Re: [PATCH RFCv2 1/3] fs: introduce helper d_path_fast()

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>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Jonathan Corbet <corbet@lwn.net>,
	Luca Coelho <luciano.coelho@intel.com>,
	Kalle Valo <kvalo@codeaurora.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Johannes Berg <johannes.berg@intel.com>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	linux-s390@vger.kernel.org
Subject: Re: [PATCH RFCv2 1/3] fs: introduce helper d_path_fast()
Date: Fri, 28 May 2021 12:44:11 +0000	[thread overview]
Message-ID: <YLDlm94+A8GcNyWL@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <20210528113951.6225-2-justin.he@arm.com>

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)...