linux-kernel - Re: [PATCH] dma-buf: insert memory barrier before updating num

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Message-ID: <86ddb4dd-6327-4a4b-8584-df366737d636@amd.com>
Date: Mon, 12 May 2025 15:16:33 +0200
From: Christian König <christian.koenig@....com>
To: Hyejeong Choi <hjeong.choi@...sung.com>, sumit.semwal@...aro.org
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-buf: insert memory barrier before updating num_fences

On 5/12/25 13:12, Hyejeong Choi wrote:
> smp_store_mb() inserts memory barrier after storing operation.
> It is different with what the comment is originally aiming so Null
> pointer dereference can be happened if memory update is reordered.
> 
> Signed-off-by: Hyejeong Choi <hjeong.choi@...sung.com>
> ---
>  drivers/dma-buf/dma-resv.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 5f8d010516f0..52af5c7430da 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -320,8 +320,9 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
>  	count++;
>  
>  	dma_resv_list_set(fobj, i, fence, usage);
> -	/* pointer update must be visible before we extend the num_fences */
> -	smp_store_mb(fobj->num_fences, count);
> +	/* fence update must be visible before we extend the num_fences */
> +	smp_wmb();
> +	WRITE_ONCE(fobj->num_fences, count);

The WRITE_ONCE isn't necessary since smp_wmb() implies a compiler barrier, but apart from that really good catch.

Can you modify the patch and re-send? I will be pushing it to -fixes ASAP.

Regards,
Christian.

>  }
>  EXPORT_SYMBOL(dma_resv_add_fence);
>  
> 
>