/*
 * Generic pidhash and scalable, time-bounded PID allocator
 *
 * (C) 2002 William Irwin, IBM
 * (C) 2002 Ingo Molnar, Red Hat
 *
 * pid-structures are backing objects for tasks sharing a given ID to chain
 * against. There is very little to them aside from hashing them and
 * parking tasks using given ID's on a list.
 *
 * The hash is always changed with the tasklist_lock write-acquired,
 * and the hash is only accessed with the tasklist_lock at least
 * read-acquired, so there's no additional SMP locking needed here.
 *
 * We have a list of bitmap pages, which bitmaps represent the PID space.
 * Allocating and freeing PIDs is completely lockless. The worst-case
 * allocation scenario when all but one out of 1 million PIDs possible are
 * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
 * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/bootmem.h>
#include <linux/hash.h>

#define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
static struct hlist_head *pid_hash[PIDTYPE_MAX];
static int pidhash_shift;

int pid_max = PID_MAX_DEFAULT;
int last_pid;

#define RESERVED_PIDS           300

#define PIDMAP_ENTRIES          (PID_MAX_LIMIT/PAGE_SIZE/8)
#define BITS_PER_PAGE           (PAGE_SIZE*8)
#define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
#define find_next_offset(map, off)                                      \
                find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

/*
 * PID-map pages start out as NULL, they get allocated upon
 * first use and are never deallocated. This way a low pid_max
 * value does not cause lots of bitmaps to be allocated, but
 * the scheme scales to up to 4 million PIDs, runtime.
 */
typedef struct pidmap {
        atomic_t nr_free;
        void *page;
} pidmap_t;

static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
         { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };

static spinlock_t pidmap_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;

static inline int mk_pid(struct pidmap *map, int off)
{
        return (map - pidmap_array)*BITS_PER_PAGE + off;
}

fastcall void free_pidmap(int pid)
{
        pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
        int offset = pid & BITS_PER_PAGE_MASK;

        clear_bit(offset, map->page);
        atomic_inc(&map->nr_free);
}

int alloc_pidmap(void)
{
        int i, offset, max_scan, pid, last = last_pid;
        pidmap_t *map;

        pid = last + 1;
        if (pid >= pid_max)
                pid = RESERVED_PIDS;
        offset = pid & BITS_PER_PAGE_MASK;
        map = &pidmap_array[pid/BITS_PER_PAGE];
        max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
        for (i = 0; i <= max_scan; ++i) {
                if (unlikely(!map->page)) {
                        void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
                        /*
                         * Free the page if someone raced with us
                         * installing it:
                         */
                        spin_lock(&pidmap_lock);
                        if (map->page)
                                kfree(page);
                        else
                                map->page = page;
                        spin_unlock(&pidmap_lock);
                        if (unlikely(!map->page))
                                break;
                }
                if (likely(atomic_read(&map->nr_free))) {
                        do {
                                if (!test_and_set_bit(offset, map->page)) {
                                        atomic_dec(&map->nr_free);
                                        last_pid = pid;
                                        return pid;
                                }
                                offset = find_next_offset(map, offset);
                                pid = mk_pid(map, offset);
                        /*
                         * find_next_offset() found a bit, the pid from it
                         * is in-bounds, and if we fell back to the last
                         * bitmap block and the final block was the same
                         * as the starting point, pid is before last_pid.
                         */
                        } while (offset < BITS_PER_PAGE && pid < pid_max &&
                                        (i != max_scan || pid < last ||
                                            !((last+1) & BITS_PER_PAGE_MASK)));
                }
                if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
                        ++map;
                        offset = 0;
                } else {
                        map = &pidmap_array[0];
                        offset = RESERVED_PIDS;
                        if (unlikely(last == offset))
                                break;
                }
                pid = mk_pid(map, offset);
        }
        return -1;
}

static int next_pidmap(int last)
{
        int offset;
        pidmap_t *map;

        offset = (last + 1) & BITS_PER_PAGE_MASK;
        map = &pidmap_array[(last + 1)/BITS_PER_PAGE];
        for (; map < &pidmap_array[PIDMAP_ENTRIES]; map++, offset = 0) {
                if (unlikely(!map->page))
                        continue;
                offset = find_next_bit(map->page, BITS_PER_PAGE, offset);
                if (offset < BITS_PER_PAGE)
                        return mk_pid(map, offset);
        }
        return -1;
}

struct pid * fastcall find_pid(enum pid_type type, int nr)
{
        struct hlist_node *elem;
        struct pid *pid;

        hlist_for_each_entry(pid, elem,
                        &pid_hash[type][pid_hashfn(nr)], pid_chain) {
                if (pid->nr == nr)
                        return pid;
        }
        return NULL;
}

int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
{
        struct pid *pid, *task_pid;

