inetpeer: seqlock optimization
David noticed : ------------------ Eric, I was profiling the non-routing-cache case and something that stuck out is the case of calling inet_getpeer() with create==0. If an entry is not found, we have to redo the lookup under a spinlock to make certain that a concurrent writer rebalancing the tree does not "hide" an existing entry from us. This makes the case of a create==0 lookup for a not-present entry really expensive. It is on the order of 600 cpu cycles on my Niagara2. I added a hack to not do the relookup under the lock when create==0 and it now costs less than 300 cycles. This is now a pretty common operation with the way we handle COW'd metrics, so I think it's definitely worth optimizing. ----------------- One solution is to use a seqlock instead of a spinlock to protect struct inet_peer_base. After a failed avl tree lookup, we can easily detect if a writer did some changes during our lookup. Taking the lock and redo the lookup is only necessary in this case. Note: Add one private rcu_deref_locked() macro to place in one spot the access to spinlock included in seqlock. Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet
David S. Miller
parent
d72751ede1
commit
65e8354ec1
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@ -81,19 +81,19 @@ static const struct inet_peer peer_fake_node = {
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struct inet_peer_base {
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struct inet_peer __rcu *root;
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spinlock_t lock;
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seqlock_t lock;
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int total;
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};
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static struct inet_peer_base v4_peers = {
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.root = peer_avl_empty_rcu,
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.lock = __SPIN_LOCK_UNLOCKED(v4_peers.lock),
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.lock = __SEQLOCK_UNLOCKED(v4_peers.lock),
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.total = 0,
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};
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static struct inet_peer_base v6_peers = {
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.root = peer_avl_empty_rcu,
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.lock = __SPIN_LOCK_UNLOCKED(v6_peers.lock),
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.lock = __SEQLOCK_UNLOCKED(v6_peers.lock),
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.total = 0,
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};
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@ -177,6 +177,9 @@ static int addr_compare(const struct inetpeer_addr *a,
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return 0;
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}
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#define rcu_deref_locked(X, BASE) \
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rcu_dereference_protected(X, lockdep_is_held(&(BASE)->lock.lock))
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/*
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* Called with local BH disabled and the pool lock held.
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*/
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@ -187,8 +190,7 @@ static int addr_compare(const struct inetpeer_addr *a,
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\
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stackptr = _stack; \
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*stackptr++ = &_base->root; \
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for (u = rcu_dereference_protected(_base->root, \
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lockdep_is_held(&_base->lock)); \
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for (u = rcu_deref_locked(_base->root, _base); \
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u != peer_avl_empty; ) { \
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int cmp = addr_compare(_daddr, &u->daddr); \
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if (cmp == 0) \
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@ -198,8 +200,7 @@ static int addr_compare(const struct inetpeer_addr *a,
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else \
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v = &u->avl_right; \
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*stackptr++ = v; \
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u = rcu_dereference_protected(*v, \
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lockdep_is_held(&_base->lock)); \
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u = rcu_deref_locked(*v, _base); \
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} \
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u; \
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})
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@ -246,13 +247,11 @@ static struct inet_peer *lookup_rcu_bh(const struct inetpeer_addr *daddr,
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struct inet_peer __rcu **v; \
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*stackptr++ = &start->avl_left; \
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v = &start->avl_left; \
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for (u = rcu_dereference_protected(*v, \
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lockdep_is_held(&base->lock)); \
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for (u = rcu_deref_locked(*v, base); \
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u->avl_right != peer_avl_empty_rcu; ) { \
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v = &u->avl_right; \
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*stackptr++ = v; \
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u = rcu_dereference_protected(*v, \
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lockdep_is_held(&base->lock)); \
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u = rcu_deref_locked(*v, base); \
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} \
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u; \
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})
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@ -271,21 +270,16 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
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while (stackend > stack) {
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nodep = *--stackend;
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node = rcu_dereference_protected(*nodep,
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lockdep_is_held(&base->lock));
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l = rcu_dereference_protected(node->avl_left,
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lockdep_is_held(&base->lock));
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r = rcu_dereference_protected(node->avl_right,
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lockdep_is_held(&base->lock));
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node = rcu_deref_locked(*nodep, base);
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l = rcu_deref_locked(node->avl_left, base);
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r = rcu_deref_locked(node->avl_right, base);
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lh = node_height(l);
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rh = node_height(r);
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if (lh > rh + 1) { /* l: RH+2 */
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struct inet_peer *ll, *lr, *lrl, *lrr;
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int lrh;
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ll = rcu_dereference_protected(l->avl_left,
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lockdep_is_held(&base->lock));
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lr = rcu_dereference_protected(l->avl_right,
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lockdep_is_held(&base->lock));
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ll = rcu_deref_locked(l->avl_left, base);
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lr = rcu_deref_locked(l->avl_right, base);
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lrh = node_height(lr);
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if (lrh <= node_height(ll)) { /* ll: RH+1 */
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RCU_INIT_POINTER(node->avl_left, lr); /* lr: RH or RH+1 */
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@ -296,10 +290,8 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
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l->avl_height = node->avl_height + 1;
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RCU_INIT_POINTER(*nodep, l);
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} else { /* ll: RH, lr: RH+1 */
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lrl = rcu_dereference_protected(lr->avl_left,
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lockdep_is_held(&base->lock)); /* lrl: RH or RH-1 */
