Re: [PATCH] fs: need to ensure that dentry is initialized before it is added to parent dentry

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Sep 11, 2019 at 01:05:37AM +0800, zhengbin wrote:
> diff --git a/fs/dcache.c b/fs/dcache.c
> index e88cf05..0a07671 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1767,6 +1767,16 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
>  	struct dentry *dentry = __d_alloc(parent->d_sb, name);
>  	if (!dentry)
>  		return NULL;
> +
> +	/*
> +	 * need to ensure that dentry->d_child.next is initialized
> +	 * (__d_alloc->INIT_LIST_HEAD) before dentry is added to
> +	 * parent->d_subdirs, Otherwise in next_positive(do not have
> +	 * spin_lock), we may visit uninitialized value because of cpu
> +	 * run optimization(first add dentry to parent->d_subdirs).
> +	 */
> +	smp_wmb();
> +
>  	spin_lock(&parent->d_lock);
>  	/*
>  	 * don't need child lock because it is not subject
> --
> 2.7.4

Unfortunately, that's not all - see the reply upthread ;-/