1 files changed, 186 insertions, 139 deletions

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index f20b7ea7c555..406b6433e467 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -62,6 +62,10 @@
  *      AV              1 Mar 1999
  *              Damn. Added missing check for ->dead in listen queues scanning.
  *
+ *      Miklos Szeredi 25 Jun 2007
+ *              Reimplement with a cycle collecting algorithm. This should
+ *              solve several problems with the previous code, like being racy
+ *              wrt receive and holding up unrelated socket operations.
  */
 
 #include <linux/kernel.h>
@@ -84,10 +88,9 @@
 
 /* Internal data structures and random procedures: */
 
-#define GC_HEAD         ((struct sock *)(-1))
-#define GC_ORPHAN       ((struct sock *)(-3))
-
-static struct sock *gc_current = GC_HEAD; /* stack of objects to mark */
+static LIST_HEAD(gc_inflight_list);
+static LIST_HEAD(gc_candidates);
+static DEFINE_SPINLOCK(unix_gc_lock);
 
 atomic_t unix_tot_inflight = ATOMIC_INIT(0);
 
@@ -122,8 +125,16 @@ void unix_inflight(struct file *fp)
 {
         struct sock *s = unix_get_socket(fp);
         if(s) {
-                atomic_inc(&unix_sk(s)->inflight);
+                struct unix_sock *u = unix_sk(s);
+                spin_lock(&unix_gc_lock);
+                if (atomic_inc_return(&u->inflight) == 1) {
+                        BUG_ON(!list_empty(&u->link));
+                        list_add_tail(&u->link, &gc_inflight_list);
+                } else {
+                        BUG_ON(list_empty(&u->link));
+                }
                 atomic_inc(&unix_tot_inflight);
+                spin_unlock(&unix_gc_lock);
         }
 }
 
@@ -131,182 +142,218 @@ void unix_notinflight(struct file *fp)
 {
         struct sock *s = unix_get_socket(fp);
         if(s) {
-                atomic_dec(&unix_sk(s)->inflight);
+                struct unix_sock *u = unix_sk(s);
+                spin_lock(&unix_gc_lock);
+                BUG_ON(list_empty(&u->link));
+                if (atomic_dec_and_test(&u->inflight))
+                        list_del_init(&u->link);
                 atomic_dec(&unix_tot_inflight);
+                spin_unlock(&unix_gc_lock);
         }
 }
 
+static inline struct sk_buff *sock_queue_head(struct sock *sk)
+{
+        return (struct sk_buff *) &sk->sk_receive_queue;
+}
 
-/*
- *      Garbage Collector Support Functions
- */
+#define receive_queue_for_each_skb(sk, next, skb) \
+        for (skb = sock_queue_head(sk)->next, next = skb->next; \
+             skb != sock_queue_head(sk); skb = next, next = skb->next)
 
-static inline struct sock *pop_stack(void)
+static void scan_inflight(struct sock *x, void (*func)(struct sock *),
+                          struct sk_buff_head *hitlist)
 {
-        struct sock *p = gc_current;
-        gc_current = unix_sk(p)->gc_tree;
-        return p;
+        struct sk_buff *skb;
+        struct sk_buff *next;
+
+        spin_lock(&x->sk_receive_queue.lock);
+        receive_queue_for_each_skb(x, next, skb) {
+                /*
+                 *      Do we have file descriptors ?
+                 */
+                if (UNIXCB(skb).fp) {
+                        bool hit = false;
+                        /*
+                         *      Process the descriptors of this socket
+                         */
+                        int nfd = UNIXCB(skb).fp->count;
+                        struct file **fp = UNIXCB(skb).fp->fp;
+                        while (nfd--) {
+                                /*
+                                 *      Get the socket the fd matches
+                                 *      if it indeed does so
+                                 */
+                                struct sock *sk = unix_get_socket(*fp++);
+                                if(sk) {
+                                        hit = true;
+                                        func(sk);
+                                }
+                        }
+                        if (hit && hitlist != NULL) {
+                                __skb_unlink(skb, &x->sk_receive_queue);
+                                __skb_queue_tail(hitlist, skb);
+                        }
+                }
+        }
+        spin_unlock(&x->sk_receive_queue.lock);
 }
 
