Re: [syzbot] [media?] [dri?] kernel BUG in sync_timeline_signal

[syzbot <syzbot+cba1a1f459230395a076@syzkaller.appspotmail.com>]

> From: Edward Aadm Davis <eadavis@sina.com>
>
> #syz test

"" does not look like a valid git branch or commit.

>
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index dd07d128adc3..4d885c8e1d4f 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -320,8 +320,9 @@ static inline void dma_fence_put(struct dma_fence *fence)
>  static inline struct dma_fence *dma_fence_get(struct dma_fence *fence)
>  {
>  	if (fence)
> -		kref_get(&fence->refcount);
> -	return fence;
> +		if (kref_get_unless_zero(&fence->refcount))
> +			return fence;
> +	return NULL;
>  }
>  
>  /**
> diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> index 8df20b0218a9..ad60ad5e5cd9 100644
> --- a/drivers/dma-buf/sw_sync.c
> +++ b/drivers/dma-buf/sw_sync.c
> @@ -221,7 +221,8 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
>  		if (!timeline_fence_signaled(&pt->base))
>  			break;
>  
> -		dma_fence_get(&pt->base);
> +		if (!dma_fence_get(&pt->base))
> +			continue;
>  
>  		list_move_tail(&pt->link, &signalled);
>  		rb_erase(&pt->node, &obj->pt_tree);