linux-kernel - Re: [PATCH RFCv2 1/3] fs: introduce helper d_path

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Date:   Fri, 28 May 2021 12:44:11 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jia He <justin.he@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Johannes Berg <johannes.berg@...el.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH RFCv2 1/3] fs: introduce helper d_path_fast()

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)...