        task_pid = &task->pids[type];
        pid = find_pid(type, nr);
        if (pid == NULL) {
                hlist_add_head(&task_pid->pid_chain,
                                &pid_hash[type][pid_hashfn(nr)]);
                INIT_LIST_HEAD(&task_pid->pid_list);
        } else {
                INIT_HLIST_NODE(&task_pid->pid_chain);
                list_add_tail(&task_pid->pid_list, &pid->pid_list);
        }
        task_pid->nr = nr;

        return 0;
}

static inline int __detach_pid(task_t *task, enum pid_type type)
{
        struct pid *pid, *pid_next;
        int nr;

        pid = &task->pids[type];
        if (!hlist_unhashed(&pid->pid_chain)) {
                hlist_del(&pid->pid_chain);
                if (!list_empty(&pid->pid_list)) {
                        pid_next = list_entry(pid->pid_list.next,
                                                struct pid, pid_list);
                        /* insert next pid from pid_list to hash */
                        hlist_add_head(&pid_next->pid_chain,
                                &pid_hash[type][pid_hashfn(pid_next->nr)]);
                }
        }
        list_del(&pid->pid_list);
        nr = pid->nr;
        pid->nr = 0;

        return nr;
}

void fastcall detach_pid(task_t *task, enum pid_type type)
{
        int nr;

        nr = __detach_pid(task, type);
        if (!nr)
                return;

        for (type = 0; type < PIDTYPE_MAX; ++type)
                if (find_pid(type, nr))
                        return;
        free_pidmap(nr);
}

task_t *find_task_by_pid_type(int type, int nr)
{
        struct pid *pid;

        pid = find_pid(type, nr);
        if (!pid)
                return NULL;

        return pid_task(&pid->pid_list, type);
}

EXPORT_SYMBOL(find_task_by_pid_type);

/*
 * This function switches the PIDs if a non-leader thread calls
 * sys_execve() - this must be done without releasing the PID.
 * (which a detach_pid() would eventually do.)
 */
void switch_exec_pids(task_t *leader, task_t *thread)
{
        __detach_pid(leader, PIDTYPE_PID);
        __detach_pid(leader, PIDTYPE_TGID);
        __detach_pid(leader, PIDTYPE_PGID);
        __detach_pid(leader, PIDTYPE_SID);

        __detach_pid(thread, PIDTYPE_PID);
        __detach_pid(thread, PIDTYPE_TGID);

        leader->pid = leader->tgid = thread->pid;
        thread->pid = thread->tgid;

        attach_pid(thread, PIDTYPE_PID, thread->pid);
        attach_pid(thread, PIDTYPE_TGID, thread->tgid);
        attach_pid(thread, PIDTYPE_PGID, thread->signal->pgrp);
        attach_pid(thread, PIDTYPE_SID, thread->signal->session);
        list_add_tail(&thread->tasks, &init_task.tasks);

        attach_pid(leader, PIDTYPE_PID, leader->pid);
        attach_pid(leader, PIDTYPE_TGID, leader->tgid);
        attach_pid(leader, PIDTYPE_PGID, leader->signal->pgrp);
        attach_pid(leader, PIDTYPE_SID, leader->signal->session);
}

/**
 * pid_alive - check that a task structure is not stale
 * @p: Task structure to be checked.
 *
 * Test if a process is not yet dead (at most zombie state)
 * If pid_alive fails, then pointers within the task structure
 * can be stale and must not be dereferenced.
 */
int pid_alive(struct task_struct *p)
{
        return p->pids[PIDTYPE_PID].nr != 0;
}

/*
 * Used by proc to find the first pid that is greater then or equal to nr.
 *
 * If there is a pid at nr this function is exactly the same as find_pid.
 */
struct pid *find_ge_pid(int nr)
{
        struct pid *pid;

        if (nr == 0)
                nr = 1;

        do {
                pid = find_pid(PIDTYPE_PID, nr);
                if (pid)
                        break;
                nr = next_pidmap(nr);
        } while (nr > 0);

        return pid;
}

/*
 * The pid hash table is scaled according to the amount of memory in the
 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 * more.
 */
void __init pidhash_init(void)
{
        int i, j, pidhash_size;
        unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);

        pidhash_shift = max(10, fls(megabytes * 4));
        pidhash_shift = min(12, pidhash_shift);
        pidhash_size = 1 << pidhash_shift;

        printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
                pidhash_size, pidhash_shift,
                PIDTYPE_MAX * pidhash_size * sizeof(struct hlist_head));

        for (i = 0; i < PIDTYPE_MAX; i++) {
                pid_hash[i] = alloc_bootmem(pidhash_size *
                                        sizeof(*(pid_hash[i])));
                if (!pid_hash[i])
                        panic("Could not alloc pidhash!\n");
                for (j = 0; j < pidhash_size; j++)
                        INIT_HLIST_HEAD(&pid_hash[i][j]);
        }
}

void __init pidmap_init(void)
{
        int i;

        pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
        set_bit(0, pidmap_array->page);
        atomic_dec(&pidmap_array->nr_free);

        /*
         * Allocate PID 0, and hash it via all PID types:
         */

        for (i = 0; i < PIDTYPE_MAX; i++)
                attach_pid(current, i, 0);
}