Linux Kernel 2.4 Internals: Process and Interrupt Management

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2. Process and Interrupt Management

2.1 Task Structure and Process Table

Every process under Linux is dynamically allocated a struct task_struct structure. The maximum number of processes which can be created on Linux is limited only by the amount of physical memory present, and is equal to (see kernel/fork.c:fork_init()):



        /*
         * The default maximum number of threads is set to a safe
         * value: the thread structures can take up at most half
         * of memory.
         */
        max_threads = mempages / (THREAD_SIZE/PAGE_SIZE) / 2;

which, on IA32 architecture, basically means num_physpages/4. As an example, on a 512M machine, you can create 32k threads. This is a considerable improvement over the 4k-epsilon limit for older (2.2 and earlier) kernels. Moreover, this can be changed at runtime using the KERN_MAX_THREADS sysctl(2), or simply using procfs interface to kernel tunables:



# cat /proc/sys/kernel/threads-max 
32764
# echo 100000 > /proc/sys/kernel/threads-max 
# cat /proc/sys/kernel/threads-max 
100000
# gdb -q vmlinux /proc/kcore
Core was generated by `BOOT_IMAGE=240ac18 ro root=306 video=matrox:vesa:0x118'.
#0  0x0 in ?? ()
(gdb) p max_threads
$1 = 100000

The set of processes on the Linux system is represented as a collection of struct task_struct structures which are linked in two ways:

as a hashtable, hashed by pid, and
as a circular, doubly-linked list using p->next_task and p->prev_task pointers.

The hashtable is called pidhash[] and is defined in include/linux/sched.h:



/* PID hashing. (shouldnt this be dynamic?) */
#define PIDHASH_SZ (4096 >> 2)
extern struct task_struct *pidhash[PIDHASH_SZ];

#define pid_hashfn(x)   ((((x) >> 8) ^ (x)) & (PIDHASH_SZ - 1))

The tasks are hashed by their pid value and the above hashing function is supposed to distribute the elements uniformly in their domain (0 to PID_MAX-1). The hashtable is used to quickly find a task by given pid, using find_task_pid() inline from include/linux/sched.h:



static inline struct task_struct *find_task_by_pid(int pid)
{
        struct task_struct *p, **htable = &pidhash[pid_hashfn(pid)];

        for(p = *htable; p && p->pid != pid; p = p->pidhash_next)
                ;

        return p;
}

The tasks on each hashlist (i.e. hashed to the same value) are linked by p->pidhash_next/pidhash_pprev which are used by hash_pid() and unhash_pid() to insert and remove a given process into the hashtable. These are done under protection of the read-write spinlock called tasklist_lock taken for WRITE.

The circular doubly-linked list that uses p->next_task/prev_task is maintained so that one could go through all tasks on the system easily. This is achieved by the for_each_task() macro from include/linux/sched.h:



#define for_each_task(p) \
        for (p = &init_task ; (p = p->next_task) != &init_task ; )

Users of for_each_task() should take tasklist_lock for READ. Note that for_each_task() is using init_task to mark the beginning (and end) of the list - this is safe because the idle task (pid 0) never exits.

The modifiers of the process hashtable or/and the process table links, notably fork(), exit() and ptrace(), must take tasklist_lock for WRITE. What is more interesting is that the writers must also disable interrupts on the local CPU. The reason for this is not trivial: the send_sigio() function walks the task list and thus takes tasklist_lock for READ, and it is called from kill_fasync() in interrupt context. This is why writers must disable interrupts while readers don't need to.

Now that we understand how the task_struct structures are linked together, let us examine the members of task_struct. They loosely correspond to the members of UNIX 'struct proc' and 'struct user' combined together.

The other versions of UNIX separated the task state information into one part which should be kept memory-resident at all times (called 'proc structure' which includes process state, scheduling information etc.) and another part which is only needed when the process is running (called 'u area' which includes file descriptor table, disk quota information etc.). The only reason for such ugly design was that memory was a very scarce resource. Modern operating systems (well, only Linux at the moment but others, e.g. FreeBSD seem to improve in this direction towards Linux) do not need such separation and therefore maintain process state in a kernel memory-resident data structure at all times.

The task_struct structure is declared in include/linux/sched.h and is currently 1680 bytes in size.

The state field is declared as:



volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */

#define TASK_RUNNING            0
#define TASK_INTERRUPTIBLE      1
#define TASK_UNINTERRUPTIBLE    2
#define TASK_ZOMBIE             4
#define TASK_STOPPED            8
#define TASK_EXCLUSIVE          32

Why is TASK_EXCLUSIVE defined as 32 and not 16? Because 16 was used up by TASK_SWAPPING and I forgot to shift TASK_EXCLUSIVE up when I removed all references to TASK_SWAPPING (sometime in 2.3.x).

The volatile in p->state declaration means it can be modified asynchronously (from interrupt handler):

TASK_RUNNING: means the task is "supposed to be" on the run queue. The reason it may not yet be on the runqueue is that marking a task as TASK_RUNNING and placing it on the runqueue is not atomic. You need to hold the runqueue_lock read-write spinlock for read in order to look at the runqueue. If you do so, you will then see that every task on the runqueue is in TASK_RUNNING state. However, the converse is not true for the reason explained above. Similarly, drivers can mark themselves (or rather the process context they run in) as TASK_INTERRUPTIBLE (or TASK_UNINTERRUPTIBLE) and then call schedule(), which will then remove it from the runqueue (unless there is a pending signal, in which case it is left on the runqueue).
TASK_INTERRUPTIBLE: means the task is sleeping but can be woken up by a signal or by expiry of a timer.
TASK_UNINTERRUPTIBLE: same as TASK_INTERRUPTIBLE, except it cannot be woken up.
TASK_ZOMBIE: task has terminated but has not had its status collected (wait()-ed for) by the parent (natural or by adoption).
TASK_STOPPED: task was stopped, either due to job control signals or due to ptrace(2).
TASK_EXCLUSIVE: this is not a separate state but can be OR-ed to either one of TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE. This means that when this task is sleeping on a wait queue with many other tasks, it will be woken up alone instead of causing "thundering herd" problem by waking up all the waiters.

