linux-kernel - Re: [PATCH] sysctl: Add a group of macro functions to initcall the sysctl table of each feature

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Date:   Mon, 6 Dec 2021 17:38:42 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Xiaoming Ni <nixiaoming@...wei.com>
Cc:     <linux-kernel@...r.kernel.org>, <mcgrof@...nel.org>,
        <viro@...iv.linux.org.uk>, <ebiederm@...ssion.com>,
        <keescook@...omium.org>, <jlayton@...nel.org>,
        <bfields@...ldses.org>, <yzaikin@...gle.com>, <wangle6@...wei.com>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] sysctl: Add a group of macro functions to initcall the
 sysctl table of each feature

On Tue, 7 Dec 2021 09:13:20 +0800 Xiaoming Ni <nixiaoming@...wei.com> wrote:

> To avoid duplicated code, add a set of macro functions to initialize the
> sysctl table for each feature.
> 
> The system initialization process is as follows:
> 
> 	start_kernel () {
> 		...
> 		/* init proc and sysctl base,
> 		 * proc_root_init()-->proc_sys_init()-->sysctl_init_bases()
> 		 */
> 		proc_root_init(); /* init proc and sysctl base */
> 		...
> 		arch_call_rest_init();
> 	}
> 
> 	arch_call_rest_init()-->rest_init()-->kernel_init()
> 	kernel_init() {
> 		...
> 		kernel_init_freeable(); /* do all initcalls */
> 		...
> 		do_sysctl_args(); /* Process the sysctl parameter: sysctl.*= */
> 	}
> 
> 	kernel_init_freeable()--->do_basic_setup()-->do_initcalls()
> 	do_initcalls() {
> 		for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++) {
> 			do_initcall_level
> 	}
> 
> The sysctl interface of each subfeature should be registered after
> sysctl_init_bases() and before do_sysctl_args(). It seems that the sysctl
> interface does not depend on initcall_levels. To prevent the sysctl
> interface from being initialized before the feature itself. The
> lowest-level late_initcall() is used as the common sysctl interface
> registration level.

I'm not normally a fan of wrapping commonly-used code sequences into
magical macros, but this one does seem to make sense.

I wonder if it is possible to cook up a checkpatch rule to tell people
to henceforth use the magic macros rather than to open-code things in
the old way.  Sounds hard.

> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -943,12 +943,7 @@ static struct ctl_table coredump_sysctls[] = {
>  	{ }
>  };
>  
> -static int __init init_fs_coredump_sysctls(void)
> -{
> -	register_sysctl_init("kernel", coredump_sysctls);
> -	return 0;
> -}
> -fs_initcall(init_fs_coredump_sysctls);
> +kernel_sysctl_initcall(coredump_sysctls);

But this and several like it are functional changes.

>  #endif /* CONFIG_SYSCTL */
>
> ...
>
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -132,12 +132,7 @@ static struct ctl_table inodes_sysctls[] = {
>  	{ }
>  };
>  
> -static int __init init_fs_inode_sysctls(void)
> -{
> -	register_sysctl_init("fs", inodes_sysctls);
> -	return 0;
> -}
> -early_initcall(init_fs_inode_sysctls);
> +fs_sysctl_initcall(inodes_sysctls);
>  #endif

Here's another, of many.

Someone made the decision to use early_initcall() here (why?) and this
patch switches it to late_initcall()!  Worrisome.  Each such stealth
conversion should be explained and justified, shouldn't it?