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

[Daniel Vetter]

On Thu, Jul 27, 2017 at 04:03:53PM -0300, Gustavo Padovan wrote:
> 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.
> 
> 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>

Do we have a testcase for this?
-Daniel

> ---
>  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);
> +		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);
> +
> +		dma_fence_put(&pt->base);
>  	}
>  
>  	spin_unlock_irq(&obj->lock);
> -- 
> 2.9.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch