net/ipv4/inetpeer.c

   1 /*
   2  *              INETPEER - A storage for permanent information about peers
   3  *
   4  *  This source is covered by the GNU GPL, the same as all kernel sources.
   5  *
   6  *  Authors:    Andrey V. Savochkin <saw@msu.ru>
   7  */
   8
   9 #include <linux/module.h>
  10 #include <linux/types.h>
  11 #include <linux/slab.h>
  12 #include <linux/interrupt.h>
  13 #include <linux/spinlock.h>
  14 #include <linux/random.h>
  15 #include <linux/timer.h>
  16 #include <linux/time.h>
  17 #include <linux/kernel.h>
  18 #include <linux/mm.h>
  19 #include <linux/net.h>
  20 #include <net/ip.h>
  21 #include <net/inetpeer.h>
  22
  23 /*
  24  *  Theory of operations.
  25  *  We keep one entry for each peer IP address.  The nodes contains long-living
  26  *  information about the peer which doesn't depend on routes.
  27  *  At this moment this information consists only of ID field for the next
  28  *  outgoing IP packet.  This field is incremented with each packet as encoded
  29  *  in inet_getid() function (include/net/inetpeer.h).
  30  *  At the moment of writing this notes identifier of IP packets is generated
  31  *  to be unpredictable using this code only for packets subjected
  32  *  (actually or potentially) to defragmentation.  I.e. DF packets less than
  33  *  PMTU in size uses a constant ID and do not use this code (see
  34  *  ip_select_ident() in include/net/ip.h).
  35  *
  36  *  Route cache entries hold references to our nodes.
  37  *  New cache entries get references via lookup by destination IP address in
  38  *  the avl tree.  The reference is grabbed only when it's needed i.e. only
  39  *  when we try to output IP packet which needs an unpredictable ID (see
  40  *  __ip_select_ident() in net/ipv4/route.c).
  41  *  Nodes are removed only when reference counter goes to 0.
  42  *  When it's happened the node may be removed when a sufficient amount of
  43  *  time has been passed since its last use.  The less-recently-used entry can
  44  *  also be removed if the pool is overloaded i.e. if the total amount of
  45  *  entries is greater-or-equal than the threshold.
  46  *
  47  *  Node pool is organised as an AVL tree.
  48  *  Such an implementation has been chosen not just for fun.  It's a way to
  49  *  prevent easy and efficient DoS attacks by creating hash collisions.  A huge
  50  *  amount of long living nodes in a single hash slot would significantly delay
  51  *  lookups performed with disabled BHs.
  52  *
  53  *  Serialisation issues.
  54  *  1.  Nodes may appear in the tree only with the pool lock held.
  55  *  2.  Nodes may disappear from the tree only with the pool lock held
  56  *      AND reference count being 0.
  57  *  3.  Nodes appears and disappears from unused node list only under
  58  *      "inet_peer_unused_lock".
  59  *  4.  Global variable peer_total is modified under the pool lock.
  60  *  5.  struct inet_peer fields modification:
  61  *              avl_left, avl_right, avl_parent, avl_height: pool lock
  62  *              unused: unused node list lock
  63  *              refcnt: atomically against modifications on other CPU;
  64  *                 usually under some other lock to prevent node disappearing
  65  *              dtime: unused node list lock
  66  *              v4daddr: unchangeable
  67  *              ip_id_count: atomic value (no lock needed)
  68  */
  69
  70 static struct kmem_cache *peer_cachep __read_mostly;
  71
  72 #define node_height(x) x->avl_height
  73
  74 #define peer_avl_empty ((struct inet_peer *)&peer_fake_node)
  75 #define peer_avl_empty_rcu ((struct inet_peer __rcu __force *)&peer_fake_node)
  76 static const struct inet_peer peer_fake_node = {
  77         .avl_left       = peer_avl_empty_rcu,
  78         .avl_right      = peer_avl_empty_rcu,
  79         .avl_height     = 0
  80 };
  81
  82 static struct inet_peer_base {
  83         struct inet_peer __rcu *root;
  84         spinlock_t      lock;
  85         int             total;
  86 } v4_peers = {
  87         .root           = peer_avl_empty_rcu,
  88         .lock           = __SPIN_LOCK_UNLOCKED(v4_peers.lock),
  89         .total          = 0,
  90 };
  91 #define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */
  92
  93 /* Exported for sysctl_net_ipv4.  */
  94 int inet_peer_threshold __read_mostly = 65536 + 128;    /* start to throw entries more
  95                                          * aggressively at this stage */
  96 int inet_peer_minttl __read_mostly = 120 * HZ;  /* TTL under high load: 120 sec */
  97 int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;      /* usual time to live: 10 min */
  98 int inet_peer_gc_mintime __read_mostly = 10 * HZ;
  99 int inet_peer_gc_maxtime __read_mostly = 120 * HZ;
 100
 101 static struct {
 102         struct list_head        list;
 103         spinlock_t              lock;
 104 } unused_peers = {
 105         .list                   = LIST_HEAD_INIT(unused_peers.list),
 106         .lock                   = __SPIN_LOCK_UNLOCKED(unused_peers.lock),
 107 };
 108
 109 static void peer_check_expire(unsigned long dummy);
 110 static DEFINE_TIMER(peer_periodic_timer, peer_check_expire, 0, 0);
 111
 112
 113 /* Called from ip_output.c:ip_init  */
 114 void __init inet_initpeers(void)
 115 {
 116         struct sysinfo si;
 117
 118         /* Use the straight interface to information about memory. */
 119         si_meminfo(&si);
 120         /* The values below were suggested by Alexey Kuznetsov
 121          * <kuznet@ms2.inr.ac.ru>.  I don't have any opinion about the values
 122          * myself.  --SAW
 123          */
 124         if (si.totalram <= (32768*1024)/PAGE_SIZE)
 125                 inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */
 126         if (si.totalram <= (16384*1024)/PAGE_SIZE)
 127                 inet_peer_threshold >>= 1; /* about 512KB */
 128         if (si.totalram <= (8192*1024)/PAGE_SIZE)
 129                 inet_peer_threshold >>= 2; /* about 128KB */
 130
 131         peer_cachep = kmem_cache_create("inet_peer_cache",
 132                         sizeof(struct inet_peer),
 133                         0, SLAB_HWCACHE_ALIGN | SLAB_PANIC,
 134                         NULL);
 135
 136         /* All the timers, started at system startup tend
 137            to synchronize. Perturb it a bit.
 138          */
 139         peer_periodic_timer.expires = jiffies
 140                 + net_random() % inet_peer_gc_maxtime
 141                 + inet_peer_gc_maxtime;
 142         add_timer(&peer_periodic_timer);
 143 }
 144
 145 /* Called with or without local BH being disabled. */
 146 static void unlink_from_unused(struct inet_peer *p)
 147 {
 148         if (!list_empty(&p->unused)) {
 149                 spin_lock_bh(&unused_peers.lock);
 150                 list_del_init(&p->unused);
 151                 spin_unlock_bh(&unused_peers.lock);
 152         }
 153 }
 154
 155 /*
 156  * Called with local BH disabled and the pool lock held.
 157  */
 158 #define lookup(_daddr, _stack, _base)                           \
 159 ({                                                              \
 160         struct inet_peer *u;                                    \
 161         struct inet_peer __rcu **v;                             \
 162                                                                 \
 163         stackptr = _stack;                                      \
 164         *stackptr++ = &_base->root;                             \
 165         for (u = rcu_dereference_protected(_base->root,         \
 166                         lockdep_is_held(&_base->lock));         \
 167              u != peer_avl_empty; ) {                           \
 168                 if (_daddr == u->v4daddr)                       \
 169                         break;                                  \
 170                 if ((__force __u32)_daddr < (__force __u32)u->v4daddr)  \
 171                         v = &u->avl_left;                       \
 172                 else                                            \
 173                         v = &u->avl_right;                      \
 174                 *stackptr++ = v;                                \
 175                 u = rcu_dereference_protected(*v,               \
 176                         lockdep_is_held(&_base->lock));         \
 177         }                                                       \
 178         u;                                                      \
 179 })
 180
 181 /*
 182  * Called with rcu_read_lock_bh()
 183  * Because we hold no lock against a writer, its quite possible we fall
 184  * in an endless loop.
 185  * But every pointer we follow is guaranteed to be valid thanks to RCU.
 186  * We exit from this function if number of links exceeds PEER_MAXDEPTH
 187  */
 188 static struct inet_peer *lookup_rcu_bh(__be32 daddr, struct inet_peer_base *base)
 189 {
 190         struct inet_peer *u = rcu_dereference_bh(base->root);
 191         int count = 0;
 192
 193         while (u != peer_avl_empty) {
 194                 if (daddr == u->v4daddr) {
 195                         /* Before taking a reference, check if this entry was
 196                          * deleted, unlink_from_pool() sets refcnt=-1 to make
 197                          * distinction between an unused entry (refcnt=0) and
 198                          * a freed one.
 199                          */
 200                         if (unlikely(!atomic_add_unless(&u->refcnt, 1, -1)))
 201                                 u = NULL;
 202                         return u;
 203                 }
 204                 if ((__force __u32)daddr < (__force __u32)u->v4daddr)
 205                         u = rcu_dereference_bh(u->avl_left);
 206                 else
 207                         u = rcu_dereference_bh(u->avl_right);
 208                 if (unlikely(++count == PEER_MAXDEPTH))
 209                         break;
 210         }
 211         return NULL;
 212 }
 213
 214 /* Called with local BH disabled and the pool lock held. */
 215 #define lookup_rightempty(start, base)                          \
 216 ({                                                              \
 217         struct inet_peer *u;                                    \
 218         struct inet_peer __rcu **v;                             \
 219         *stackptr++ = &start->avl_left;                         \
 220         v = &start->avl_left;                                   \
 221         for (u = rcu_dereference_protected(*v,                  \
 222                         lockdep_is_held(&base->lock));          \
 223              u->avl_right != peer_avl_empty_rcu; ) {            \
 224                 v = &u->avl_right;                              \
 225                 *stackptr++ = v;                                \
 226                 u = rcu_dereference_protected(*v,               \
 227                         lockdep_is_held(&base->lock));          \
 228         }                                                       \
 229         u;                                                      \
 230 })
 231
 232 /* Called with local BH disabled and the pool lock held.
 233  * Variable names are the proof of operation correctness.
 234  * Look into mm/map_avl.c for more detail description of the ideas.
 235  */
 236 static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
 237                                struct inet_peer __rcu ***stackend,
 238                                struct inet_peer_base *base)
 239 {
 240         struct inet_peer __rcu **nodep;
 241         struct inet_peer *node, *l, *r;
 242         int lh, rh;
 243
 244         while (stackend > stack) {
 245                 nodep = *--stackend;
 246                 node = rcu_dereference_protected(*nodep,
 247                                 lockdep_is_held(&base->lock));
 248                 l = rcu_dereference_protected(node->avl_left,
 249                                 lockdep_is_held(&base->lock));
 250                 r = rcu_dereference_protected(node->avl_right,
 251                                 lockdep_is_held(&base->lock));
 252                 lh = node_height(l);
 253                 rh = node_height(r);
 254                 if (lh > rh + 1) { /* l: RH+2 */
 255                         struct inet_peer *ll, *lr, *lrl, *lrr;
 256                         int lrh;
 257                         ll = rcu_dereference_protected(l->avl_left,
 258                                 lockdep_is_held(&base->lock));
 259                         lr = rcu_dereference_protected(l->avl_right,
 260                                 lockdep_is_held(&base->lock));
 261                         lrh = node_height(lr);
 262                         if (lrh <= node_height(ll)) {   /* ll: RH+1 */
 263                                 RCU_INIT_POINTER(node->avl_left, lr);   /* lr: RH or RH+1 */
 264                                 RCU_INIT_POINTER(node->avl_right, r);   /* r: RH */
 265                                 node->avl_height = lrh + 1; /* RH+1 or RH+2 */
 266                                 RCU_INIT_POINTER(l->avl_left, ll);       /* ll: RH+1 */
 267                                 RCU_INIT_POINTER(l->avl_right, node);   /* node: RH+1 or RH+2 */
 268                                 l->avl_height = node->avl_height + 1;
 269                                 RCU_INIT_POINTER(*nodep, l);
 270                         } else { /* ll: RH, lr: RH+1 */
 271                                 lrl = rcu_dereference_protected(lr->avl_left,
 272                                         lockdep_is_held(&base->lock));  /* lrl: RH or RH-1 */
 273                                 lrr = rcu_dereference_protected(lr->avl_right,
 274                                         lockdep_is_held(&base->lock));  /* lrr: RH or RH-1 */
 275                                 RCU_INIT_POINTER(node->avl_left, lrr);  /* lrr: RH or RH-1 */
 276                                 RCU_INIT_POINTER(node->avl_right, r);   /* r: RH */
 277                                 node->avl_height = rh + 1; /* node: RH+1 */
 278                                 RCU_INIT_POINTER(l->avl_left, ll);      /* ll: RH */
 279                                 RCU_INIT_POINTER(l->avl_right, lrl);    /* lrl: RH or RH-1 */
 280                                 l->avl_height = rh + 1; /* l: RH+1 */
 281                                 RCU_INIT_POINTER(lr->avl_left, l);      /* l: RH+1 */
 282                                 RCU_INIT_POINTER(lr->avl_right, node);  /* node: RH+1 */
 283                                 lr->avl_height = rh + 2;
 284                                 RCU_INIT_POINTER(*nodep, lr);
 285                         }
 286                 } else if (rh > lh + 1) { /* r: LH+2 */
 287                         struct inet_peer *rr, *rl, *rlr, *rll;
 288                         int rlh;
 289                         rr = rcu_dereference_protected(r->avl_right,
 290                                 lockdep_is_held(&base->lock));
 291                         rl = rcu_dereference_protected(r->avl_left,
 292                                 lockdep_is_held(&base->lock));
 293                         rlh = node_height(rl);
 294                         if (rlh <= node_height(rr)) {   /* rr: LH+1 */
 295                                 RCU_INIT_POINTER(node->avl_right, rl);  /* rl: LH or LH+1 */
 296                                 RCU_INIT_POINTER(node->avl_left, l);    /* l: LH */
 297                                 node->avl_height = rlh + 1; /* LH+1 or LH+2 */
 298                                 RCU_INIT_POINTER(r->avl_right, rr);     /* rr: LH+1 */
 299                                 RCU_INIT_POINTER(r->avl_left, node);    /* node: LH+1 or LH+2 */
 300                                 r->avl_height = node->avl_height + 1;
 301                                 RCU_INIT_POINTER(*nodep, r);
 302                         } else { /* rr: RH, rl: RH+1 */
 303                                 rlr = rcu_dereference_protected(rl->avl_right,
 304                                         lockdep_is_held(&base->lock));  /* rlr: LH or LH-1 */
 305                                 rll = rcu_dereference_protected(rl->avl_left,
 306                                         lockdep_is_held(&base->lock));  /* rll: LH or LH-1 */
 307                                 RCU_INIT_POINTER(node->avl_right, rll); /* rll: LH or LH-1 */
 308                                 RCU_INIT_POINTER(node->avl_left, l);    /* l: LH */
 309                                 node->avl_height = lh + 1; /* node: LH+1 */
 310                                 RCU_INIT_POINTER(r->avl_right, rr);     /* rr: LH */
 311                                 RCU_INIT_POINTER(r->avl_left, rlr);     /* rlr: LH or LH-1 */
 312                                 r->avl_height = lh + 1; /* r: LH+1 */
 313                                 RCU_INIT_POINTER(rl->avl_right, r);     /* r: LH+1 */
 314                                 RCU_INIT_POINTER(rl->avl_left, node);   /* node: LH+1 */
 315                                 rl->avl_height = lh + 2;
 316                                 RCU_INIT_POINTER(*nodep, rl);
 317                         }
 318                 } else {
 319                         node->avl_height = (lh > rh ? lh : rh) + 1;
 320                 }
 321         }
 322 }
 323
 324 /* Called with local BH disabled and the pool lock held. */
 325 #define link_to_pool(n, base)                                   \
 326 do {                                                            \
 327         n->avl_height = 1;                                      \
 328         n->avl_left = peer_avl_empty_rcu;                       \
 329         n->avl_right = peer_avl_empty_rcu;                      \
 330         /* lockless readers can catch us now */                 \
 331         rcu_assign_pointer(**--stackptr, n);                    \
 332         peer_avl_rebalance(stack, stackptr, base);              \
 333 } while (0)
 334
 335 static void inetpeer_free_rcu(struct rcu_head *head)
 336 {
 337         kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
 338 }
 339
 340 /* May be called with local BH enabled. */
 341 static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
 342 {
 343         int do_free;
 344
 345         do_free = 0;
 346
 347         spin_lock_bh(&base->lock);
 348         /* Check the reference counter.  It was artificially incremented by 1
 349          * in cleanup() function to prevent sudden disappearing.  If we can
 350          * atomically (because of lockless readers) take this last reference,
 351          * it's safe to remove the node and free it later.
 352          * We use refcnt=-1 to alert lockless readers this entry is deleted.
 353          */
 354         if (atomic_cmpxchg(&p->refcnt, 1, -1) == 1) {
 355                 struct inet_peer __rcu **stack[PEER_MAXDEPTH];
 356                 struct inet_peer __rcu ***stackptr, ***delp;
 357                 if (lookup(p->v4daddr, stack, base) != p)
 358                         BUG();
 359                 delp = stackptr - 1; /* *delp[0] == p */
 360                 if (p->avl_left == peer_avl_empty_rcu) {
 361                         *delp[0] = p->avl_right;
 362                         --stackptr;
 363                 } else {
 364                         /* look for a node to insert instead of p */
 365                         struct inet_peer *t;
 366                         t = lookup_rightempty(p, base);
 367                         BUG_ON(rcu_dereference_protected(*stackptr[-1],
 368                                         lockdep_is_held(&base->lock)) != t);
 369                         **--stackptr = t->avl_left;
 370                         /* t is removed, t->v4daddr > x->v4daddr for any
 371                          * x in p->avl_left subtree.
 372                          * Put t in the old place of p. */
 373                         RCU_INIT_POINTER(*delp[0], t);
 374                         t->avl_left = p->avl_left;
 375                         t->avl_right = p->avl_right;
 376                         t->avl_height = p->avl_height;
 377                         BUG_ON(delp[1] != &p->avl_left);
 378                         delp[1] = &t->avl_left; /* was &p->avl_left */
 379                 }
 380                 peer_avl_rebalance(stack, stackptr, base);
 381                 base->total--;
 382                 do_free = 1;
 383         }
 384         spin_unlock_bh(&base->lock);
 385
 386         if (do_free)
 387                 call_rcu_bh(&p->rcu, inetpeer_free_rcu);
 388         else
 389                 /* The node is used again.  Decrease the reference counter
 390                  * back.  The loop "cleanup -> unlink_from_unused
 391                  *   -> unlink_from_pool -> putpeer -> link_to_unused
 392                  *   -> cleanup (for the same node)"
 393                  * doesn't really exist because the entry will have a
 394                  * recent deletion time and will not be cleaned again soon.
 395                  */
 396                 inet_putpeer(p);
 397 }
 398
 399 static struct inet_peer_base *peer_to_base(struct inet_peer *p)
 400 {
 401         return &v4_peers;
 402 }
 403
 404 /* May be called with local BH enabled. */
 405 static int cleanup_once(unsigned long ttl)
 406 {
 407         struct inet_peer *p = NULL;
 408
 409         /* Remove the first entry from the list of unused nodes. */
 410         spin_lock_bh(&unused_peers.lock);
 411         if (!list_empty(&unused_peers.list)) {
 412                 __u32 delta;
 413
 414                 p = list_first_entry(&unused_peers.list, struct inet_peer, unused);
 415                 delta = (__u32)jiffies - p->dtime;
 416
 417                 if (delta < ttl) {
 418                         /* Do not prune fresh entries. */
 419                         spin_unlock_bh(&unused_peers.lock);
 420                         return -1;
 421                 }
 422
 423                 list_del_init(&p->unused);
 424
 425                 /* Grab an extra reference to prevent node disappearing
 426                  * before unlink_from_pool() call. */
 427                 atomic_inc(&p->refcnt);
 428         }
 429         spin_unlock_bh(&unused_peers.lock);
 430
 431         if (p == NULL)
 432                 /* It means that the total number of USED entries has
 433                  * grown over inet_peer_threshold.  It shouldn't really
 434                  * happen because of entry limits in route cache. */
 435                 return -1;
 436
 437         unlink_from_pool(p, peer_to_base(p));
 438         return 0;
 439 }
 440
 441 static struct inet_peer_base *family_to_base(int family)
 442 {
 443         return &v4_peers;
 444 }
 445
 446 /* Called with or without local BH being disabled. */
 447 struct inet_peer *inet_getpeer(__be32 daddr, int create)
 448 {
 449         struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
 450         struct inet_peer_base *base = family_to_base(AF_INET);
 451         struct inet_peer *p;
 452
 453         /* Look up for the address quickly, lockless.
