FreeBSD/Linux Kernel Cross Reference
sys/kernel/pid.c

     /*
      * Generic pidhash and scalable, time-bounded PID allocator
      *
      * (C) 2002-2003 Nadia Yvette Chambers, IBM
      * (C) 2004 Nadia Yvette Chambers, 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).
     *
     * Pid namespaces:
     *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
     *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
     *     Many thanks to Oleg Nesterov for comments and help
     *
     */
    
    #include <linux/mm.h>
    #include <linux/export.h>
    #include <linux/slab.h>
    #include <linux/init.h>
    #include <linux/rculist.h>
    #include <linux/bootmem.h>
    #include <linux/hash.h>
    #include <linux/pid_namespace.h>
    #include <linux/init_task.h>
    #include <linux/syscalls.h>
    #include <linux/proc_fs.h>
    
    #define pid_hashfn(nr, ns)      \
            hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
    static struct hlist_head *pid_hash;
    static unsigned int pidhash_shift = 4;
    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,
            .level = 0,
            .child_reaper = &init_task,
            .user_ns = &init_user_ns,
            .proc_inum = PROC_PID_INIT_INO,
    };
    EXPORT_SYMBOL_GPL(init_pid_ns);
    
    /*
     * 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 void free_pidmap(struct upid *upid)
   {
           int nr = upid->nr;
           struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
           int offset = nr & BITS_PER_PAGE_MASK;
   
           clear_bit(offset, map->page);
           atomic_inc(&map->nr_free);
   }
   
   /*
    * If we started walking pids at 'base', is 'a' seen before 'b'?
    */
   static int pid_before(int base, int a, int b)
   {
           /*
            * This is the same as saying
            *
            * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
            * and that mapping orders 'a' and 'b' with respect to 'base'.
            */
           return (unsigned)(a - base) < (unsigned)(b - base);
   }
   
   /*
    * We might be racing with someone else trying to set pid_ns->last_pid
    * at the pid allocation time (there's also a sysctl for this, but racing
    * with this one is OK, see comment in kernel/pid_namespace.c about it).
    * We want the winner to have the "later" value, because if the
    * "earlier" value prevails, then a pid may get reused immediately.
    *
    * Since pids rollover, it is not sufficient to just pick the bigger
    * value.  We have to consider where we started counting from.
    *
    * 'base' is the value of pid_ns->last_pid that we observed when
    * we started looking for a pid.
    *
    * 'pid' is the pid that we eventually found.
    */
   static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
   {
           int prev;
           int last_write = base;
           do {
                   prev = last_write;
                   last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
           } while ((prev != last_write) && (pid_before(base, last_write, pid)));
   }
   
   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];
           /*
            * If last_pid points into the middle of the map->page we
            * want to scan this bitmap block twice, the second time
            * we start with offset == 0 (or RESERVED_PIDS).
            */
           max_scan = DIV_ROUND_UP(pid_max, 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) {
                                   map->page = page;
                                   page = NULL;
                           }
                           spin_unlock_irq(&pidmap_lock);
                           kfree(page);
                           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);
                                           set_last_pid(pid_ns, last, pid);
                                           return pid;
                                   }
                                   offset = find_next_offset(map, offset);
                                   pid = mk_pid(pid_ns, map, offset);
                           } while (offset < BITS_PER_PAGE && pid < pid_max);
                   }
                   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;
   }
   
   int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
   {
           int offset;
           struct pidmap *map, *end;
   
           if (last >= PID_MAX_LIMIT)
                   return -1;
   
           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;
   }
   
   void put_pid(struct pid *pid)
   {
           struct pid_namespace *ns;
   
           if (!pid)
                   return;
   
           ns = pid->numbers[pid->level].ns;
           if ((atomic_read(&pid->count) == 1) ||
                atomic_dec_and_test(&pid->count)) {
                   kmem_cache_free(ns->pid_cachep, pid);
                   put_pid_ns(ns);
           }
   }
   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);
   }
   
   void free_pid(struct pid *pid)
   {
           /* We can be called with write_lock_irq(&tasklist_lock) held */
           int i;
           unsigned long flags;
   