Task flags contain information about the process states which are not mutually exclusive:



unsigned long flags;    /* per process flags, defined below */
/*
 * Per process flags
 */
#define PF_ALIGNWARN    0x00000001      /* Print alignment warning msgs */
                                        /* Not implemented yet, only for 486*/
#define PF_STARTING     0x00000002      /* being created */
#define PF_EXITING      0x00000004      /* getting shut down */
#define PF_FORKNOEXEC   0x00000040      /* forked but didn't exec */
#define PF_SUPERPRIV    0x00000100      /* used super-user privileges */
#define PF_DUMPCORE     0x00000200      /* dumped core */
#define PF_SIGNALED     0x00000400      /* killed by a signal */
#define PF_MEMALLOC     0x00000800      /* Allocating memory */
#define PF_VFORK        0x00001000      /* Wake up parent in mm_release */
#define PF_USEDFPU      0x00100000      /* task used FPU this quantum (SMP) */

The fields p->has_cpu, p->processor, p->counter, p->priority, p->policy and p->rt_priority are related to the scheduler and will be looked at later.

The fields p->mm and p->active_mm point respectively to the process' address space described by mm_struct structure and to the active address space if the process doesn't have a real one (e.g. kernel threads). This helps minimise TLB flushes on switching address spaces when the task is scheduled out. So, if we are scheduling-in the kernel thread (which has no p->mm) then its next->active_mm will be set to the prev->active_mm of the task that was scheduled-out, which will be the same as prev->mm if prev->mm != NULL. The address space can be shared between threads if CLONE_VM flag is passed to the clone(2) system call or by means of vfork(2) system call.

The fields p->exec_domain and p->personality relate to the personality of the task, i.e. to the way certain system calls behave in order to emulate the "personality" of foreign flavours of UNIX.

The field p->fs contains filesystem information, which under Linux means three pieces of information:

root directory's dentry and mountpoint,
alternate root directory's dentry and mountpoint,
current working directory's dentry and mountpoint.

This structure also includes a reference count because it can be shared between cloned tasks when CLONE_FS flag is passed to the clone(2) system call.

The field p->files contains the file descriptor table. This too can be shared between tasks, provided CLONE_FILES is specified with clone(2) system call.

The field p->sig contains signal handlers and can be shared between cloned tasks by means of CLONE_SIGHAND.

2.2 Creation and termination of tasks and kernel threads

Different books on operating systems define a "process" in different ways, starting from "instance of a program in execution" and ending with "that which is produced by clone(2) or fork(2) system calls". Under Linux, there are three kinds of processes:

the idle thread(s),
kernel threads,
user tasks.

The idle thread is created at compile time for the first CPU; it is then "manually" created for each CPU by means of arch-specific fork_by_hand() in arch/i386/kernel/smpboot.c, which unrolls the fork(2) system call by hand (on some archs). Idle tasks share one init_task structure but have a private TSS structure, in the per-CPU array init_tss. Idle tasks all have pid = 0 and no other task can share pid, i.e. use CLONE_PID flag to clone(2).

Kernel threads are created using kernel_thread() function which invokes the clone(2) system call in kernel mode. Kernel threads usually have no user address space, i.e. p->mm = NULL, because they explicitly do exit_mm(), e.g. via daemonize() function. Kernel threads can always access kernel address space directly. They are allocated pid numbers in the low range. Running at processor's ring 0 (on x86, that is) implies that the kernel threads enjoy all I/O privileges and cannot be pre-empted by the scheduler.

User tasks are created by means of clone(2) or fork(2) system calls, both of which internally invoke kernel/fork.c:do_fork().

Let us understand what happens when a user process makes a fork(2) system call. Although fork(2) is architecture-dependent due to the different ways of passing user stack and registers, the actual underlying function do_fork() that does the job is portable and is located at kernel/fork.c.

The following steps are done:

Local variable retval is set to -ENOMEM, as this is the value which errno should be set to if fork(2) fails to allocate a new task structure.
If CLONE_PID is set in clone_flags then return an error (-EPERM), unless the caller is the idle thread (during boot only). So, normal user threads cannot pass CLONE_PID to clone(2) and expect it to succeed. For fork(2), this is irrelevant as clone_flags is set to SIFCHLD - this is only relevant when do_fork() is invoked from sys_clone() which passes the clone_flags from the value requested from userspace.
current->vfork_sem is initialised (it is later cleared in the child). This is used by sys_vfork() (vfork(2) system call, corresponds to clone_flags = CLONE_VFORK|CLONE_VM|SIGCHLD) to make the parent sleep until the child does mm_release(), for example as a result of exec()ing another program or exit(2)-ing.
A new task structure is allocated using arch-dependent alloc_task_struct() macro. On x86 it is just a gfp at GFP_KERNEL priority. This is the first reason why fork(2) system call may sleep. If this allocation fails, we return -ENOMEM.
All the values from current process' task structure are copied into the new one, using structure assignment *p = *current. Perhaps this should be replaced by a memcpy? Later on, the fields that should not be inherited by the child are set to the correct values.
Big kernel lock is taken as the rest of the code would otherwise be non-reentrant.
If the parent has user resources (a concept of UID, Linux is flexible enough to make it a question rather than a fact), then verify if the user exceeded RLIMIT_NPROC soft limit - if so, fail with -EAGAIN, if not, increment the count of processes by given uid p->user->count.
If the system-wide number of tasks exceeds the value of the tunable max_threads, fail with -EAGAIN.
If the binary being executed belongs to a modularised execution domain, increment the corresponding module's reference count.
If the binary being executed belongs to a modularised binary format, increment the corresponding module's reference count.
The child is marked as 'has not execed' (p->did_exec = 0)
The child is marked as 'not-swappable' (p->swappable = 0)
The child is put into 'uninterruptible sleep' state, i.e. p->state = TASK_UNINTERRUPTIBLE (TODO: why is this done? I think it's not needed - get rid of it, Linus confirms it is not needed)
The child's p->flags are set according to the value of clone_flags; for plain fork(2), this will be p->flags = PF_FORKNOEXEC.
The child's pid p->pid is set using the fast algorithm in kernel/fork.c:get_pid() (TODO: lastpid_lock spinlock can be made redundant since get_pid() is always called under big kernel lock from do_fork(), also remove flags argument of get_pid(), patch sent to Alan on 20/06/2000 - followup later).
The rest of the code in do_fork() initialises the rest of child's task structure. At the very end, the child's task structure is hashed into the pidhash hashtable and the child is woken up (TODO: wake_up_process(p) sets p->state = TASK_RUNNING and adds the process to the runq, therefore we probably didn't need to set p->state to TASK_RUNNING earlier on in do_fork()). The interesting part is setting p->exit_signal to clone_flags & CSIGNAL, which for fork(2) means just SIGCHLD and setting p->pdeath_signal to 0. The pdeath_signal is used when a process 'forgets' the original parent (by dying) and can be set/get by means of PR_GET/SET_PDEATHSIG commands of prctl(2) system call (You might argue that the way the value of pdeath_signal is returned via userspace pointer argument in prctl(2) is a bit silly - mea culpa, after Andries Brouwer updated the manpage it was too late to fix ;)

Thus tasks are created. There are several ways for tasks to terminate:

by making exit(2) system call;
by being delivered a signal with default disposition to die;
by being forced to die under certain exceptions;
by calling bdflush(2) with func == 1 (this is Linux-specific, for compatibility with old distributions that still had the 'update' line in /etc/inittab - nowadays the work of update is done by kernel thread kupdate).

