How does the Linux kernel determine the order of __init calls?
All the init magic are implemented in files:
- include/asm-generic/vmlinux.lds.h
- include/linux/init.h
- init/main.c
Firstly, look at include/asm-generic/vmlinux.lds.h that contains the following:
13 * . = START;
14 * __init_begin = .;
15 * HEAD_TEXT_SECTION
16 * INIT_TEXT_SECTION(PAGE_SIZE)
17 * INIT_DATA_SECTION(...)
18 * PERCPU_SECTION(CACHELINE_SIZE)
19 * __init_end = .;
Where INIT_TEXT_SECTION and INIT_DATA_SECTION defined as follow:
790 #define INIT_TEXT_SECTION(inittext_align) \
791 . = ALIGN(inittext_align); \
792 .init.text : AT(ADDR(.init.text) - LOAD_OFFSET) { \
793 VMLINUX_SYMBOL(_sinittext) = .; \
794 INIT_TEXT \
795 VMLINUX_SYMBOL(_einittext) = .; \
796 }
797
798 #define INIT_DATA_SECTION(initsetup_align) \
799 .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { \
800 INIT_DATA \
801 INIT_SETUP(initsetup_align) \
802 INIT_CALLS \
803 CON_INITCALL \
804 SECURITY_INITCALL \
805 INIT_RAM_FS \
806 }
Let's look at INIT_CALLS defines for example:
628 #define INIT_CALLS_LEVEL(level) \
629 VMLINUX_SYMBOL(__initcall##level##_start) = .; \
630 *(.initcall##level##.init) \
631 *(.initcall##level##s.init)
633 #define INIT_CALLS \
634 VMLINUX_SYMBOL(__initcall_start) = .; \
635 *(.initcallearly.init) \
636 INIT_CALLS_LEVEL(0) \
637 INIT_CALLS_LEVEL(1) \
638 INIT_CALLS_LEVEL(2) \
639 INIT_CALLS_LEVEL(3) \
640 INIT_CALLS_LEVEL(4) \
641 INIT_CALLS_LEVEL(5) \
642 INIT_CALLS_LEVEL(rootfs) \
643 INIT_CALLS_LEVEL(6) \
644 INIT_CALLS_LEVEL(7) \
645 VMLINUX_SYMBOL(__initcall_end) = .;
You can see the this defines the sections names that marked with.initcall.... And all the marked data gets into the __initcall_start .. __initcall_end range.
Now let's look at the [include/linux/init.h that contains the following:
44 #define __init __section(.init.text) __cold notrace
45 #define __initdata __section(.init.data)
And further:
189 #define __define_initcall(level,fn,id) \
190 static initcall_t __initcall_##fn##id __used \
191 __attribute__((__section__(".initcall" level ".init"))) = fn
...
220 #define device_initcall(fn) __define_initcall("6",fn,6)
...
225 #define __initcall(fn) device_initcall(fn)
...
271 /**
272 * module_init() - driver initialization entry point
273 * @x: function to be run at kernel boot time or module insertion
274 *
275 * module_init() will either be called during do_initcalls() (if
276 * builtin) or at module insertion time (if a module). There can only
277 * be one per module.
278 */
279 #define module_init(x) __initcall(x);
So you can see that module_init defined as __initcall that defined as device_initcall that defined as __define_initcall("6",fn,6). Six here means initcall level. See below...
init/main.c contains the following:
711 extern initcall_t __initcall_start[];
712 extern initcall_t __initcall0_start[];
713 extern initcall_t __initcall1_start[];
714 extern initcall_t __initcall2_start[];
715 extern initcall_t __initcall3_start[];
716 extern initcall_t __initcall4_start[];
717 extern initcall_t __initcall5_start[];
718 extern initcall_t __initcall6_start[];
719 extern initcall_t __initcall7_start[];
720 extern initcall_t __initcall_end[];
721
722 static initcall_t *initcall_levels[] __initdata = {
723 __initcall0_start,
724 __initcall1_start,
725 __initcall2_start,
726 __initcall3_start,
727 __initcall4_start,
728 __initcall5_start,
729 __initcall6_start,
730 __initcall7_start,
731 __initcall_end,
732 };
733
734 /* Keep these in sync with initcalls in include/linux/init.h */
735 static char *initcall_level_names[] __initdata = {
736 "early",
737 "core",
738 "postcore",
739 "arch",
740 "subsys",
741 "fs",
742 "device",
743 "late",
744 };
745
746 static void __init do_initcall_level(int level)
747 {
748 extern const struct kernel_param __start___param[], __stop___param[];
749 initcall_t *fn;
750
751 strcpy(static_command_line, saved_command_line);
752 parse_args(initcall_level_names[level],
753 static_command_line, __start___param,
754 __stop___param - __start___param,
755 level, level,
756 &repair_env_string);
757
758 for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++)
759 do_one_initcall(*fn);
760 }
761
762 static void __init do_initcalls(void)
763 {
764 int level;
765
766 for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++)
767 do_initcall_level(level);
768 }
As you can see do_initcall simply iterates over all the initcall levels and calls do_initcall_level for each one that calls do_one_initcall for each level's entry.
Let's note also that kernel discards all the __init functions after execution. So they don't take place in memory after the kernel loads.
That's all.