Re: [PATCH] bpf: Use refcount_t instead of atomic_t for mmap_count

[Jiri Olsa]

On Mon, Dec 30, 2024 at 03:16:55PM +0800, Pei Xiao wrote:
> Use an API that resembles more the actual use of mmap_count.

I'm not sure I understand the issue, could you provide more details?

thanks,
jirka

> 
> Found by cocci:
> kernel/bpf/arena.c:245:6-25: WARNING: atomic_dec_and_test variation before object free at line 249.
> 
> Fixes: b90d77e5fd78 ("bpf: Fix remap of arena.")
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202412292037.LXlYSHKl-lkp@xxxxxxxxx/
> Signed-off-by: Pei Xiao <xiaopei01@xxxxxxxxxx>
> ---
>  kernel/bpf/arena.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 945a5680f6a5..8caf56a308d9 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -218,7 +218,7 @@ static u64 arena_map_mem_usage(const struct bpf_map *map)
>  struct vma_list {
>  	struct vm_area_struct *vma;
>  	struct list_head head;
> -	atomic_t mmap_count;
> +	refcount_t mmap_count;
>  };
>  
>  static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
> @@ -228,7 +228,7 @@ static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
>  	vml = kmalloc(sizeof(*vml), GFP_KERNEL);
>  	if (!vml)
>  		return -ENOMEM;
> -	atomic_set(&vml->mmap_count, 1);
> +	refcount_set(&vml->mmap_count, 1);
>  	vma->vm_private_data = vml;
>  	vml->vma = vma;
>  	list_add(&vml->head, &arena->vma_list);
> @@ -239,7 +239,7 @@ static void arena_vm_open(struct vm_area_struct *vma)
>  {
>  	struct vma_list *vml = vma->vm_private_data;
>  
> -	atomic_inc(&vml->mmap_count);
> +	refcount_inc(&vml->mmap_count);
>  }
>  
>  static void arena_vm_close(struct vm_area_struct *vma)
> @@ -248,7 +248,7 @@ static void arena_vm_close(struct vm_area_struct *vma)
>  	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
>  	struct vma_list *vml = vma->vm_private_data;
>  
> -	if (!atomic_dec_and_test(&vml->mmap_count))
> +	if (!refcount_dec_and_test(&vml->mmap_count))
>  		return;
>  	guard(mutex)(&arena->lock);
>  	/* update link list under lock */
> -- 
> 2.25.1
> 
>