Functions implementing system calls under Linux are prefixed with sys_, but they are usually concerned only with argument checking or arch-specific ways to pass some information and the actual work is done by do_ functions. So it is with sys_exit() which calls do_exit() to do the work. Although, other parts of the kernel sometimes invoke sys_exit() while they should really call do_exit().

The function do_exit() is found in kernel/exit.c. The points to note about do_exit():

Uses global kernel lock (locks but doesn't unlock).
Calls schedule() at the end, which never returns.
Sets the task state to TASK_ZOMBIE.
Notifies any child with current->pdeath_signal, if not 0.
Notifies the parent with a current->exit_signal, which is usually equal to SIGCHLD.
Releases resources allocated by fork, closes open files etc.
On architectures that use lazy FPU switching (ia64, mips, mips64) (TODO: remove 'flags' argument of sparc, sparc64), do whatever the hardware requires to pass the FPU ownership (if owned by current) to "none".

2.3 Linux Scheduler

The job of a scheduler is to arbitrate access to the current CPU between multiple processes. The scheduler is implemented in the 'main kernel file' kernel/sched.c. The corresponding header file include/linux/sched.h is included (either explicitly or indirectly) by virtually every kernel source file.

The fields of task structure relevant to scheduler include:

p->need_resched: this field is set if schedule() should be invoked at the 'next opportunity'.
p->counter: number of clock ticks left to run in this scheduling slice, decremented by a timer. When this field becomes lower than or equal to zero, it is reset to 0 and p->need_resched is set. This is also sometimes called 'dynamic priority' of a process because it can change by itself.
p->priority: the process' static priority, only changed through well-known system calls like nice(2), POSIX.1b sched_setparam(2) or 4.4BSD/SVR4 setpriority(2).
p->rt_priority: realtime priority
p->policy: the scheduling policy, specifies which scheduling class the task belongs to. Tasks can change their scheduling class using the sched_setscheduler(2) system call. The valid values are SCHED_OTHER (traditional UNIX process), SCHED_FIFO (POSIX.1b FIFO realtime process) and SCHED_RR (POSIX round-robin realtime process). One can also OR SCHED_YIELD to any of these values to signify that the process decided to yield the CPU, for example by calling sched_yield(2) system call. A FIFO realtime process will run until either a) it blocks on I/O, b) it explicitly yields the CPU or c) it is preempted by another realtime process with a higher p->rt_priority value. SCHED_RR is the same as SCHED_FIFO, except that when its timeslice expires it goes back to the end of the runqueue.

The scheduler's algorithm is simple, despite the great apparent complexity of the schedule() function. The function is complex because it implements three scheduling algorithms in one and also because of the subtle SMP-specifics.

The apparently 'useless' gotos in schedule() are there for a purpose - to generate the best optimised (for i386) code. Also, note that scheduler (like most of the kernel) was completely rewritten for 2.4, therefore the discussion below does not apply to 2.2 or earlier kernels.

Let us look at the function in detail:

If current->active_mm == NULL then something is wrong. Current process, even a kernel thread (current->mm == NULL) must have a valid p->active_mm at all times.
If there is something to do on the tq_scheduler task queue, process it now. Task queues provide a kernel mechanism to schedule execution of functions at a later time. We shall look at it in details elsewhere.
Initialise local variables prev and this_cpu to current task and current CPU respectively.
Check if schedule() was invoked from interrupt handler (due to a bug) and panic if so.
Release the global kernel lock.
If there is some work to do via softirq mechanism, do it now.
Initialise local pointer struct schedule_data *sched_data to point to per-CPU (cacheline-aligned to prevent cacheline ping-pong) scheduling data area, which contains the TSC value of last_schedule and the pointer to last scheduled task structure (TODO: sched_data is used on SMP only but why does init_idle() initialises it on UP as well?).
runqueue_lock spinlock is taken. Note that we use spin_lock_irq() because in schedule() we guarantee that interrupts are enabled. Therefore, when we unlock runqueue_lock, we can just re-enable them instead of saving/restoring eflags (spin_lock_irqsave/restore variant).
task state machine: if the task is in TASK_RUNNING state, it is left alone; if it is in TASK_INTERRUPTIBLE state and a signal is pending, it is moved into TASK_RUNNING state. In all other cases, it is deleted from the runqueue.
next (best candidate to be scheduled) is set to the idle task of this cpu. However, the goodness of this candidate is set to a very low value (-1000), in hope that there is someone better than that.
If the prev (current) task is in TASK_RUNNING state, then the current goodness is set to its goodness and it is marked as a better candidate to be scheduled than the idle task.
Now the runqueue is examined and a goodness of each process that can be scheduled on this cpu is compared with current value; the process with highest goodness wins. Now the concept of "can be scheduled on this cpu" must be clarified: on UP, every process on the runqueue is eligible to be scheduled; on SMP, only process not already running on another cpu is eligible to be scheduled on this cpu. The goodness is calculated by a function called goodness(), which treats realtime processes by making their goodness very high (1000 + p->rt_priority), this being greater than 1000 guarantees that no SCHED_OTHER process can win; so they only contend with other realtime processes that may have a greater p->rt_priority. The goodness function returns 0 if the process' time slice (p->counter) is over. For non-realtime processes, the initial value of goodness is set to p->counter - this way, the process is less likely to get CPU if it already had it for a while, i.e. interactive processes are favoured more than CPU bound number crunchers. The arch-specific constant PROC_CHANGE_PENALTY attempts to implement "cpu affinity" (i.e. give advantage to a process on the same CPU). It also gives a slight advantage to processes with mm pointing to current active_mm or to processes with no (user) address space, i.e. kernel threads.
if the current value of goodness is 0 then the entire list of processes (not just the ones on the runqueue!) is examined and their dynamic priorities are recalculated using simple algorithm:
recalculate: { struct task_struct *p; spin_unlock_irq(&runqueue_lock); read_lock(&tasklist_lock); for_each_task(p) p->counter = (p->counter >> 1) + p->priority; read_unlock(&tasklist_lock); spin_lock_irq(&runqueue_lock); }
Note that the we drop the runqueue_lock before we recalculate. The reason is that we go through entire set of processes; this can take a long time, during which the schedule() could be called on another CPU and select a process with goodness good enough for that CPU, whilst we on this CPU were forced to recalculate. Ok, admittedly this is somewhat inconsistent because while we (on this CPU) are selecting a process with the best goodness, schedule() running on another CPU could be recalculating dynamic priorities.
From this point on it is certain that next points to the task to be scheduled, so we initialise next->has_cpu to 1 and next->processor to this_cpu. The runqueue_lock can now be unlocked.
If we are switching back to the same task (next == prev) then we can simply reacquire the global kernel lock and return, i.e. skip all the hardware-level (registers, stack etc.) and VM-related (switch page directory, recalculate active_mm etc.) stuff.
The macro switch_to() is architecture specific. On i386, it is concerned with a) FPU handling, b) LDT handling, c) reloading segment registers, d) TSS handling and e) reloading debug registers.

