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Date:   Fri, 28 May 2021 12:44:11 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jia He <justin.he@arm.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
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Subject: Re: [PATCH RFCv2 1/3] fs: introduce helper d_path_fast()
Message-ID: <YLDlm94+A8GcNyWL@zeniv-ca.linux.org.uk>
References: <20210528113951.6225-1-justin.he@arm.com>
 <20210528113951.6225-2-justin.he@arm.com>
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On Fri, May 28, 2021 at 07:39:49PM +0800, Jia He wrote:

> +/**
> + * d_path_fast - fast return the full path of a dentry without taking
> + * any seqlock/spinlock. This helper is typical for debugging purpose
> + */
> +char *d_path_fast(const struct path *path, char *buf, int buflen)
> +{
> +	struct path root;
> +	struct mount *mnt = real_mount(path->mnt);
> +	DECLARE_BUFFER(b, buf, buflen);
> +
> +	rcu_read_lock();
> +	get_fs_root_rcu(current->fs, &root);
> +
> +	prepend(&b, "", 1);
> +	__prepend_path(path->dentry, mnt, &root, &b);
> +	rcu_read_unlock();
> +
> +	return extract_string(&b);
> +}
> +EXPORT_SYMBOL(d_path_fast);

Umm...  I'd suggest failing if __prepend_path() returns 3 (at least)...