From aa1039e73cc2cf834e99c09d2033d5d2675357b9 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 15 Jun 2010 08:23:14 +0000 Subject: [PATCH] inetpeer: RCU conversion inetpeer currently uses an AVL tree protected by an rwlock. It's possible to make most lookups use RCU 1) Add a struct rcu_head to struct inet_peer 2) add a lookup_rcu_bh() helper to perform lockless and opportunistic lookup. This is a normal function, not a macro like lookup(). 3) Add a limit to number of links followed by lookup_rcu_bh(). This is needed in case we fall in a loop. 4) add an smp_wmb() in link_to_pool() right before node insert. 5) make unlink_from_pool() use atomic_cmpxchg() to make sure it can take last reference to an inet_peer, since lockless readers could increase refcount, even while we hold peers.lock. 6) Delay struct inet_peer freeing after rcu grace period so that lookup_rcu_bh() cannot crash. 7) inet_getpeer() first attempts lockless lookup. Note this lookup can fail even if target is in AVL tree, but a concurrent writer can let tree in a non correct form. If this attemps fails, lock is taken a regular lookup is performed again. 8) convert peers.lock from rwlock to a spinlock 9) Remove SLAB_HWCACHE_ALIGN when peer_cachep is created, because rcu_head adds 16 bytes on 64bit arches, doubling effective size (64 -> 128 bytes) In a future patch, this is probably possible to revert this part, if rcu field is put in an union to share space with rid, ip_id_count, tcp_ts & tcp_ts_stamp. These fields being manipulated only with refcnt > 0. Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- include/net/inetpeer.h | 1 + net/ipv4/inetpeer.c | 164 ++++++++++++++++++++++++----------------- 2 files changed, 96 insertions(+), 69 deletions(-) diff --git a/include/net/inetpeer.h b/include/net/inetpeer.h index 87b1df0d4d8c..617404730422 100644 --- a/include/net/inetpeer.h +++ b/include/net/inetpeer.h @@ -26,6 +26,7 @@ struct inet_peer { atomic_t ip_id_count; /* IP ID for the next packet */ __u32 tcp_ts; __u32 tcp_ts_stamp; + struct rcu_head rcu; }; void inet_initpeers(void) __init; diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c index 035673fd42d4..58fbc7e2475e 100644 --- a/net/ipv4/inetpeer.c +++ b/net/ipv4/inetpeer.c @@ -51,8 +51,8 @@ * lookups performed with disabled BHs. * * Serialisation issues. - * 1. Nodes may appear in the tree only with the pool write lock held. - * 2. Nodes may disappear from the tree only with the pool write lock held + * 1. Nodes may appear in the tree only with the pool lock held. + * 2. Nodes may disappear from the tree only with the pool lock held * AND reference count being 0. * 3. Nodes appears and disappears from unused node list only under * "inet_peer_unused_lock". @@ -80,11 +80,11 @@ static const struct inet_peer peer_fake_node = { static struct { struct inet_peer *root; - rwlock_t lock; + spinlock_t lock; int total; } peers = { .root = peer_avl_empty, - .lock = __RW_LOCK_UNLOCKED(peers.lock), + .lock = __SPIN_LOCK_UNLOCKED(peers.lock), .total = 0, }; #define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */ @@ -129,7 +129,7 @@ void __init inet_initpeers(void) peer_cachep = kmem_cache_create("inet_peer_cache", sizeof(struct inet_peer), - 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, + 0, SLAB_PANIC, NULL); /* All the timers, started at system startup tend @@ -153,16 +153,13 @@ static void unlink_from_unused(struct inet_peer *p) /* * Called with local BH disabled and the pool lock held. - * _stack is known to be NULL or not at compile time, - * so compiler will optimize the if (_stack) tests. */ #define lookup(_daddr, _stack) \ ({ \ struct inet_peer *u, **v; \ - if (_stack != NULL) { \ - stackptr = _stack; \ - *stackptr++ = &peers.root; \ - } \ + \ + stackptr = _stack; \ + *stackptr++ = &peers.root; \ for (u = peers.root; u != peer_avl_empty; ) { \ if (_daddr == u->v4daddr) \ break; \ @@ -170,14 +167,41 @@ static void unlink_from_unused(struct inet_peer *p) v = &u->avl_left; \ else \ v = &u->avl_right; \ - if (_stack != NULL) \ - *stackptr++ = v; \ + *stackptr++ = v; \ u = *v; \ } \ u; \ }) -/* Called with local BH disabled and the pool write lock held. */ +/* + * Called with rcu_read_lock_bh() + * Because we hold no lock against a writer, its quite possible we fall + * in an endless loop. + * But every pointer we follow is guaranteed to be valid thanks to RCU. + * We exit from this function if number of links exceeds PEER_MAXDEPTH + */ +static struct inet_peer *lookup_rcu_bh(__be32 daddr) +{ + struct inet_peer *u = rcu_dereference_bh(peers.root); + int count = 0; + + while (u != peer_avl_empty) { + if (daddr == u->v4daddr) { + if (unlikely(!atomic_inc_not_zero(&u->refcnt))) + u = NULL; + return u; + } + if ((__force __u32)daddr < (__force __u32)u->v4daddr) + u = rcu_dereference_bh(u->avl_left); + else + u = rcu_dereference_bh(u->avl_right); + if (unlikely(++count == PEER_MAXDEPTH)) + break; + } + return NULL; +} + +/* Called with local BH disabled and the pool lock held. */ #define lookup_rightempty(start) \ ({ \ struct inet_peer *u, **v; \ @@ -191,9 +215,10 @@ static void unlink_from_unused(struct inet_peer *p) u; \ }) -/* Called