/* * 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> #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift) static struct hlist_head *pid_hash; static int pidhash_shift; static kmem_cache_t *pid_cachep; int pid_max = PID_MAX_DEFAULT; int last_pid; #define RESERVED_PIDS 300 int pid_max_min = RESERVED_PIDS + 1; int pid_max_max = PID_MAX_LIMIT; #define PIDMAP_ENTRIES ((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8) #define BITS_PER_PAGE (PAGE_SIZE*8) #define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1) #define mk_pid(map, off) (((map) - pidmap_array)*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. */ 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 } }; /* * 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(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); } static 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)) { unsigned long page = get_zeroed_page(GFP_KERNEL); /* * Free the page if someone raced with us * installing it: */ spin_lock_irq(&pidmap_lock); if (map->page) free_page(page); else map->page = (void *)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); 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; } 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); } 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(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(); 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; } int fastcall attach_pid(task_t *task, enum pid_type type, int nr) { struct pid_link *link; struct pid *pid; WARN_ON(!task->pid); /* to be removed soon */ WARN_ON(!nr); /* to be removed soon */ link = &task->pids[type]; link->pid = pid = find_pid(nr); hlist_add_head_rcu(&link->node, &pid->tasks[type]); return 0; } void fastcall detach_pid(task_t *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); } 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. */ task_t *find_task_by_pid_type(int type, int nr) { return pid_task(find_pid(nr), type); } EXPORT_SYMBOL(find_task_by_pid_type); 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; } /* * 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)\n", 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!\n"); for (i = 0; i < pidhash_size; i++) INIT_HLIST_HEAD(&pid_hash[i]); } void __init pidmap_init(void) { pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL); /* Reserve PID 0. We never call free_pidmap(0) */ set_bit(0, pidmap_array->page); atomic_dec(&pidmap_array->nr_free); pid_cachep = kmem_cache_create("pid", sizeof(struct pid), __alignof__(struct pid), SLAB_PANIC, NULL, NULL); }