The initcall mechanism

Introduction

As you may understand from the title, this part will cover an interesting and important concept in the Linux kernel which is called initcall. We already saw definitions like these:

early_param("debug", debug_kernel);

or

arch_initcall(init_pit_clocksource);

in some parts of the Linux kernel. Before we will see how this mechanism is implemented in the Linux kernel, we must know actually what is it and how the Linux kernel uses it. Definitions like these represent a callback function which is called during Linux kernel initialization. Actually the main point of the initcall mechanism is to determine correct order of the built-in modules and subsystems initialization. For example let's look at the following function:

static int __init nmi_warning_debugfs(void)
{
debugfs_create_u64("nmi_longest_ns", 0644,
arch_debugfs_dir, &nmi_longest_ns);
return 0;
}
fs_initcall(nmi_warning_debugfs);

from the arch/x86/kernel/nmi.c source code file. As we may see it just creates the nmi_longest_ns debugfs file in the arch_debugfs_dir directory. Actually, this debugfs file may be created only after the arch_debugfs_dir will be created. Creation of this directory occurs during the architecture-specific initialization of the Linux kernel. Actually this directory will be created in the arch_kdebugfs_init function from the arch/x86/kernel/kdebugfs.c source code file. Note that the arch_kdebugfs_init function is marked as initcall too:

arch_initcall(arch_kdebugfs_init);

The Linux kernel calls all architecture-specific initcalls before the fs related initcalls. So, our nmi_longest_ns file will be created only after the arch_kdebugfs_dir directory will be created. Actually, the Linux kernel provides eight levels of main initcalls:

  • early;

  • core;

  • postcore;

  • arch;

  • subsys;

  • fs;

  • device;

  • late.

All of their names are represented by the initcall_level_names array which is defined in the init/main.c source code file:

static char *initcall_level_names[] __initdata = {
"early",
"core",
"postcore",
"arch",
"subsys",
"fs",
"device",
"late",
};

All functions which are marked as initcall by these identifiers, will be called in the same order presented in the initcall_level_names array, in other words, at first early initcalls will be called, then core initcalls and so forth. From this moment we know a little about initcall mechanism, so we can start to dive into the source code of the Linux kernel to see how this mechanism is implemented.

Implementation initcall mechanism in the Linux kernel

The Linux kernel provides a set of macros from the include/linux/init.h header file to mark a given function as initcall. All of these macros are pretty simple:

#define early_initcall(fn) __define_initcall(fn, early)
#define core_initcall(fn) __define_initcall(fn, 1)
#define postcore_initcall(fn) __define_initcall(fn, 2)
#define arch_initcall(fn) __define_initcall(fn, 3)
#define subsys_initcall(fn) __define_initcall(fn, 4)
#define fs_initcall(fn) __define_initcall(fn, 5)
#define device_initcall(fn) __define_initcall(fn, 6)
#define late_initcall(fn) __define_initcall(fn, 7)

and as we may see these macros just expand to the call of the __define_initcall macro from the same header file. Moreover, the __define_initcall macro takes two arguments:

  • fn - callback function which will be called during call of initcalls of the certain level;

  • id - identifier to identify initcall to prevent error when two the same initcalls point to the same handler.

The implementation of the __define_initcall macro looks like:

#define __define_initcall(fn, id) \
static initcall_t __initcall_##fn##id __used \
__attribute__((__section__(".initcall" #id ".init"))) = fn; \
LTO_REFERENCE_INITCALL(__initcall_##fn##id)

To understand the __define_initcall macro, first of all let's look at the initcall_t type. This type is defined in the same header file and it represents pointer to a function which returns integer which will be result of the initcall:

typedef int (*initcall_t)(void);

Now let's return to the __define_initcall macro. The ## provides ability to concatenate two symbols. In our case, the first line of the __define_initcall macro produces the definition of a given function, __initcall_<function-name>_<id>, which is located in the .initcall <id> .init ELF section and marked with the __user gcc attribute. If we look at include/asm-generic/vmlinux.lds.h header file, which represents data for the kernel linker script, we will see that all of initcalls sections will be placed in the .data section:

#define INIT_CALLS \
VMLINUX_SYMBOL(__initcall_start) = .; \
*(.initcallearly.init) \
INIT_CALLS_LEVEL(0) \
INIT_CALLS_LEVEL(1) \
INIT_CALLS_LEVEL(2) \
INIT_CALLS_LEVEL(3) \
INIT_CALLS_LEVEL(4) \
INIT_CALLS_LEVEL(5) \
INIT_CALLS_LEVEL(rootfs) \
INIT_CALLS_LEVEL(6) \
INIT_CALLS_LEVEL(7) \
VMLINUX_SYMBOL(__initcall_end) = .;
#define INIT_DATA_SECTION(initsetup_align) \
.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { \
... \
INIT_CALLS \
... \
}

and their names are going to be as follows (got from System.map):

...
ffffffff8320ce60 t __initcall_arch_kdebugfs_init3
...
ffffffff8320d0e0 t __initcall_nmi_warning_debugfs5
...

