nest/locks.c

   1 /*
   2  *      BIRD Object Locks
   3  *
   4  *      (c) 1999 Martin Mares <mj@ucw.cz>
   5  *
   6  *      Can be freely distributed and used under the terms of the GNU GPL.
   7  */
   8
   9 /**
  10  * DOC: Object locks
  11  *
  12  * The lock module provides a simple mechanism for avoiding conflicts between
  13  * various protocols which would like to use a single physical resource (for
  14  * example a network port). It would be easy to say that such collisions can
  15  * occur only when the user specifies an invalid configuration and therefore
  16  * he deserves to get what he has asked for, but unfortunately they can also
  17  * arise legitimately when the daemon is reconfigured and there exists (although
  18  * for a short time period only) an old protocol instance being shut down and a new one
  19  * willing to start up on the same interface.
  20  *
  21  * The solution is very simple: when any protocol wishes to use a network port
  22  * or some other non-shareable resource, it asks the core to lock it and it doesn't
  23  * use the resource until it's notified that it has acquired the lock.
  24  *
  25  * Object locks are represented by &object_lock structures which are in turn a
  26  * kind of resource. Lockable resources are uniquely determined by resource type
  27  * (%OBJLOCK_UDP for a UDP port etc.), IP address (usually a broadcast or
  28  * multicast address the port is bound to), port number, interface and optional
  29  * instance ID.
  30  */
  31
  32 #undef LOCAL_DEBUG
  33
  34 #include "nest/bird.h"
  35 #include "lib/resource.h"
  36 #include "nest/locks.h"
  37 #include "nest/iface.h"
  38
  39 static list olock_list;
  40 static event *olock_event;
  41
  42 static inline int
  43 olock_same(struct object_lock *x, struct object_lock *y)
  44 {
  45   return
  46     x->type == y->type &&
  47     x->iface == y->iface &&
  48     x->port == y->port &&
  49     x->inst == y->inst &&
  50     ipa_equal(x->addr, y->addr);
  51 }
  52
  53 static void
  54 olock_free(resource *r)
  55 {
  56   struct object_lock *q, *l = (struct object_lock *) r;
  57   node *n;
  58
  59   DBG("olock: Freeing %p\n", l);
  60   switch (l->state)
  61     {
  62     case OLOCK_STATE_FREE:
  63       break;
  64     case OLOCK_STATE_LOCKED:
  65     case OLOCK_STATE_EVENT:
  66       rem_node(&l->n);
  67       n = HEAD(l->waiters);
  68       if (n->next)
  69         {
  70           DBG("olock: -> %p becomes locked\n", n);
  71           q = SKIP_BACK(struct object_lock, n, n);
  72           rem_node(n);
  73           add_tail_list(&q->waiters, &l->waiters);
  74           q->state = OLOCK_STATE_EVENT;
  75           add_head(&olock_list, n);
  76           ev_schedule(olock_event);
  77         }
  78       break;
  79     case OLOCK_STATE_WAITING:
  80       rem_node(&l->n);
  81       break;
  82     default:
  83       ASSERT(0);
  84     }
  85 }
  86
  87 static void
  88 olock_dump(resource *r)
  89 {
  90   struct object_lock *l = (struct object_lock *) r;
  91   static char *olock_states[] = { "free", "locked", "waiting", "event" };
  92
  93   debug("(%d:%s:%I:%d:%d) [%s]\n", l->type, (l->iface ? l->iface->name : "?"), l->addr, l->port, l->inst, olock_states[l->state]);
  94   if (!EMPTY_LIST(l->waiters))
  95     debug(" [wanted]\n");
  96 }
  97
  98 static struct resclass olock_class = {
  99   "ObjLock",
 100   sizeof(struct object_lock),
 101   olock_free,
 102   olock_dump,
 103   NULL
 104 };
 105
 106 /**
 107  * olock_new - create an object lock
 108  * @p: resource pool to create the lock in.
 109  *
 110  * The olock_new() function creates a new resource of type &object_lock
 111  * and returns a pointer to it. After filling in the structure, the caller
 112  * should call olock_acquire() to do the real locking.
 113  */
 114 struct object_lock *
 115 olock_new(pool *p)
 116 {
 117   struct object_lock *l = ralloc(p, &olock_class);
 118
 119   l->state = OLOCK_STATE_FREE;
 120   init_list(&l->waiters);
 121   return l;
 122 }
 123
 124 /**
 125  * olock_acquire - acquire a lock
 126  * @l: the lock to acquire
 127  *
 128  * This function attempts to acquire exclusive access to the non-shareable
 129  * resource described by the lock @l. It returns immediately, but as soon
 130  * as the resource becomes available, it calls the hook() function set up
 131  * by the caller.
 132  *
 133  * When you want to release the resource, just rfree() the lock.
 134  */
 135 void
 136 olock_acquire(struct object_lock *l)
 137 {
 138   node *n;
 139   struct object_lock *q;
 140
 141   WALK_LIST(n, olock_list)
 142     {
 143       q = SKIP_BACK(struct object_lock, n, n);
 144       if (olock_same(q, l))
 145         {
 146           l->state = OLOCK_STATE_WAITING;
 147           add_tail(&q->waiters, &l->n);
 148           DBG("olock: %p waits\n", l);
 149           return;
 150         }
 151     }
 152   DBG("olock: %p acquired immediately\n", l);
 153   l->state = OLOCK_STATE_EVENT;
 154   add_head(&olock_list, &l->n);
 155   ev_schedule(olock_event);
 156 }
 157
 158 static void
 159 olock_run_event(void *unused UNUSED)
 160 {
 161   node *n;
 162   struct object_lock *q;
 163
 164   DBG("olock: Processing events\n");
 165   for(;;)
 166     {
 167       n = HEAD(olock_list);
 168       if (!n->next)
 169         break;
 170       q = SKIP_BACK(struct object_lock, n, n);
 171       if (q->state != OLOCK_STATE_EVENT)
 172         break;
 173       DBG("olock: %p locked\n", q);
 174       q->state = OLOCK_STATE_LOCKED;
 175       rem_node(&q->n);
 176       add_tail(&olock_list, &q->n);
 177       q->hook(q);
 178     }
 179 }
 180
 181 /**
 182  * olock_init - initialize the object lock mechanism
 183  *
 184  * This function is called during BIRD startup. It initializes
 185  * all the internal data structures of the lock module.
 186  */
 187 void
 188 olock_init(void)
 189 {
 190   DBG("olock: init\n");
 191   init_list(&olock_list);
 192   olock_event = ev_new(&root_pool);
 193   olock_event->hook = olock_run_event;
 194 }