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lrr = rcu_dereference_protected(lr->avl_right,
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lockdep_is_held(&base->lock)); /* lrr: RH or RH-1 */
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lrl = rcu_deref_locked(lr->avl_left, base);/* lrl: RH or RH-1 */
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lrr = rcu_deref_locked(lr->avl_right, base);/* lrr: RH or RH-1 */
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RCU_INIT_POINTER(node->avl_left, lrr); /* lrr: RH or RH-1 */
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RCU_INIT_POINTER(node->avl_right, r); /* r: RH */
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node->avl_height = rh + 1; /* node: RH+1 */
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@ -314,10 +306,8 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
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} else if (rh > lh + 1) { /* r: LH+2 */
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struct inet_peer *rr, *rl, *rlr, *rll;
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int rlh;
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rr = rcu_dereference_protected(r->avl_right,
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lockdep_is_held(&base->lock));
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rl = rcu_dereference_protected(r->avl_left,
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lockdep_is_held(&base->lock));
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rr = rcu_deref_locked(r->avl_right, base);
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rl = rcu_deref_locked(r->avl_left, base);
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rlh = node_height(rl);
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if (rlh <= node_height(rr)) { /* rr: LH+1 */
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RCU_INIT_POINTER(node->avl_right, rl); /* rl: LH or LH+1 */
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@ -328,10 +318,8 @@ static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
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r->avl_height = node->avl_height + 1;
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RCU_INIT_POINTER(*nodep, r);
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} else { /* rr: RH, rl: RH+1 */
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rlr = rcu_dereference_protected(rl->avl_right,
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lockdep_is_held(&base->lock)); /* rlr: LH or LH-1 */
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rll = rcu_dereference_protected(rl->avl_left,
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lockdep_is_held(&base->lock)); /* rll: LH or LH-1 */
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rlr = rcu_deref_locked(rl->avl_right, base);/* rlr: LH or LH-1 */
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rll = rcu_deref_locked(rl->avl_left, base);/* rll: LH or LH-1 */
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RCU_INIT_POINTER(node->avl_right, rll); /* rll: LH or LH-1 */
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RCU_INIT_POINTER(node->avl_left, l); /* l: LH */
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node->avl_height = lh + 1; /* node: LH+1 */
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@ -372,7 +360,7 @@ static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
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do_free = 0;
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spin_lock_bh(&base->lock);
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write_seqlock_bh(&base->lock);
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/* Check the reference counter. It was artificially incremented by 1
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* in cleanup() function to prevent sudden disappearing. If we can
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* atomically (because of lockless readers) take this last reference,
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@ -392,8 +380,7 @@ static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
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/* look for a node to insert instead of p */
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struct inet_peer *t;
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t = lookup_rightempty(p, base);
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BUG_ON(rcu_dereference_protected(*stackptr[-1],
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lockdep_is_held(&base->lock)) != t);
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BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
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**--stackptr = t->avl_left;
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/* t is removed, t->daddr > x->daddr for any
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* x in p->avl_left subtree.
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@ -409,7 +396,7 @@ static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base)
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base->total--;
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do_free = 1;
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}
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spin_unlock_bh(&base->lock);
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write_sequnlock_bh(&base->lock);
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if (do_free)
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call_rcu_bh(&p->rcu, inetpeer_free_rcu);
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@ -477,12 +464,16 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
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struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
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struct inet_peer_base *base = family_to_base(daddr->family);
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struct inet_peer *p;
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unsigned int sequence;
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int invalidated;
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/* Look up for the address quickly, lockless.
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* Because of a concurrent writer, we might not find an existing entry.
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*/
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rcu_read_lock_bh();
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sequence = read_seqbegin(&base->lock);
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p = lookup_rcu_bh(daddr, base);
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invalidated = read_seqretry(&base->lock, sequence);
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rcu_read_unlock_bh();
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if (p) {
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@ -493,14 +484,18 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
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return p;
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}
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/* If no writer did a change during our lookup, we can return early. */
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if (!create && !invalidated)
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return NULL;
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/* retry an exact lookup, taking the lock before.
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* At least, nodes should be hot in our cache.
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*/
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spin_lock_bh(&base->lock);
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write_seqlock_bh(&base->lock);
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p = lookup(daddr, stack, base);
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if (p != peer_avl_empty) {
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atomic_inc(&p->refcnt);
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spin_unlock_bh(&base->lock);
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write_sequnlock_bh(&base->lock);
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/* Remove the entry from unused list if it was there. */
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unlink_from_unused(p);
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return p;
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@ -524,7 +519,7 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
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link_to_pool(p, base);
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base->total++;
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}
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spin_unlock_bh(&base->lock);
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write_sequnlock_bh(&base->lock);
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if (base->total >= inet_peer_threshold)
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/* Remove one less-recently-used entry. */
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