linux-kernel - DEFINE_FREE/CLASS && code readability (Was: [PATCH v2 2/6] Provide __free(argv) for argv

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Message-ID: <20250105141422.GB23059@redhat.com>
Date: Sun, 5 Jan 2025 15:14:22 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"David S . Miller" <davem@...emloft.net>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tzvetomir Stoyanov <tz.stoyanov@...il.com>,
	Naveen N Rao <naveen@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Jason Baron <jbaron@...mai.com>, Ard Biesheuvel <ardb@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: DEFINE_FREE/CLASS && code readability (Was: [PATCH v2 2/6] Provide
 __free(argv) for argv_split() users)

Masami,

Sorry for abusing this thread. Your patches look fine to me, it is not
that I suggest to change them. I will use your patch as an example for
off-topic discussion.

On 01/05, Masami Hiramatsu (Google) wrote:
>
> +DEFINE_FREE(argv, char **, if (!IS_ERR_OR_NULL(_T)) argv_free(_T))

(IS_ERR looks unneeded but this is cosmetic).

OK, so it can be used as

	void func(void)
	{
		char **argv __free(argv) = argv_split(...);
		do_something(argv);
		return;
	}

And I cry every time when I read the code like this ;)

Because, to understand this code, I need to do the "nontrivial" grep to find
"DEFINE_FREE(argv,".

Perhaps we can establish a simple rule that every DEFINE_FREE() or DEFINE_CLASS()
should add another #define? I mean something like

	DEFINE_FREE(argv, char **, if (!IS_ERR_OR_NULL(_T)) argv_free(_T))
	#define __FREE_ARGV	__free(argv)

	void func(void)
	{
		char **argv __FREE_ARGV = argv_split(...);
		do_something(argv);
		return;
	}

This way I can press Ctrl-] and see what the cleanup code actually does.
Can save a second or two. Important when you try to read the code you are
not familiar with.

Same for DEFINE_CLASS. For example,

	int ksys_fchown(unsigned int fd, uid_t user, gid_t group)
	{
		CLASS(fd, f)(fd);

		if (fd_empty(f))
			return -EBADF;

		return vfs_fchown(fd_file(f), user, group);
	}

If you are not familiar with this code, it looks mysterious until you find
DEFINE_CLASS(fd, ...) in include/linux/file.h.

Oleg.