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@samsung.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..b1ef4546346d 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();
+	fobj->num_fences = count;
 }
 EXPORT_SYMBOL(dma_resv_add_fence);
 
-- 
2.31.1

On 5/13/25 04:06, 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@samsung.com>

I've reviewed, add CC stable and Fixes tags and pushed it to drm-misc-fixes.

Thanks,
Christian.

> ---
>  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..b1ef4546346d 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();
> +	fobj->num_fences = count;
>  }
>  EXPORT_SYMBOL(dma_resv_add_fence);
>  
> 
>