The attribute __used is defined in the include/linux/compiler-gcc.h header file and it expands to the definition of the following gcc attribute:

#define __used __attribute__((__used__))

which prevents variable defined but not used warning. The last line of the __define_initcall macro is:

LTO_REFERENCE_INITCALL(__initcall_##fn##id)

depends on the CONFIG_LTO kernel configuration option and just provides stub for the compiler Link time optimization:

#ifdef CONFIG_LTO
#define LTO_REFERENCE_INITCALL(x) \
static __used __exit void *reference_##x(void) \
{ \
return &x; \
}
#else
#define LTO_REFERENCE_INITCALL(x)
#endif

In order to prevent any problem when there is no reference to a variable in a module, it will be moved to the end of the program. That's all about the __define_initcall macro. So, all of the *_initcall macros will be expanded during compilation of the Linux kernel, and all initcalls will be placed in their sections and all of them will be available from the .data section and the Linux kernel will know where to find a certain initcall to call it during initialization process.

As initcalls can be called by the Linux kernel, let's look how the Linux kernel does this. This process starts in the do_basic_setup function from the init/main.c source code file:

static void __init do_basic_setup(void)
{
...
...
...
do_initcalls();
...
...
...
}

which is called during the initialization of the Linux kernel, right after main steps of initialization like memory manager related initialization, CPU subsystem and others are already finished. The do_initcalls function just goes through the array of initcall levels and call the do_initcall_level function for each level:

static void __init do_initcalls(void)
{
int level;
for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++)
do_initcall_level(level);
}

The initcall_levels array is defined in the same source code file and contains pointers to the sections which were defined in the __define_initcall macro:

static initcall_t *initcall_levels[] __initdata = {
__initcall0_start,
__initcall1_start,
__initcall2_start,
__initcall3_start,
__initcall4_start,
__initcall5_start,
__initcall6_start,
__initcall7_start,
__initcall_end,
};

If you are interested, you can find these sections in the arch/x86/kernel/vmlinux.lds linker script which is generated after the Linux kernel compilation:

.init.data : AT(ADDR(.init.data) - 0xffffffff80000000) {
...
...
...
...
__initcall_start = .;
*(.initcallearly.init)
__initcall0_start = .;
*(.initcall0.init)
*(.initcall0s.init)
__initcall1_start = .;
...
...
}

If you are not familiar with this then you can know more about linkers in the special part of this book.

As we just saw, the do_initcall_level function takes one parameter - level of initcall - and does the following two things:

  • parses the initcall_command_line which is copy of usual kernel command line which may contain parameters for modules with the parse_args function from the kernel/params.c source code file;

  • call the do_on_initcall function for each level:

for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++)
do_one_initcall(*fn);

The do_one_initcall does the main job for us. As we may see, this function takes one parameter which represent initcall callback function and does the call of the given callback:

int __init_or_module do_one_initcall(initcall_t fn)
{
int count = preempt_count();
int ret;
char msgbuf[64];
if (initcall_blacklisted(fn))
return -EPERM;
if (initcall_debug)
ret = do_one_initcall_debug(fn);
else
ret = fn();
msgbuf[0] = 0;
if (preempt_count() != count) {
sprintf(msgbuf, "preemption imbalance ");
preempt_count_set(count);
}
if (irqs_disabled()) {
strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));
local_irq_enable();
}
WARN(msgbuf[0], "initcall %pF returned with %s\n", fn, msgbuf);
return ret;
}

Let's try to understand what does the do_one_initcall function does. First of all we increase preemption counter so that we can check it later to be sure that it is not imbalanced. After this step we can see the call of the initcall_backlist function which goes over the blacklisted_initcalls list which stores blacklisted initcalls and releases the given initcall if it is located in this list:

list_for_each_entry(entry, &blacklisted_initcalls, next) {
if (!strcmp(fn_name, entry->buf)) {
pr_debug("initcall %s blacklisted\n", fn_name);
kfree(fn_name);
return true;
}
}

This blacklist is filled during early Linux kernel initialization from the Linux kernel command line.