 454          * Because of a concurrent writer, we might not find an existing entry.
 455          */
 456         rcu_read_lock_bh();
 457         p = lookup_rcu_bh(daddr, base);
 458         rcu_read_unlock_bh();
 459
 460         if (p) {
 461                 /* The existing node has been found.
 462                  * Remove the entry from unused list if it was there.
 463                  */
 464                 unlink_from_unused(p);
 465                 return p;
 466         }
 467
 468         /* retry an exact lookup, taking the lock before.
 469          * At least, nodes should be hot in our cache.
 470          */
 471         spin_lock_bh(&base->lock);
 472         p = lookup(daddr, stack, base);
 473         if (p != peer_avl_empty) {
 474                 atomic_inc(&p->refcnt);
 475                 spin_unlock_bh(&base->lock);
 476                 /* Remove the entry from unused list if it was there. */
 477                 unlink_from_unused(p);
 478                 return p;
 479         }
 480         p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
 481         if (p) {
 482                 p->v4daddr = daddr;
 483                 atomic_set(&p->refcnt, 1);
 484                 atomic_set(&p->rid, 0);
 485                 atomic_set(&p->ip_id_count, secure_ip_id(daddr));
 486                 p->tcp_ts_stamp = 0;
 487                 INIT_LIST_HEAD(&p->unused);
 488
 489
 490                 /* Link the node. */
 491                 link_to_pool(p, base);
 492                 base->total++;
 493         }
 494         spin_unlock_bh(&base->lock);
 495
 496         if (base->total >= inet_peer_threshold)
 497                 /* Remove one less-recently-used entry. */
 498                 cleanup_once(0);
 499
 500         return p;
 501 }
 502
 503 static int compute_total(void)
 504 {
 505         return v4_peers.total;
 506 }
 507
 508 /* Called with local BH disabled. */
 509 static void peer_check_expire(unsigned long dummy)
 510 {
 511         unsigned long now = jiffies;
 512         int ttl, total;
 513
 514         total = compute_total();
 515         if (total >= inet_peer_threshold)
 516                 ttl = inet_peer_minttl;
 517         else
 518                 ttl = inet_peer_maxttl
 519                                 - (inet_peer_maxttl - inet_peer_minttl) / HZ *
 520                                         total / inet_peer_threshold * HZ;
 521         while (!cleanup_once(ttl)) {
 522                 if (jiffies != now)
 523                         break;
 524         }
 525
 526         /* Trigger the timer after inet_peer_gc_mintime .. inet_peer_gc_maxtime
 527          * interval depending on the total number of entries (more entries,
 528          * less interval). */
 529         total = compute_total();
 530         if (total >= inet_peer_threshold)
 531                 peer_periodic_timer.expires = jiffies + inet_peer_gc_mintime;
 532         else
 533                 peer_periodic_timer.expires = jiffies
 534                         + inet_peer_gc_maxtime
 535                         - (inet_peer_gc_maxtime - inet_peer_gc_mintime) / HZ *
 536                                 total / inet_peer_threshold * HZ;
 537         add_timer(&peer_periodic_timer);
 538 }
 539
 540 void inet_putpeer(struct inet_peer *p)
 541 {
 542         local_bh_disable();
 543
 544         if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
 545                 list_add_tail(&p->unused, &unused_peers.list);
 546                 p->dtime = (__u32)jiffies;
 547                 spin_unlock(&unused_peers.lock);
 548         }
 549
 550         local_bh_enable();
 551 }