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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 <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2010-06-15 08:23:14 +00:00 committed by David S. Miller
parent 7b34a4644b
commit aa1039e73c
2 changed files with 95 additions and 68 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/net/inetpeer.h
View File

@ -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;

162
net/ipv4/inetpeer.c
View File

@ -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 != peer_avl_empty) {
/* The existing node has been found. */
/* Remove the entry from unused list if it was there. */
if (p) {
/* The existing node has been found.
* 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. */
/* 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)
goto out_free;
if (p != peer_avl_empty) {
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;
}
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(n);
INIT_LIST_HEAD(&n->unused);
peers.total++;
write_unlock_bh(&peers.lock);
/* 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;
}