After the blacklisted initcalls are handled, the next part of code directly calls the initcall callback:

if (initcall_debug)
ret = do_one_initcall_debug(fn);
else
ret = fn();

initcall_debug variable defines if the call should be handled through the debug codepath (with more information being printed to the kernel log buffer) or not, where the callback will finally be executed. The initcall_debug variable is defined in the same source code file:

bool initcall_debug;

The value of the variable can be set from the kernel commands via the initcall_debug parameter, as we can read from the documentation of the Linux kernel command line:

initcall_debug [KNL] Trace initcalls as they are executed. Useful
for working out where the kernel is dying during
startup.

And that's true. If we will look at the implementation of the do_one_initcall_debug function, we will see that it does the same as the do_one_initcall function, i.e. the do_one_initcall_debug function calls the given initcall and prints some information (like the pid of the currently running task, duration of execution of the initcall and etc.) related to the execution of the given initcall:

static int __init_or_module do_one_initcall_debug(initcall_t fn)
{
ktime_t calltime, delta, rettime;
unsigned long long duration;
int ret;
printk(KERN_DEBUG "calling %pF @ %i\n", fn, task_pid_nr(current));
calltime = ktime_get();
ret = fn();
rettime = ktime_get();
delta = ktime_sub(rettime, calltime);
duration = (unsigned long long) ktime_to_ns(delta) >> 10;
printk(KERN_DEBUG "initcall %pF returned %d after %lld usecs\n",
fn, ret, duration);
return ret;
}

As an initcall was called by the one of the do_one_initcall or do_one_initcall_debug functions, we may see two checks in the end of the do_one_initcall function. The first one checks the amount of possible __preempt_count_add and __preempt_count_sub calls inside of the executed initcall, and if this value is not equal to the previous value of the preemptible counter, we add the preemption imbalance string to the message buffer and set correct value of the preemptible counter:

if (preempt_count() != count) {
sprintf(msgbuf, "preemption imbalance ");
preempt_count_set(count);
}

The last check the state of local IRQs and if they are disabled, we add the disabled interrupts strings to log buffer and enable IRQs for the current processor to make sure that IRQs are enabled after each initcall is completed (in case the callback disabled it and didn't enable before exiting):

if (irqs_disabled()) {
strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));
local_irq_enable();
}

That's all. In this way the Linux kernel does initialization of many subsystems in a correct order. From now on, we know what is the initcall mechanism in the Linux kernel. In this part, we covered main general portion of the initcall mechanism but we left some important concepts. Let's make a short look at these concepts.

First of all, we have missed one level of initcalls, this is rootfs initcalls. You can find definition of the rootfs_initcall in the include/linux/init.h header file along with all similar macros which we saw in this part:

#define rootfs_initcall(fn) __define_initcall(fn, rootfs)

As we may understand from the macro's name, its main purpose is to store callbacks which are related to the rootfs. Besides this goal, it may be useful to initialize other components after initialization related to filesystems level was already done, but before devices related initcalls. For example, the decompression of the initramfs which occurred in the populate_rootfs function from the init/initramfs.c source code file:

rootfs_initcall(populate_rootfs);

From this place, we may see familiar output:

[ 0.199960] Unpacking initramfs...

Besides the rootfs_initcall level, there are additional console_initcall, security_initcall and other secondary initcall levels. The last thing that we have missed is the set of the *_initcall_sync levels. Almost each *_initcall macro that we have seen in this part, has macro companion with the _sync prefix:

#define core_initcall_sync(fn) __define_initcall(fn, 1s)
#define postcore_initcall_sync(fn) __define_initcall(fn, 2s)
#define arch_initcall_sync(fn) __define_initcall(fn, 3s)
#define subsys_initcall_sync(fn) __define_initcall(fn, 4s)
#define fs_initcall_sync(fn) __define_initcall(fn, 5s)
#define device_initcall_sync(fn) __define_initcall(fn, 6s)
#define late_initcall_sync(fn) __define_initcall(fn, 7s)

The main goal of these additional levels is to wait for completion of all modules related initialization routines for a certain level.

Another point worthy of mention is the module_init(x) macro, defined at include/linux/module.h as:

#define module_init(x) __initcall(x);

If we follow and check what's the definition of __initcall(x) at include/linux/init.h we can see that it's being set as an device_initcall:

#define __initcall(fn) device_initcall(fn)

With that we can conclude that when a function set as __init of certain module isn't explicitly added to a specific initcall category, but using module_init() macro, it is added to device initcall list by default.

That's all.

Conclusion

In this part we saw the important mechanism of the Linux kernel which allows to call a function which depends on the current state of the Linux kernel during its initialization.

If you have questions or suggestions, feel free to ping me in twitter 0xAX, drop me email or just create issue.

Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to linux-insides.

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