Re: [PATCH] erofs: initialized fields can only be observed after bit is set

[Chao Yu]

Hi Xiang,

On 2021/2/9 21:06, Gao Xiang via Linux-erofs wrote:
> From: Gao Xiang <hsiangkao@xxxxxxxxxx>
>
> Currently, although set_bit() & test_bit() pairs are used as a fast-
> path for initialized configurations. However, these atomic ops are
> actually relaxed forms. Instead, load-acquire & store-release form is
> needed to make sure uninitialized fields won't be observed in advance
> here (yet no such corresponding bitops so use full barriers instead.)
>
> Fixes: 62dc45979f3f ("staging: erofs: fix race of initializing xattrs of a inode at the same time")
> Fixes: 152a333a5895 ("staging: erofs: add compacted compression indexes support")
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.3+
> Reported-by: Huang Jianan <huangjianan@xxxxxxxx>
> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxx>
> ---
>   fs/erofs/xattr.c | 10 +++++++++-
>   fs/erofs/zmap.c  | 10 +++++++++-
>   2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/fs/erofs/xattr.c b/fs/erofs/xattr.c
> index 5bde77d70852..47314a26767a 100644
> --- a/fs/erofs/xattr.c
> +++ b/fs/erofs/xattr.c
> @@ -48,8 +48,14 @@ static int init_inode_xattrs(struct inode *inode)
>   	int ret = 0;
>   
>   	/* the most case is that xattrs of this inode are initialized. */
> -	if (test_bit(EROFS_I_EA_INITED_BIT, &vi->flags))
> +	if (test_bit(EROFS_I_EA_INITED_BIT, &vi->flags)) {
> +		/*
> +		 * paired with smp_mb() at the end of the function to ensure
> +		 * fields will only be observed after the bit is set.
> +		 */
> +		smp_mb();

I can understand below usage, since w/o smp_mb(), xattr initialization
could be done after set_bit(EROFS_I_EA_INITED_BIT), then other apps could
see out-of-update xattr info after that bit check.

So what out-of-order execution do we need to avoid by adding above barrier?

Thanks,

> +	/* paired with smp_mb() at the beginning of the function. */
> +	smp_mb();
>   	set_bit(EROFS_I_EA_INITED_BIT, &vi->flags);