2.4 Linux linked list implementation

Before we go on to examine implementation of wait queues, we must acquaint ourselves with the Linux standard doubly-linked list implementation. Wait queues (as well as everything else in Linux) make heavy use of them and they are called in jargon "list.h implementation" because the most relevant file is include/linux/list.h.

The fundamental data structure here is struct list_head:



struct list_head {
        struct list_head *next, *prev;
};

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
        struct list_head name = LIST_HEAD_INIT(name)

#define INIT_LIST_HEAD(ptr) do { \
        (ptr)->next = (ptr); (ptr)->prev = (ptr); \
} while (0)

#define list_entry(ptr, type, member) \
        ((type *)((char *)(ptr)-(unsigned long)(&((type *)0)->member)))

#define list_for_each(pos, head) \
        for (pos = (head)->next; pos != (head); pos = pos->next)

The first three macros are for initialising an empty list by pointing both next and prev pointers to itself. It is obvious from C syntactical restrictions which ones should be used where - for example, LIST_HEAD_INIT() can be used for structure's element initialisation in declaration, the second can be used for static variable initialising declarations and the third can be used inside a function.

The macro list_entry() gives access to individual list element, for example (from fs/file_table.c:fs_may_remount_ro()):



struct super_block {
   ...
   struct list_head s_files;
   ...
} *sb = &some_super_block;

struct file {
   ...
   struct list_head f_list;
   ...
} *file;

struct list_head *p;

for (p = sb->s_files.next; p != &sb->s_files; p = p->next) {
     struct file *file = list_entry(p, struct file, f_list);
     do something to 'file'
}

A good example of the use of list_for_each() macro is in the scheduler where we walk the runqueue looking for the process with highest goodness:



static LIST_HEAD(runqueue_head);
struct list_head *tmp;
struct task_struct *p;

list_for_each(tmp, &runqueue_head) {
    p = list_entry(tmp, struct task_struct, run_list);
    if (can_schedule(p)) {
        int weight = goodness(p, this_cpu, prev->active_mm);
        if (weight > c)
            c = weight, next = p;
    }
}

Here, p->run_list is declared as struct list_head run_list inside task_struct structure and serves as anchor to the list. Removing an element from the list and adding (to head or tail of the list) is done by list_del()/list_add()/list_add_tail() macros. The examples below are adding and removing a task from runqueue:



static inline void del_from_runqueue(struct task_struct * p)
{
        nr_running--;
        list_del(&p->run_list);
        p->run_list.next = NULL;
}

static inline void add_to_runqueue(struct task_struct * p)
{
        list_add(&p->run_list, &runqueue_head);
        nr_running++;
}

static inline void move_last_runqueue(struct task_struct * p)
{
        list_del(&p->run_list);
        list_add_tail(&p->run_list, &runqueue_head);
}

static inline void move_first_runqueue(struct task_struct * p)
{
        list_del(&p->run_list);
        list_add(&p->run_list, &runqueue_head);
}

2.5 Wait Queues

When a process requests the kernel to do something which is currently impossible but that may become possible later, the process is put to sleep and is woken up when the request is more likely to be satisfied. One of the kernel mechanisms used for this is called a 'wait queue'.

Linux implementation allows wake-on semantics using TASK_EXCLUSIVE flag. With waitqueues, you can either use a well-known queue and then simply sleep_on/sleep_on_timeout/interruptible_sleep_on/interruptible_sleep_on_timeout, or you can define your own waitqueue and use add/remove_wait_queue to add and remove yourself from it and wake_up/wake_up_interruptible to wake up when needed.

An example of the first usage of waitqueues is interaction between the page allocator (in mm/page_alloc.c:__alloc_pages()) and the kswapd kernel daemon (in mm/vmscan.c:kswap()), by means of wait queue kswapd_wait, declared in mm/vmscan.c; the kswapd daemon sleeps on this queue, and it is woken up whenever the page allocator needs to free up some pages.

An example of autonomous waitqueue usage is interaction between user process requesting data via read(2) system call and kernel running in the interrupt context to supply the data. An interrupt handler might look like (simplified drivers/char/rtc_interrupt()):



static DECLARE_WAIT_QUEUE_HEAD(rtc_wait);

void rtc_interrupt(int irq, void *dev_id, struct pt_regs *regs)
{
        spin_lock(&rtc_lock);       
        rtc_irq_data = CMOS_READ(RTC_INTR_FLAGS);
        spin_unlock(&rtc_lock);     
        wake_up_interruptible(&rtc_wait);
}

So, the interrupt handler obtains the data by reading from some device-specific I/O port (CMOS_READ() macro turns into a couple outb/inb) and then wakes up whoever is sleeping on the rtc_wait wait queue.