with local BH disabled and the pool write lock held. +/* Called with local BH disabled and the pool lock held. * Variable names are the proof of operation correctness. - * Look into mm/map_avl.c for more detail description of the ideas. */ + * Look into mm/map_avl.c for more detail description of the ideas. + */ static void peer_avl_rebalance(struct inet_peer **stack[], struct inet_peer ***stackend) { @@ -269,16 +294,22 @@ static void peer_avl_rebalance(struct inet_peer **stack[], } } -/* Called with local BH disabled and the pool write lock held. */ +/* Called with local BH disabled and the pool lock held. */ #define link_to_pool(n) \ do { \ n->avl_height = 1; \ n->avl_left = peer_avl_empty; \ n->avl_right = peer_avl_empty; \ + smp_wmb(); /* lockless readers can catch us now */ \ **--stackptr = n; \ peer_avl_rebalance(stack, stackptr); \ } while (0) +static void inetpeer_free_rcu(struct rcu_head *head) +{ + kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu)); +} + /* May be called with local BH enabled. */ static void unlink_from_pool(struct inet_peer *p) { @@ -286,13 +317,13 @@ static void unlink_from_pool(struct inet_peer *p) do_free = 0; - write_lock_bh(&peers.lock); + spin_lock_bh(&peers.lock); /* Check the reference counter. It was artificially incremented by 1 - * in cleanup() function to prevent sudden disappearing. If the - * reference count is still 1 then the node is referenced only as `p' - * here and from the pool. So under the exclusive pool lock it's safe - * to remove the node and free it later. */ - if (atomic_read(&p->refcnt) == 1) { + * in cleanup() function to prevent sudden disappearing. If we can + * atomically (because of lockless readers) take this last reference, + * it's safe to remove the node and free it later. + */ + if (atomic_cmpxchg(&p->refcnt, 1, 0) == 1) { struct inet_peer **stack[PEER_MAXDEPTH]; struct inet_peer ***stackptr, ***delp; if (lookup(p->v4daddr, stack) != p) @@ -321,17 +352,18 @@ static void unlink_from_pool(struct inet_peer *p) peers.total--; do_free = 1; } - write_unlock_bh(&peers.lock); + spin_unlock_bh(&peers.lock); if (do_free) - kmem_cache_free(peer_cachep, p); + call_rcu_bh(&p->rcu, inetpeer_free_rcu); else /* The node is used again. Decrease the reference counter * back. The loop "cleanup -> unlink_from_unused * -> unlink_from_pool -> putpeer -> link_to_unused * -> cleanup (for the same node)" * doesn't really exist because the entry will have a - * recent deletion time and will not be cleaned again soon. */ + * recent deletion time and will not be cleaned again soon. + */ inet_putpeer(p); } @@ -375,62 +407,56 @@ static int cleanup_once(unsigned long ttl) /* Called with or without local BH being disabled. */ struct inet_peer *inet_getpeer(__be32 daddr, int create) { - struct inet_peer *p, *n; + struct inet_peer *p; struct inet_peer **stack[PEER_MAXDEPTH], ***stackptr; - /* Look up for the address quickly. */ - read_lock_bh(&peers.lock); - p = lookup(daddr, NULL); - if (p != peer_avl_empty) - atomic_inc(&p->refcnt); - read_unlock_bh(&peers.lock); + /* Look up for the address quickly, lockless. + * Because of a concurrent writer, we might not find an existing entry. + */ + rcu_read_lock_bh(); + p = lookup_rcu_bh(daddr); + rcu_read_unlock_bh(); + + if (p) { + /* The existing node has been found. + * Remove the entry from unused list if it was there. + */ + unlink_from_unused(p); + return p; + } + /* retry an exact lookup, taking the lock before. + * At least, nodes should be hot in our cache. + */ + spin_lock_bh(&peers.lock); + p = lookup(daddr, stack); if (p != peer_avl_empty) { - /* The existing node has been found. */ + atomic_inc(&p->refcnt); + spin_unlock_bh(&peers.lock); /* Remove the entry from unused list if it was there. */ unlink_from_unused(p); return p; } - - if (!create) - return NULL; - - /* Allocate the space outside the locked region. */ - n = kmem_cache_alloc(peer_cachep, GFP_ATOMIC); - if (n == NULL) - return NULL; - n->v4daddr = daddr; - atomic_set(&n->refcnt, 1); - atomic_set(&n->rid, 0); - atomic_set(&n->ip_id_count, secure_ip_id(daddr)); - n->tcp_ts_stamp = 0; - - write_lock_bh(&peers.lock); - /* Check if an entry has suddenly appeared. */ - p = lookup(daddr, stack); - if (p != peer_avl_empty) - goto out_free; - - /* Link the node. */ - link_to_pool(n); - INIT_LIST_HEAD(&n->unused); - peers.total++; - write_unlock_bh(&peers.lock); + p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL; + if (p) { + p->v4daddr = daddr; + atomic_set(&p->refcnt, 1); + atomic_set(&p->rid, 0); + atomic_set(&p->ip_id_count, secure_ip_id(daddr)); + p->tcp_ts_stamp = 0; + INIT_LIST_HEAD(&p->unused); + + + /* Link the node. */ + link_to_pool(p); + peers.total++; + } + spin_unlock_bh(&peers.lock); if (peers.total >= inet_peer_threshold) /* Remove one less-recently-used entry. */ cleanup_once(0); - return n; - -out_free: - /* The appropriate node is already in the pool. */ - atomic_inc(&p->refcnt); - write_unlock_bh(&peers.lock); - /* Remove the entry from unused list if it was there. */ - unlink_from_unused(p); - /* Free preallocated the preallocated node. */ - kmem_cache_free(peer_cachep, n); return p; } -- 2.20.1