/*
   * Generic pidhash and scalable, time-bounded PID allocator
   *
   * (C) 2002-2003 William Irwin, IBM
   * (C) 2004 William Irwin, Oracle
   * (C) 2002-2004 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>

  #include <linux/pid_namespace.h>

  #include <linux/init_task.h>

  
  #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)

  static struct hlist_head *pid_hash;

  static int pidhash_shift;

  static struct kmem_cache *pid_cachep;

  struct pid init_struct_pid = INIT_STRUCT_PID;

  
  int pid_max = PID_MAX_DEFAULT;

  
  #define RESERVED_PIDS		300
  
  int pid_max_min = RESERVED_PIDS + 1;
  int pid_max_max = PID_MAX_LIMIT;

  #define BITS_PER_PAGE		(PAGE_SIZE*8)
  #define BITS_PER_PAGE_MASK	(BITS_PER_PAGE-1)

  static inline int mk_pid(struct pid_namespace *pid_ns,
  		struct pidmap *map, int off)

  {

  	return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;

  }

  #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.
   */

  struct pid_namespace init_pid_ns = {

  	.kref = {
  		.refcount       = ATOMIC_INIT(2),
  	},

  	.pidmap = {
  		[ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  	},

  	.last_pid = 0,
  	.child_reaper = &init_task

  };

  /*
   * Note: disable interrupts while the pidmap_lock is held as an
   * interrupt might come in and do read_lock(&tasklist_lock).
   *
   * If we don't disable interrupts there is a nasty deadlock between
   * detach_pid()->free_pid() and another cpu that does
   * spin_lock(&pidmap_lock) followed by an interrupt routine that does
   * read_lock(&tasklist_lock);
   *
   * After we clean up the tasklist_lock and know there are no
   * irq handlers that take it we can leave the interrupts enabled.
   * For now it is easier to be safe than to prove it can't happen.
   */

  static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

  static fastcall void free_pidmap(struct pid_namespace *pid_ns, int pid)

  {

  	struct pidmap *map = pid_ns->pidmap + pid / BITS_PER_PAGE;

  	int offset = pid & BITS_PER_PAGE_MASK;
  
  	clear_bit(offset, map->page);
  	atomic_inc(&map->nr_free);
  }

  static int alloc_pidmap(struct pid_namespace *pid_ns)

  {

  	int i, offset, max_scan, pid, last = pid_ns->last_pid;

  	struct pidmap *map;

  
  	pid = last + 1;
  	if (pid >= pid_max)
  		pid = RESERVED_PIDS;
  	offset = pid & BITS_PER_PAGE_MASK;

  	map = &pid_ns->pidmap[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_irq(&pidmap_lock);

  			if (map->page)

  				kfree(page);

  			else

  				map->page = page;

  			spin_unlock_irq(&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);

  					pid_ns->last_pid = pid;

  					return pid;
  				}
  				offset = find_next_offset(map, offset);

  				pid = mk_pid(pid_ns, 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 < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {

  			++map;
  			offset = 0;
  		} else {

  			map = &pid_ns->pidmap[0];

  			offset = RESERVED_PIDS;
  			if (unlikely(last == offset))
  				break;
  		}

  		pid = mk_pid(pid_ns, map, offset);

  	}
  	return -1;
  }

  static int next_pidmap(struct pid_namespace *pid_ns, int last)

  {
  	int offset;

  	struct pidmap *map, *end;

  
  	offset = (last + 1) & BITS_PER_PAGE_MASK;

  	map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
  	end = &pid_ns->pidmap[PIDMAP_ENTRIES];

  	for (; map < end; 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(pid_ns, map, offset);

  	}
  	return -1;
  }

  fastcall void put_pid(struct pid *pid)
  {
  	if (!pid)
  		return;
  	if ((atomic_read(&pid->count) == 1) ||
  	     atomic_dec_and_test(&pid->count))
  		kmem_cache_free(pid_cachep, pid);
  }

  EXPORT_SYMBOL_GPL(put_pid);

  
  static void delayed_put_pid(struct rcu_head *rhp)
  {
  	struct pid *pid = container_of(rhp, struct pid, rcu);
  	put_pid(pid);
  }
  
  fastcall void free_pid(struct pid *pid)
  {
  	/* We can be called with write_lock_irq(&tasklist_lock) held */
  	unsigned long flags;
  
  	spin_lock_irqsave(&pidmap_lock, flags);
  	hlist_del_rcu(&pid->pid_chain);
  	spin_unlock_irqrestore(&pidmap_lock, flags);

  	free_pidmap(&init_pid_ns, pid->nr);

  	call_rcu(&pid->rcu, delayed_put_pid);
  }
  
  struct pid *alloc_pid(void)
  {
  	struct pid *pid;
  	enum pid_type type;
  	int nr = -1;
  
  	pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
  	if (!pid)
  		goto out;

  	nr = alloc_pidmap(current->nsproxy->pid_ns);

  	if (nr < 0)
  		goto out_free;
  
  	atomic_set(&pid->count, 1);
  	pid->nr = nr;
  	for (type = 0; type < PIDTYPE_MAX; ++type)
  		INIT_HLIST_HEAD(&pid->tasks[type]);
  
  	spin_lock_irq(&pidmap_lock);
  	hlist_add_head_rcu(&pid->pid_chain, &pid_hash[pid_hashfn(pid->nr)]);
  	spin_unlock_irq(&pidmap_lock);
  
  out:
  	return pid;
  
  out_free:
  	kmem_cache_free(pid_cachep, pid);
  	pid = NULL;
  	goto out;
  }
  
  struct pid * fastcall find_pid(int nr)