Now, the read(2) system call could be implemented as:



ssize_t rtc_read(struct file file, char *buf, size_t count, loff_t *ppos)
{
        DECLARE_WAITQUEUE(wait, current);
        unsigned long data;
        ssize_t retval;

        add_wait_queue(&rtc_wait, &wait);
        current->state = TASK_INTERRUPTIBLE;
        do {
                spin_lock_irq(&rtc_lock);
                data = rtc_irq_data;
                rtc_irq_data = 0;
                spin_unlock_irq(&rtc_lock);

                if (data != 0)
                        break;

                if (file->f_flags & O_NONBLOCK) {
                        retval = -EAGAIN;
                        goto out;
                }
                if (signal_pending(current)) {
                        retval = -ERESTARTSYS;
                        goto out;
                }
                schedule();
        } while(1);
        retval = put_user(data, (unsigned long *)buf);  
        if (!retval)
                retval = sizeof(unsigned long);

out:
        current->state = TASK_RUNNING;
        remove_wait_queue(&rtc_wait, &wait);
        return retval;
}

What happens in rtc_read() is this:

We declare a wait queue element pointing to current process context.
We add this element to the rtc_wait waitqueue.
We mark current context as TASK_INTERRUPTIBLE which means it will not be rescheduled after the next time it sleeps.
We check if there is no data available; if there is we break out, copy data to user buffer, mark ourselves as TASK_RUNNING, remove ourselves from the wait queue and return
If there is no data yet, we check whether the user specified non-blocking I/O and if so we fail with EAGAIN (which is the same as EWOULDBLOCK)
We also check if a signal is pending and if so inform the "higher layers" to restart the system call if necessary. By "if necessary" I meant the details of signal disposition as specified in sigaction(2) system call.
Then we "switch out", i.e. fall asleep, until woken up by the interrupt handler. If we didn't mark ourselves as TASK_INTERRUPTIBLE then the scheduler could schedule us sooner than when the data is available, thus causing unneeded processing.

It is also worth pointing out that, using wait queues, it is rather easy to implement the poll(2) system call:



static unsigned int rtc_poll(struct file *file, poll_table *wait)
{
        unsigned long l;

        poll_wait(file, &rtc_wait, wait);

        spin_lock_irq(&rtc_lock);
        l = rtc_irq_data;
        spin_unlock_irq(&rtc_lock);

        if (l != 0)
                return POLLIN | POLLRDNORM;
        return 0;
}

All the work is done by the device-independent function poll_wait() which does the necessary waitqueue manipulations; all we need to do is point it to the waitqueue which is woken up by our device-specific interrupt handler.

2.6 Kernel Timers

Now let us turn our attention to kernel timers. Kernel timers are used to dispatch execution of a particular function (called 'timer handler') at a specified time in the future. The main data structure is struct timer_list declared in include/linux/timer.h:



struct timer_list {
        struct list_head list;
        unsigned long expires;
        unsigned long data;
        void (*function)(unsigned long);
        volatile int running;
};

The list field is for linking into the internal list, protected by the timerlist_lock spinlock. The expires field is the value of jiffies when the function handler should be invoked with data passed as a parameter. The running field is used on SMP to test if the timer handler is currently running on another CPU.

The functions add_timer() and del_timer() add and remove a given timer to the list. When a timer expires, it is removed automatically. Before a timer is used, it MUST be initialised by means of init_timer() function. And before it is added, the fields function and expires must be set.

2.7 Bottom Halves

Sometimes it is reasonable to split the amount of work to be performed inside an interrupt handler into immediate work (e.g. acknowledging the interrupt, updating the stats etc.) and work which can be postponed until later, when interrupts are enabled (e.g. to do some postprocessing on data, wake up processes waiting for this data, etc).

Bottom halves are the oldest mechanism for deferred execution of kernel tasks and have been available since Linux 1.x. In Linux 2.0, a new mechanism was added, called 'task queues', which will be the subject of next section.

Bottom halves are serialised by the global_bh_lock spinlock, i.e. there can only be one bottom half running on any CPU at a time. However, when attempting to execute the handler, if global_bh_lock is not available, the bottom half is marked (i.e. scheduled) for execution - so processing can continue, as opposed to a busy loop on global_bh_lock.

There can only be 32 bottom halves registered in total. The functions required to manipulate bottom halves are as follows (all exported to modules):

void init_bh(int nr, void (*routine)(void)): installs a bottom half handler pointed to by routine argument into slot nr. The slot ought to be enumerated in include/linux/interrupt.h in the form XXXX_BH, e.g. TIMER_BH or TQUEUE_BH. Typically, a subsystem's initialisation routine (init_module() for modules) installs the required bottom half using this function.
void remove_bh(int nr): does the opposite of init_bh(), i.e. de-installs bottom half installed at slot nr. There is no error checking performed there, so, for example remove_bh(32) will panic/oops the system. Typically, a subsystem's cleanup routine (cleanup_module() for modules) uses this function to free up the slot that can later be reused by some other subsystem. (TODO: wouldn't it be nice to have /proc/bottom_halves list all registered bottom halves on the system? That means global_bh_lock must be made read/write, obviously)
void mark_bh(int nr): marks bottom half in slot nr for execution. Typically, an interrupt handler will mark its bottom half (hence the name!) for execution at a "safer time".

Bottom halves are globally locked tasklets, so the question "when are bottom half handlers executed?" is really "when are tasklets executed?". And the answer is, in two places: a) on each schedule() and b) on each interrupt/syscall return path in entry.S (TODO: therefore, the schedule() case is really boring - it like adding yet another very very slow interrupt, why not get rid of handle_softirq label from schedule() altogether?).

2.8 Task Queues

Task queues can be though of as a dynamic extension to old bottom halves. In fact, in the source code they are sometimes referred to as "new" bottom halves. More specifically, the old bottom halves discussed in previous section have these limitations:

There are only a fixed number (32) of them.
Each bottom half can only be associated with one handler function.
Bottom halves are consumed with a spinlock held so they cannot block.

So, with task queues, arbitrary number of functions can be chained and processed one after another at a later time. One creates a new task queue using the DECLARE_TASK_QUEUE() macro and queues a task onto it using the queue_task() function. The task queue then can be processed using run_task_queue(). Instead of creating your own task queue (and having to consume it manually) you can use one of Linux' predefined task queues which are consumed at well-known points:

tq_timer: the timer task queue, run on each timer interrupt and when releasing a tty device (closing or releasing a half-opened terminal device). Since the timer handler runs in interrupt context, the tq_timer tasks also run in interrupt context and thus cannot block.
tq_scheduler: the scheduler task queue, consumed by the scheduler (and also when closing tty devices, like tq_timer). Since the scheduler executed in the context of the process being re-scheduled, the tq_scheduler tasks can do anything they like, i.e. block, use process context data (but why would they want to), etc.
tq_immediate: this is really a bottom half IMMEDIATE_BH, so drivers can queue_task(task, &tq_immediate) and then mark_bh(IMMEDIATE_BH) to be consumed in interrupt context.
tq_disk: used by low level block device access (and RAID) to start the actual requests. This task queue is exported to modules but shouldn't be used except for the special purposes which it was designed for.

