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->vrf == y->vrf &&
  49     x->port == y->port &&
  50     x->inst == y->inst &&
  51     ipa_equal(x->addr, y->addr);
  52 }
  53
  54 static void
  55 olock_free(resource *r)
  56 {
  57   struct object_lock *q, *l = (struct object_lock *) r;
  58   node *n;
  59
  60   DBG("olock: Freeing %p\n", l);
  61   switch (l->state)
  62     {
  63     case OLOCK_STATE_FREE:
  64       break;
  65     case OLOCK_STATE_LOCKED:
  66     case OLOCK_STATE_EVENT:
  67       rem_node(&l->n);
  68       n = HEAD(l->waiters);
  69       if (n->next)
  70         {
  71           DBG("olock: -> %p becomes locked\n", n);
  72           q = SKIP_BACK(struct object_lock, n, n);
  73           rem_node(n);
  74           add_tail_list(&q->waiters, &l->waiters);
  75           q->state = OLOCK_STATE_EVENT;
  76           add_head(&olock_list, n);
  77           ev_schedule(olock_event);
  78         }
  79       break;
  80     case OLOCK_STATE_WAITING:
  81       rem_node(&l->n);
  82       break;
  83     default:
  84       ASSERT(0);
  85     }
  86 }
  87
  88 static void
  89 olock_dump(resource *r)
  90 {
  91   struct object_lock *l = (struct object_lock *) r;
  92   static char *olock_states[] = { "free", "locked", "waiting", "event" };
  93
  94   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]);
  95   if (!EMPTY_LIST(l->waiters))
  96     debug(" [wanted]\n");
  97 }
  98
  99 static struct resclass olock_class = {
 100   "ObjLock",
 101   sizeof(struct object_lock),
 102   olock_free,
 103   olock_dump,
 104   NULL,
 105   NULL,
 106 };
 107
 108 /**
 109  * olock_new - create an object lock
 110  * @p: resource pool to create the lock in.
 111  *
 112  * The olock_new() function creates a new resource of type &object_lock
 113  * and returns a pointer to it. After filling in the structure, the caller
 114  * should call olock_acquire() to do the real locking.
 115  */
 116 struct object_lock *
 117 olock_new(pool *p)
 118 {
 119   struct object_lock *l = ralloc(p, &olock_class);
 120
 121   l->state = OLOCK_STATE_FREE;
 122   init_list(&l->waiters);
 123   return l;
 124 }
 125
 126 /**
 127  * olock_acquire - acquire a lock
 128  * @l: the lock to acquire
 129  *
 130  * This function attempts to acquire exclusive access to the non-shareable
 131  * resource described by the lock @l. It returns immediately, but as soon
 132  * as the resource becomes available, it calls the hook() function set up
 133  * by the caller.
 134  *
 135  * When you want to release the resource, just rfree() the lock.
 136  */
 137 void
 138 olock_acquire(struct object_lock *l)
 139 {
 140   node *n;
 141   struct object_lock *q;
 142
 143   WALK_LIST(n, olock_list)
 144     {
 145       q = SKIP_BACK(struct object_lock, n, n);
 146       if (olock_same(q, l))
 147         {
 148           l->state = OLOCK_STATE_WAITING;
 149           add_tail(&q->waiters, &l->n);
 150           DBG("olock: %p waits\n", l);
 151           return;
 152         }
 153     }
 154   DBG("olock: %p acquired immediately\n", l);
 155   l->state = OLOCK_STATE_EVENT;
 156   add_head(&olock_list, &l->n);
 157   ev_schedule(olock_event);
 158 }
 159
 160 static void
 161 olock_run_event(void *unused UNUSED)
 162 {
 163   node *n;
 164   struct object_lock *q;
 165
 166   DBG("olock: Processing events\n");
 167   for(;;)
 168     {
 169       n = HEAD(olock_list);
 170       if (!n->next)
 171         break;
 172       q = SKIP_BACK(struct object_lock, n, n);
 173       if (q->state != OLOCK_STATE_EVENT)
 174         break;
 175       DBG("olock: %p locked\n", q);
 176       q->state = OLOCK_STATE_LOCKED;
 177       rem_node(&q->n);
 178       add_tail(&olock_list, &q->n);
 179       q->hook(q);
 180     }
 181 }
 182
 183 /**
 184  * olock_init - initialize the object lock mechanism
 185  *
 186  * This function is called during BIRD startup. It initializes
 187  * all the internal data structures of the lock module.
 188  */
 189 void
 190 olock_init(void)
 191 {
 192   DBG("olock: init\n");
 193   init_list(&olock_list);
 194   olock_event = ev_new(&root_pool);
 195   olock_event->hook = olock_run_event;
 196 }