  {
  	struct hlist_node *elem;
  	struct pid *pid;

  	hlist_for_each_entry_rcu(pid, elem,

  			&pid_hash[pid_hashfn(nr)], pid_chain) {

  		if (pid->nr == nr)
  			return pid;
  	}
  	return NULL;
  }

  EXPORT_SYMBOL_GPL(find_pid);

  /*
   * attach_pid() must be called with the tasklist_lock write-held.
   */
  int fastcall attach_pid(struct task_struct *task, enum pid_type type,
  		struct pid *pid)

  {

  	struct pid_link *link;

  	link = &task->pids[type];

  	link->pid = pid;

  	hlist_add_head_rcu(&link->node, &pid->tasks[type]);

  
  	return 0;
  }

  void fastcall detach_pid(struct task_struct *task, enum pid_type type)

  {

  	struct pid_link *link;
  	struct pid *pid;
  	int tmp;

  	link = &task->pids[type];
  	pid = link->pid;

  	hlist_del_rcu(&link->node);
  	link->pid = NULL;

  	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
  		if (!hlist_empty(&pid->tasks[tmp]))
  			return;

  	free_pid(pid);

  }

  /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
  void fastcall transfer_pid(struct task_struct *old, struct task_struct *new,
  			   enum pid_type type)
  {
  	new->pids[type].pid = old->pids[type].pid;
  	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
  	old->pids[type].pid = NULL;
  }

  struct task_struct * fastcall pid_task(struct pid *pid, enum pid_type type)

  {

  	struct task_struct *result = NULL;
  	if (pid) {
  		struct hlist_node *first;
  		first = rcu_dereference(pid->tasks[type].first);
  		if (first)
  			result = hlist_entry(first, struct task_struct, pids[(type)].node);
  	}
  	return result;
  }

  /*
   * Must be called under rcu_read_lock() or with tasklist_lock read-held.
   */

  struct task_struct *find_task_by_pid_type(int type, int nr)

  {
  	return pid_task(find_pid(nr), type);
  }

  EXPORT_SYMBOL(find_task_by_pid_type);

  struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
  {
  	struct pid *pid;
  	rcu_read_lock();
  	pid = get_pid(task->pids[type].pid);
  	rcu_read_unlock();
  	return pid;
  }

  struct task_struct *fastcall get_pid_task(struct pid *pid, enum pid_type type)
  {
  	struct task_struct *result;
  	rcu_read_lock();
  	result = pid_task(pid, type);
  	if (result)
  		get_task_struct(result);
  	rcu_read_unlock();
  	return result;

  }

  struct pid *find_get_pid(pid_t nr)

  {
  	struct pid *pid;

  	rcu_read_lock();
  	pid = get_pid(find_pid(nr));
  	rcu_read_unlock();

  	return pid;

  }

  /*

   * 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;
  
  	do {
  		pid = find_pid(nr);
  		if (pid)
  			break;

  		nr = next_pidmap(current->nsproxy->pid_ns, nr);

  	} while (nr > 0);
  
  	return pid;
  }

  EXPORT_SYMBOL_GPL(find_get_pid);

  struct pid_namespace *copy_pid_ns(unsigned long flags, struct pid_namespace *old_ns)

  {

  	BUG_ON(!old_ns);

  	get_pid_ns(old_ns);

  	return old_ns;

  }
  
  void free_pid_ns(struct kref *kref)
  {
  	struct pid_namespace *ns;
  
  	ns = container_of(kref, struct pid_namespace, kref);
  	kfree(ns);
  }

  /*

   * 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, pidhash_size;

  	unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
  
  	pidhash_shift = max(4, fls(megabytes * 4));
  	pidhash_shift = min(12, pidhash_shift);
  	pidhash_size = 1 << pidhash_shift;
  
  	printk("PID hash table entries: %d (order: %d, %Zd bytes)
  ",
  		pidhash_size, pidhash_shift,

  		pidhash_size * sizeof(struct hlist_head));
  
  	pid_hash = alloc_bootmem(pidhash_size *	sizeof(*(pid_hash)));
  	if (!pid_hash)
  		panic("Could not alloc pidhash!
  ");
  	for (i = 0; i < pidhash_size; i++)
  		INIT_HLIST_HEAD(&pid_hash[i]);

  }
  
  void __init pidmap_init(void)
  {

  	init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);

  	/* Reserve PID 0. We never call free_pidmap(0) */

  	set_bit(0, init_pid_ns.pidmap[0].page);
  	atomic_dec(&init_pid_ns.pidmap[0].nr_free);

  	pid_cachep = KMEM_CACHE(pid, SLAB_PANIC);

  }