Unless a driver uses its own task queues, it does not need to call run_tasks_queues() to process the queue, except under circumstances explained below.

The reason tq_timer/tq_scheduler task queues are consumed not only in the usual places but elsewhere (closing tty device is but one example) becomes clear if one remembers that the driver can schedule tasks on the queue, and these tasks only make sense while a particular instance of the device is still valid - which usually means until the application closes it. So, the driver may need to call run_task_queue() to flush the tasks it (and anyone else) has put on the queue, because allowing them to run at a later time may make no sense - i.e. the relevant data structures may have been freed/reused by a different instance. This is the reason you see run_task_queue() on tq_timer and tq_scheduler in places other than timer interrupt and schedule() respectively.

2.9 Tasklets

Not yet, will be in future revision.

2.10 Softirqs

Not yet, will be in future revision.

2.11 How System Calls Are Implemented on i386 Architecture?

There are two mechanisms under Linux for implementing system calls:

lcall7/lcall27 call gates;
int 0x80 software interrupt.

Native Linux programs use int 0x80 whilst binaries from foreign flavours of UNIX (Solaris, UnixWare 7 etc.) use the lcall7 mechanism. The name 'lcall7' is historically misleading because it also covers lcall27 (e.g. Solaris/x86), but the handler function is called lcall7_func.

When the system boots, the function arch/i386/kernel/traps.c:trap_init() is called which sets up the IDT so that vector 0x80 (of type 15, dpl 3) points to the address of system_call entry from arch/i386/kernel/entry.S.

When a userspace application makes a system call, the arguments are passed via registers and the application executes 'int 0x80' instruction. This causes a trap into kernel mode and processor jumps to system_call entry point in entry.S. What this does is:

Save registers.
Set %ds and %es to KERNEL_DS, so that all data (and extra segment) references are made in kernel address space.
If the value of %eax is greater than NR_syscalls (currently 256), fail with ENOSYS error.
If the task is being ptraced (tsk->ptrace & PF_TRACESYS), do special processing. This is to support programs like strace (analogue of SVR4 truss(1)) or debuggers.
Call sys_call_table+4*(syscall_number from %eax). This table is initialised in the same file (arch/i386/kernel/entry.S) to point to individual system call handlers which under Linux are (usually) prefixed with sys_, e.g. sys_open, sys_exit, etc. These C system call handlers will find their arguments on the stack where SAVE_ALL stored them.
Enter 'system call return path'. This is a separate label because it is used not only by int 0x80 but also by lcall7, lcall27. This is concerned with handling tasklets (including bottom halves), checking if a schedule() is needed (tsk->need_resched != 0), checking if there are signals pending and if so handling them.

Linux supports up to 6 arguments for system calls. They are passed in %ebx, %ecx, %edx, %esi, %edi (and %ebp used temporarily, see _syscall6() in asm-i386/unistd.h). The system call number is passed via %eax.

2.12 Atomic Operations

There are two types of atomic operations: bitmaps and atomic_t. Bitmaps are very convenient for maintaining a concept of "allocated" or "free" units from some large collection where each unit is identified by some number, for example free inodes or free blocks. They are also widely used for simple locking, for example to provide exclusive access to open a device. An example of this can be found in arch/i386/kernel/microcode.c:



/*
 *  Bits in microcode_status. (31 bits of room for future expansion)
 */
#define MICROCODE_IS_OPEN       0       /* set if device is in use */

static unsigned long microcode_status;

There is no need to initialise microcode_status to 0 as BSS is zero-cleared under Linux explicitly.



/*
 * We enforce only one user at a time here with open/close.
 */
static int microcode_open(struct inode *inode, struct file *file)
{
        if (!capable(CAP_SYS_RAWIO))
                return -EPERM;

        /* one at a time, please */
        if (test_and_set_bit(MICROCODE_IS_OPEN, &microcode_status))
                return -EBUSY;

        MOD_INC_USE_COUNT;
        return 0;
}

The operations on bitmaps are:

void set_bit(int nr, volatile void *addr): set bit nr in the bitmap pointed to by addr.
void clear_bit(int nr, volatile void *addr): clear bit nr in the bitmap pointed to by addr.
void change_bit(int nr, volatile void *addr): toggle bit nr (if set clear, if clear set) in the bitmap pointed to by addr.
int test_and_set_bit(int nr, volatile void *addr): atomically set bit nr and return the old bit value.
int test_and_clear_bit(int nr, volatile void *addr): atomically clear bit nr and return the old bit value.
int test_and_change_bit(int nr, volatile void *addr): atomically toggle bit nr and return the old bit value.

These operations use the LOCK_PREFIX macro, which on SMP kernels evaluates to bus lock instruction prefix and to nothing on UP. This guarantees atomicity of access in SMP environment.

Sometimes bit manipulations are not convenient, but instead we need to perform arithmetic operations - add, subtract, increment decrement. The typical cases are reference counts (e.g. for inodes). This facility is provided by the atomic_t data type and the following operations:

atomic_read(&v): read the value of atomic_t variable v.
atomic_set(&v, i): set the value of atomic_t variable v to integer i.
void atomic_add(int i, volatile atomic_t *v): add integer i to the value of atomic variable pointed to by v.
void atomic_sub(int i, volatile atomic_t *v): subtract integer i from the value of atomic variable pointed to by v.
int atomic_sub_and_test(int i, volatile atomic_t *v): subtract integer i from the value of atomic variable pointed to by v; return 1 if the new value is 0, return 0 otherwise.
void atomic_inc(volatile atomic_t *v): increment the value by 1.
void atomic_dec(volatile atomic_t *v): decrement the value by 1.
int atomic_dec_and_test(volatile atomic_t *v): decrement the value; return 1 if the new value is 0, return 0 otherwise.
int atomic_inc_and_test(volatile atomic_t *v): increment the value; return 1 if the new value is 0, return 0 otherwise.
int atomic_add_negative(int i, volatile atomic_t *v): add the value of i to v and return 1 if the result is negative. Return 0 if the result is greater than or equal to 0. This operation is used for implementing semaphores.

2.13 Spinlocks, Read-write Spinlocks and Big-Reader Spinlocks

Since the early days of Linux support (early 90s, this century), developers were faced with the classical problem of accessing shared data between different types of context (user process vs interrupt) and different instances of the same context from multiple cpus.

SMP support was added to Linux 1.3.42 on 15 Nov 1995 (the original patch was made to 1.3.37 in October the same year).

