hold the lock to remove the fence from the list.

So the issue here is that call to dma_fence_is_signaled_lock() is
triggering the unreference?
 
> We need to grab a reference to the fence before calling into this chain if
> we want to avoid this issue.
> 
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Gustavo Padovan <gustavo.padovan at collabora.com>
> ---
>  drivers/dma-buf/sw_sync.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> index af1bc84..8291434 100644
> --- a/drivers/dma-buf/sw_sync.c
> +++ b/drivers/dma-buf/sw_sync.c
> @@ -144,11 +144,16 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
>         obj->value += inc;
>  
>         list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
> -               if (!dma_fence_is_signaled_locked(&pt->base))
> +               dma_fence_get(&pt->base);

This would need to be dma_fence_get_rcu() to avoid grabbing the fence
when its refcount has hit 0.

> +               if (!dma_fence_is_signaled_locked(&pt->base)) {
> +                       dma_fence_put(&pt->base);
>                         break;
> +               }
>  
>                 list_del_init(&pt->link);
>                 rb_erase(&pt->node, &obj->pt_tree);

But if I understand correctly, we just need to unlink first, then
signal.

list_for_each_entry_safe() {
	if (!timeline_fence_signaled(&pt->base))
		break;

	list_del_init(&pt->link);
	rb_erase(&pt->node, &obj->pt_tree);

	dma_fence_signal_locked(&pt->base);
}

The challenge is in writing the comment to explain the open-coding.
-Chris