-static inline int empty_stack(void)
+static void scan_children(struct sock *x, void (*func)(struct sock *),
+                          struct sk_buff_head *hitlist)
 {
-        return gc_current == GC_HEAD;
+        if (x->sk_state != TCP_LISTEN)
+                scan_inflight(x, func, hitlist);
+        else {
+                struct sk_buff *skb;
+                struct sk_buff *next;
+                struct unix_sock *u;
+                LIST_HEAD(embryos);
+
+                /*
+                 * For a listening socket collect the queued embryos
+                 * and perform a scan on them as well.
+                 */
+                spin_lock(&x->sk_receive_queue.lock);
+                receive_queue_for_each_skb(x, next, skb) {
+                        u = unix_sk(skb->sk);
+
+                        /*
+                         * An embryo cannot be in-flight, so it's safe
+                         * to use the list link.
+                         */
+                        BUG_ON(!list_empty(&u->link));
+                        list_add_tail(&u->link, &embryos);
+                }
+                spin_unlock(&x->sk_receive_queue.lock);
+
+                while (!list_empty(&embryos)) {
+                        u = list_entry(embryos.next, struct unix_sock, link);
+                        scan_inflight(&u->sk, func, hitlist);
+                        list_del_init(&u->link);
+                }
+        }
 }
 
-static void maybe_unmark_and_push(struct sock *x)
+static void dec_inflight(struct sock *sk)
 {
-        struct unix_sock *u = unix_sk(x);
+        atomic_dec(&unix_sk(sk)->inflight);
+}
 
-        if (u->gc_tree != GC_ORPHAN)
-                return;
-        sock_hold(x);
-        u->gc_tree = gc_current;
-        gc_current = x;
+static void inc_inflight(struct sock *sk)
+{
+        atomic_inc(&unix_sk(sk)->inflight);
 }
 
+static void inc_inflight_move_tail(struct sock *sk)
+{
+        struct unix_sock *u = unix_sk(sk);
+
+        atomic_inc(&u->inflight);
+        /*
+         * If this is still a candidate, move it to the end of the
+         * list, so that it's checked even if it was already passed
+         * over
+         */
+        if (u->gc_candidate)
+                list_move_tail(&u->link, &gc_candidates);
+}
 
 /* The external entry point: unix_gc() */
 
 void unix_gc(void)
 {
-        static DEFINE_MUTEX(unix_gc_sem);
-        int i;
-        struct sock *s;
-        struct sk_buff_head hitlist;
-        struct sk_buff *skb;
+        static bool gc_in_progress = false;
 
-        /*
-         *      Avoid a recursive GC.
-         */
+        struct unix_sock *u;
+        struct unix_sock *next;
+        struct sk_buff_head hitlist;
+        struct list_head cursor;
 
-        if (!mutex_trylock(&unix_gc_sem))
-                return;
+        spin_lock(&unix_gc_lock);
 
-        spin_lock(&unix_table_lock);
+        /* Avoid a recursive GC. */
+        if (gc_in_progress)
+                goto out;
 
-        forall_unix_sockets(i, s)
-        {
-                unix_sk(s)->gc_tree = GC_ORPHAN;
-        }
+        gc_in_progress = true;
         /*
-         *      Everything is now marked
-         */
-
-        /* Invariant to be maintained:
-                - everything unmarked is either:
-                -- (a) on the stack, or
-                -- (b) has all of its children unmarked
-                - everything on the stack is always unmarked
-                - nothing is ever pushed onto the stack twice, because:
-                -- nothing previously unmarked is ever pushed on the stack
+         * First, select candidates for garbage collection.  Only
+         * in-flight sockets are considered, and from those only ones
+         * which don't have any external reference.
+         *
+         * Holding unix_gc_lock will protect these candidates from
+         * being detached, and hence from gaining an external
+         * reference.  This also means, that since there are no
+         * possible receivers, the receive queues of these sockets are
+         * static during the GC, even though the dequeue is done
+         * before the detach without atomicity guarantees.
          */
+        list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
+                int total_refs;
+                int inflight_refs;
+
+                total_refs = file_count(u->sk.sk_socket->file);
+                inflight_refs = atomic_read(&u->inflight);
+
+                BUG_ON(inflight_refs < 1);
+                BUG_ON(total_refs < inflight_refs);
+                if (total_refs == inflight_refs) {
+                        list_move_tail(&u->link, &gc_candidates);
+                        u->gc_candidate = 1;
+                }
+        }
 