If the critical region of code may be executed by either process context and interrupt context, then the way to protect it using cli/sti instructions on UP is:



unsigned long flags;

save_flags(flags);
cli();
/* critical code */
restore_flags(flags);

While this is ok on UP, it obviously is of no use on SMP because the same code sequence may be executed simultaneously on another cpu, and while cli() provides protection against races with interrupt context on each CPU individually, it provides no protection at all against races between contexts running on different CPUs. This is where spinlocks are useful for.

There are three types of spinlocks: vanilla (basic), read-write and big-reader spinlocks. Read-write spinlocks should be used when there is a natural tendency of 'many readers and few writers'. Example of this is access to the list of registered filesystems (see fs/super.c). The list is guarded by the file_systems_lock read-write spinlock because one needs exclusive access only when registering/unregistering a filesystem, but any process can read the file /proc/filesystems or use the sysfs(2) system call to force a read-only scan of the file_systems list. This makes it sensible to use read-write spinlocks. With read-write spinlocks, one can have multiple readers at a time but only one writer and there can be no readers while there is a writer. Btw, it would be nice if new readers would not get a lock while there is a writer trying to get a lock, i.e. if Linux could correctly deal with the issue of potential writer starvation by multiple readers. This would mean that readers must be blocked while there is a writer attempting to get the lock. This is not currently the case and it is not obvious whether this should be fixed - the argument to the contrary is - readers usually take the lock for a very short time so should they really be starved while the writer takes the lock for potentially longer periods?

Big-reader spinlocks are a form of read-write spinlocks heavily optimised for very light read access, with a penalty for writes. There is a limited number of big-reader spinlocks - currently only two exist, of which one is used only on sparc64 (global irq) and the other is used for networking. In all other cases where the access pattern does not fit into any of these two scenarios, one should use basic spinlocks. You cannot block while holding any kind of spinlock.

Spinlocks come in three flavours: plain, _irq() and _bh().

Plain spin_lock()/spin_unlock(): if you know the interrupts are always disabled or if you do not race with interrupt context (e.g. from within interrupt handler), then you can use this one. It does not touch interrupt state on the current CPU.
spin_lock_irq()/spin_unlock_irq(): if you know that interrupts are always enabled then you can use this version, which simply disables (on lock) and re-enables (on unlock) interrupts on the current CPU. For example, rtc_read() uses spin_lock_irq(&rtc_lock) (interrupts are always enabled inside read()) whilst rtc_interrupt() uses spin_lock(&rtc_lock) (interrupts are always disabled inside interrupt handler). Note that rtc_read() uses spin_lock_irq() and not the more generic spin_lock_irqsave() because on entry to any system call interrupts are always enabled.
spin_lock_irqsave()/spin_unlock_irqrestore(): the strongest form, to be used when the interrupt state is not known, but only if interrupts matter at all, i.e. there is no point in using it if our interrupt handlers don't execute any critical code.

The reason you cannot use plain spin_lock() if you race against interrupt handlers is because if you take it and then an interrupt comes in on the same CPU, it will busy wait for the lock forever: the lock holder, having been interrupted, will not continue until the interrupt handler returns.

The most common usage of a spinlock is to access a data structure shared between user process context and interrupt handlers:



spinlock_t my_lock = SPIN_LOCK_UNLOCKED;

my_ioctl()
{
        spin_lock_irq(&my_lock);
        /* critical section */
        spin_unlock_irq(&my_lock);
}

my_irq_handler()
{
        spin_lock(&lock);
        /* critical section */
        spin_unlock(&lock);
}

There are a couple of things to note about this example:

The process context, represented here as a typical driver method - ioctl() (arguments and return values omitted for clarity), must use spin_lock_irq() because it knows that interrupts are always enabled while executing the device ioctl() method.
Interrupt context, represented here by my_irq_handler() (again arguments omitted for clarity) can use plain spin_lock() form because interrupts are disabled inside an interrupt handler.

2.14 Semaphores and read/write Semaphores

Sometimes, while accessing a shared data structure, one must perform operations that can block, for example copy data to userspace. The locking primitive available for such scenarios under Linux is called a semaphore. There are two types of semaphores: basic and read-write semaphores. Depending on the initial value of the semaphore, they can be used for either mutual exclusion (initial value of 1) or to provide more sophisticated type of access.

Read-write semaphores differ from basic semaphores in the same way as read-write spinlocks differ from basic spinlocks: one can have multiple readers at a time but only one writer and there can be no readers while there are writers - i.e. the writer blocks all readers and new readers block while a writer is waiting.

Also, basic semaphores can be interruptible - just use the operations down/up_interruptible() instead of the plain down()/up() and check the value returned from down_interruptible(): it will be non zero if the operation was interrupted.

Using semaphores for mutual exclusion is ideal in situations where a critical code section may call by reference unknown functions registered by other subsystems/modules, i.e. the caller cannot know apriori whether the function blocks or not.

A simple example of semaphore usage is in kernel/sys.c, implementation of gethostname(2)/sethostname(2) system calls.



asmlinkage long sys_sethostname(char *name, int len)
{
        int errno;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        if (len < 0 || len > __NEW_UTS_LEN)
                return -EINVAL;
        down_write(&uts_sem);
        errno = -EFAULT;
        if (!copy_from_user(system_utsname.nodename, name, len)) {
                system_utsname.nodename[len] = 0;
                errno = 0;
        }
        up_write(&uts_sem);
        return errno;
}

asmlinkage long sys_gethostname(char *name, int len)
{
        int i, errno;

        if (len < 0)
                return -EINVAL;
        down_read(&uts_sem);
        i = 1 + strlen(system_utsname.nodename);
        if (i > len)
                i = len;
        errno = 0;
        if (copy_to_user(name, system_utsname.nodename, i))
                errno = -EFAULT;
        up_read(&uts_sem);
        return errno;
}

The points to note about this example are:

The functions may block while copying data from/to userspace in copy_from_user()/copy_to_user(). Therefore they could not use any form of spinlock here.
The semaphore type chosen is read-write as opposed to basic because there may be lots of concurrent gethostname(2) requests which need not be mutually exclusive.

Although Linux implementation of semaphores and read-write semaphores is very sophisticated, there are possible scenarios one can think of which are not yet implemented, for example there is no concept of interruptible read-write semaphores. This is obviously because there are no real-world situations which require these exotic flavours of the primitives.

2.15 Kernel Support for Loading Modules

Linux is a monolithic operating system and despite all the modern hype about some "advantages" offered by operating systems based on micro-kernel design, the truth remains (quoting Linus Torvalds himself):

... message passing as the fundamental operation of the OS is just an exercise in computer science masturbation. It may feel good, but you don't actually get anything DONE.