           spin_lock_irqsave(&pidmap_lock, flags);
           for (i = 0; i <= pid->level; i++) {
                   struct upid *upid = pid->numbers + i;
                   struct pid_namespace *ns = upid->ns;
                   hlist_del_rcu(&upid->pid_chain);
                   switch(--ns->nr_hashed) {
                   case 1:
                           /* When all that is left in the pid namespace
                            * is the reaper wake up the reaper.  The reaper
                            * may be sleeping in zap_pid_ns_processes().
                            */
                           wake_up_process(ns->child_reaper);
                           break;
                   case 0:
                           schedule_work(&ns->proc_work);
                           break;
                   }
           }
           spin_unlock_irqrestore(&pidmap_lock, flags);
   
           for (i = 0; i <= pid->level; i++)
                   free_pidmap(pid->numbers + i);
   
           call_rcu(&pid->rcu, delayed_put_pid);
   }
   
   struct pid *alloc_pid(struct pid_namespace *ns)
   {
           struct pid *pid;
           enum pid_type type;
           int i, nr;
           struct pid_namespace *tmp;
           struct upid *upid;
   
           pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
           if (!pid)
                   goto out;
   
           tmp = ns;
           pid->level = ns->level;
           for (i = ns->level; i >= 0; i--) {
                   nr = alloc_pidmap(tmp);
                   if (nr < 0)
                           goto out_free;
   
                   pid->numbers[i].nr = nr;
                   pid->numbers[i].ns = tmp;
                   tmp = tmp->parent;
           }
   
           if (unlikely(is_child_reaper(pid))) {
                   if (pid_ns_prepare_proc(ns))
                           goto out_free;
           }
   
           get_pid_ns(ns);
           atomic_set(&pid->count, 1);
           for (type = 0; type < PIDTYPE_MAX; ++type)
                   INIT_HLIST_HEAD(&pid->tasks[type]);
   
           upid = pid->numbers + ns->level;
           spin_lock_irq(&pidmap_lock);
           if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
                   goto out_unlock;
           for ( ; upid >= pid->numbers; --upid) {
                   hlist_add_head_rcu(&upid->pid_chain,
                                   &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
                   upid->ns->nr_hashed++;
           }
           spin_unlock_irq(&pidmap_lock);
   
   out:
           return pid;
   
   out_unlock:
           spin_unlock(&pidmap_lock);
   out_free:
           while (++i <= ns->level)
                   free_pidmap(pid->numbers + i);
   
           kmem_cache_free(ns->pid_cachep, pid);
           pid = NULL;
           goto out;
   }
   
   void disable_pid_allocation(struct pid_namespace *ns)
   {
           spin_lock_irq(&pidmap_lock);
           ns->nr_hashed &= ~PIDNS_HASH_ADDING;
           spin_unlock_irq(&pidmap_lock);
   }
   
   struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
   {
           struct hlist_node *elem;
           struct upid *pnr;
   
           hlist_for_each_entry_rcu(pnr, elem,
                           &pid_hash[pid_hashfn(nr, ns)], pid_chain)
                   if (pnr->nr == nr && pnr->ns == ns)
                           return container_of(pnr, struct pid,
                                           numbers[ns->level]);
   
           return NULL;
   }
   EXPORT_SYMBOL_GPL(find_pid_ns);
   
   struct pid *find_vpid(int nr)
   {
           return find_pid_ns(nr, task_active_pid_ns(current));
   }
   EXPORT_SYMBOL_GPL(find_vpid);
   
   /*
    * attach_pid() must be called with the tasklist_lock write-held.
    */
   void 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]);
   }
   
   static void __change_pid(struct task_struct *task, enum pid_type type,
                           struct pid *new)
   {
           struct pid_link *link;
           struct pid *pid;
           int tmp;
   
           link = &task->pids[type];
           pid = link->pid;
   
           hlist_del_rcu(&link->node);
           link->pid = new;
   
           for (tmp = PIDTYPE_MAX; --tmp >= 0; )
                   if (!hlist_empty(&pid->tasks[tmp]))
                           return;
   
           free_pid(pid);
   }
   
   void detach_pid(struct task_struct *task, enum pid_type type)
   {
           __change_pid(task, type, NULL);
   }
   
   void change_pid(struct task_struct *task, enum pid_type type,
                   struct pid *pid)
   {
           __change_pid(task, type, pid);
           attach_pid(task, type, pid);
   }
   
   /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
   void 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);
   }
   
   struct task_struct *pid_task(struct pid *pid, enum pid_type type)
   {
           struct task_struct *result = NULL;
           if (pid) {
                   struct hlist_node *first;
                   first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
                                                 lockdep_tasklist_lock_is_held());
                   if (first)
                           result = hlist_entry(first, struct task_struct, pids[(type)].node);
           }
           return result;
   }
   EXPORT_SYMBOL(pid_task);
   
   /*
    * Must be called under rcu_read_lock().
    */
   struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
   {
           rcu_lockdep_assert(rcu_read_lock_held(),
                              "find_task_by_pid_ns() needs rcu_read_lock()"
                              " protection");
           return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
   }
   
   struct task_struct *find_task_by_vpid(pid_t vnr)
   {
           return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
   }
   
   struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
   {
           struct pid *pid;
           rcu_read_lock();
           if (type != PIDTYPE_PID)
                   task = task->group_leader;
           pid = get_pid(task->pids[type].pid);
           rcu_read_unlock();
           return pid;
   }
   EXPORT_SYMBOL_GPL(get_task_pid);
   
   struct task_struct *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;
   }
   EXPORT_SYMBOL_GPL(get_pid_task);
   
   struct pid *find_get_pid(pid_t nr)
   {
           struct pid *pid;
   
           rcu_read_lock();
           pid = get_pid(find_vpid(nr));
           rcu_read_unlock();
   
           return pid;
   }
   EXPORT_SYMBOL_GPL(find_get_pid);
   
   pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
   {
           struct upid *upid;
           pid_t nr = 0;
   
           if (pid && ns->level <= pid->level) {
                   upid = &pid->numbers[ns->level];
                   if (upid->ns == ns)
                           nr = upid->nr;
           }
           return nr;
   }
   EXPORT_SYMBOL_GPL(pid_nr_ns);
   
   pid_t pid_vnr(struct pid *pid)
   {
           return pid_nr_ns(pid, task_active_pid_ns(current));
   }
   EXPORT_SYMBOL_GPL(pid_vnr);
   
   pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
                           struct pid_namespace *ns)
   {
           pid_t nr = 0;
   
           rcu_read_lock();
           if (!ns)
                   ns = task_active_pid_ns(current);
           if (likely(pid_alive(task))) {
                   if (type != PIDTYPE_PID)
                           task = task->group_leader;
                   nr = pid_nr_ns(task->pids[type].pid, ns);
           }
           rcu_read_unlock();
   
           return nr;
   }
   EXPORT_SYMBOL(__task_pid_nr_ns);
   
   pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
   {
           return pid_nr_ns(task_tgid(tsk), ns);
   }
   EXPORT_SYMBOL(task_tgid_nr_ns);
   
   struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
   {
           return ns_of_pid(task_pid(tsk));
   }
   EXPORT_SYMBOL_GPL(task_active_pid_ns);
   
   /*
    * Used by proc to find the first pid that is greater than or equal to nr.
    *
    * If there is a pid at nr this function is exactly the same as find_pid_ns.
    */
   struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
   {
           struct pid *pid;
   
           do {
                   pid = find_pid_ns(nr, ns);
                   if (pid)
                           break;
                   nr = next_pidmap(ns, 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)
   {
           unsigned int i, pidhash_size;
   
           pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
                                              HASH_EARLY | HASH_SMALL,
                                              &pidhash_shift, NULL,
                                              0, 4096);
           pidhash_size = 1U << pidhash_shift;
   
           for (i = 0; i < pidhash_size; i++)
                   INIT_HLIST_HEAD(&pid_hash[i]);
   }
   
   void __init pidmap_init(void)
   {
           /* Veryify no one has done anything silly */
           BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
   
           /* bump default and minimum pid_max based on number of cpus */
           pid_max = min(pid_max_max, max_t(int, pid_max,
                                   PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
           pid_max_min = max_t(int, pid_max_min,
                                   PIDS_PER_CPU_MIN * num_possible_cpus());
           pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
   
           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);
           init_pid_ns.nr_hashed = PIDNS_HASH_ADDING;
   
           init_pid_ns.pid_cachep = KMEM_CACHE(pid,
                           SLAB_HWCACHE_ALIGN | SLAB_PANIC);
   }

Cache object: ebba0c1894b4bd2e2bda67fddb8bc686