Therefore, Linux is and will always be based on a monolithic design, which means that all subsystems run in the same privileged mode and share the same address space; communication between them is achieved by the usual C function call means.

However, although separating kernel functionality into separate "processes" as is done in micro-kernels is definitely a bad idea, separating it into dynamically loadable on demand kernel modules is desirable in some circumstances (e.g. on machines with low memory or for installation kernels which could otherwise contain ISA auto-probing device drivers that are mutually exclusive). The decision whether to include support for loadable modules is made at compile time and is determined by the CONFIG_MODULES option. Support for module autoloading via request_module() mechanism is a separate compilation option (CONFIG_KMOD).

The following functionality can be implemented as loadable modules under Linux:

Character and block device drivers, including misc device drivers.
Terminal line disciplines.
Virtual (regular) files in /proc and in devfs (e.g. /dev/cpu/microcode vs /dev/misc/microcode).
Binary file formats (e.g. ELF, aout, etc).
Execution domains (e.g. Linux, UnixWare7, Solaris, etc).
Filesystems.
System V IPC.

There a few things that cannot be implemented as modules under Linux (probably because it makes no sense for them to be modularised):

Scheduling algorithms.
VM policies.
Buffer cache, page cache and other caches.

Linux provides several system calls to assist in loading modules:

caddr_t create_module(const char *name, size_t size): allocates size bytes using vmalloc() and maps a module structure at the beginning thereof. This new module is then linked into the list headed by module_list. Only a process with CAP_SYS_MODULE can invoke this system call, others will get EPERM returned.
long init_module(const char *name, struct module *image): loads the relocated module image and causes the module's initialisation routine to be invoked. Only a process with CAP_SYS_MODULE can invoke this system call, others will get EPERM returned.
long delete_module(const char *name): attempts to unload the module. If name == NULL, attempt is made to unload all unused modules.
long query_module(const char *name, int which, void *buf, size_t bufsize, size_t *ret): returns information about a module (or about all modules).

The command interface available to users consists of:

insmod: insert a single module.
modprobe: insert a module including all other modules it depends on.
rmmod: remove a module.
modinfo: print some information about a module, e.g. author, description, parameters the module accepts, etc.

Apart from being able to load a module manually using either insmod or modprobe, it is also possible to have the module inserted automatically by the kernel when a particular functionality is required. The kernel interface for this is the function called request_module(name) which is exported to modules, so that modules can load other modules as well. The request_module(name) internally creates a kernel thread which execs the userspace command modprobe -s -k module_name, using the standard exec_usermodehelper() kernel interface (which is also exported to modules). The function returns 0 on success, however it is usually not worth checking the return code from request_module(). Instead, the programming idiom is:



if (check_some_feature() == NULL)
    request_module(module);
if (check_some_feature() == NULL)
    return -ENODEV;

For example, this is done by fs/block_dev.c:get_blkfops() to load a module block-major-N when attempt is made to open a block device with major N. Obviously, there is no such module called block-major-N (Linux developers only chose sensible names for their modules) but it is mapped to a proper module name using the file /etc/modules.conf. However, for most well-known major numbers (and other kinds of modules) the modprobe/insmod commands know which real module to load without needing an explicit alias statement in /etc/modules.conf.

A good example of loading a module is inside the mount(2) system call. The mount(2) system call accepts the filesystem type as a string which fs/super.c:do_mount() then passes on to fs/super.c:get_fs_type():



static struct file_system_type *get_fs_type(const char *name)
{
        struct file_system_type *fs;

        read_lock(&file_systems_lock);
        fs = *(find_filesystem(name));
        if (fs && !try_inc_mod_count(fs->owner))
                fs = NULL;
        read_unlock(&file_systems_lock);
        if (!fs && (request_module(name) == 0)) {
                read_lock(&file_systems_lock);
                fs = *(find_filesystem(name));
                if (fs && !try_inc_mod_count(fs->owner))
                        fs = NULL;
                read_unlock(&file_systems_lock);
        }
        return fs;
}

A few things to note in this function:

First we attempt to find the filesystem with the given name amongst those already registered. This is done under protection of file_systems_lock taken for read (as we are not modifying the list of registered filesystems).
If such a filesystem is found then we attempt to get a new reference to it by trying to increment its module's hold count. This always returns 1 for statically linked filesystems or for modules not presently being deleted. If try_inc_mod_count() returned 0 then we consider it a failure - i.e. if the module is there but is being deleted, it is as good as if it were not there at all.
We drop the file_systems_lock because what we are about to do next (request_module()) is a blocking operation, and therefore we can't hold a spinlock over it. Actually, in this specific case, we would have to drop file_systems_lock anyway, even if request_module() were guaranteed to be non-blocking and the module loading were executed in the same context atomically. The reason for this is that the module's initialisation function will try to call register_filesystem(), which will take the same file_systems_lock read-write spinlock for write.
If the attempt to load was successful, then we take the file_systems_lock spinlock and try to locate the newly registered filesystem in the list. Note that this is slightly wrong because it is in principle possible for a bug in modprobe command to cause it to coredump after it successfully loaded the requested module, in which case request_module() will fail even though the new filesystem will be registered, and yet get_fs_type() won't find it.
If the filesystem is found and we are able to get a reference to it, we return it. Otherwise we return NULL.

When a module is loaded into the kernel, it can refer to any symbols that are exported as public by the kernel using EXPORT_SYMBOL() macro or by other currently loaded modules. If the module uses symbols from another module, it is marked as depending on that module during dependency recalculation, achieved by running depmod -a command on boot (e.g. after installing a new kernel).

Usually, one must match the set of modules with the version of the kernel interfaces they use, which under Linux simply means the "kernel version" as there is no special kernel interface versioning mechanism in general. However, there is a limited functionality called "module versioning" or CONFIG_MODVERSIONS which allows to avoid recompiling modules when switching to a new kernel. What happens here is that the kernel symbol table is treated differently for internal access and for access from modules. The elements of public (i.e. exported) part of the symbol table are built by 32bit checksumming the C declaration. So, in order to resolve a symbol used by a module during loading, the loader must match the full representation of the symbol that includes the checksum; it will refuse to load the module if these symbols differ. This only happens when both the kernel and the module are compiled with module versioning enabled. If either one of them uses the original symbol names, the loader simply tries to match the kernel version declared by the module and the one exported by the kernel and refuses to load if they differ.

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