         /*
-         *      Push root set
+         * Now remove all internal in-flight reference to children of
+         * the candidates.
          */
-
-        forall_unix_sockets(i, s)
-        {
-                int open_count = 0;
-
-                /*
-                 *      If all instances of the descriptor are not
-                 *      in flight we are in use.
-                 *
-                 *      Special case: when socket s is embrion, it may be
-                 *      hashed but still not in queue of listening socket.
-                 *      In this case (see unix_create1()) we set artificial
-                 *      negative inflight counter to close race window.
-                 *      It is trick of course and dirty one.
-                 */
-                if (s->sk_socket && s->sk_socket->file)
-                        open_count = file_count(s->sk_socket->file);
-                if (open_count > atomic_read(&unix_sk(s)->inflight))
-                        maybe_unmark_and_push(s);
-        }
+        list_for_each_entry(u, &gc_candidates, link)
+                scan_children(&u->sk, dec_inflight, NULL);
 
         /*
-         *      Mark phase
+         * Restore the references for children of all candidates,
+         * which have remaining references.  Do this recursively, so
+         * only those remain, which form cyclic references.
+         *
+         * Use a "cursor" link, to make the list traversal safe, even
+         * though elements might be moved about.
          */
+        list_add(&cursor, &gc_candidates);
+        while (cursor.next != &gc_candidates) {
+                u = list_entry(cursor.next, struct unix_sock, link);
 
-        while (!empty_stack())
-        {
-                struct sock *x = pop_stack();
-                struct sock *sk;
-
-                spin_lock(&x->sk_receive_queue.lock);
-                skb = skb_peek(&x->sk_receive_queue);
-
-                /*
-                 *      Loop through all but first born
-                 */
+                /* Move cursor to after the current position. */
+                list_move(&cursor, &u->link);
 
-                while (skb && skb != (struct sk_buff *)&x->sk_receive_queue) {
-                        /*
-                         *      Do we have file descriptors ?
-                         */
-                        if(UNIXCB(skb).fp)
-                        {
-                                /*
-                                 *      Process the descriptors of this socket
-                                 */
-                                int nfd=UNIXCB(skb).fp->count;
-                                struct file **fp = UNIXCB(skb).fp->fp;
-                                while(nfd--)
-                                {
-                                        /*
-                                         *      Get the socket the fd matches if
-                                         *      it indeed does so
-                                         */
-                                        if((sk=unix_get_socket(*fp++))!=NULL)
-                                        {
-                                                maybe_unmark_and_push(sk);
-                                        }
-                                }
-                        }
-                        /* We have to scan not-yet-accepted ones too */
-                        if (x->sk_state == TCP_LISTEN)
-                                maybe_unmark_and_push(skb->sk);
-                        skb=skb->next;
+                if (atomic_read(&u->inflight) > 0) {
+                        list_move_tail(&u->link, &gc_inflight_list);
+                        u->gc_candidate = 0;
+                        scan_children(&u->sk, inc_inflight_move_tail, NULL);
                 }
-                spin_unlock(&x->sk_receive_queue.lock);
-                sock_put(x);
         }
+        list_del(&cursor);
 
+        /*
+         * Now gc_candidates contains only garbage.  Restore original
+         * inflight counters for these as well, and remove the skbuffs
+         * which are creating the cycle(s).
+         */
         skb_queue_head_init(&hitlist);
+        list_for_each_entry(u, &gc_candidates, link)
+                scan_children(&u->sk, inc_inflight, &hitlist);
 
-        forall_unix_sockets(i, s)
-        {
-                struct unix_sock *u = unix_sk(s);
+        spin_unlock(&unix_gc_lock);
 
-                if (u->gc_tree == GC_ORPHAN) {
-                        struct sk_buff *nextsk;
+        /* Here we are. Hitlist is filled. Die. */
+        __skb_queue_purge(&hitlist);
 
-                        spin_lock(&s->sk_receive_queue.lock);
-                        skb = skb_peek(&s->sk_receive_queue);
-                        while (skb &&
-                               skb != (struct sk_buff *)&s->sk_receive_queue) {
-                                nextsk = skb->next;
-                                /*
-                                 *      Do we have file descriptors ?
-                                 */
-                                if (UNIXCB(skb).fp) {
-                                        __skb_unlink(skb,
-                                                     &s->sk_receive_queue);
-                                        __skb_queue_tail(&hitlist, skb);
-                                }
-                                skb = nextsk;
-                        }
-                        spin_unlock(&s->sk_receive_queue.lock);
-                }
-                u->gc_tree = GC_ORPHAN;
-        }
-        spin_unlock(&unix_table_lock);
+        spin_lock(&unix_gc_lock);
 
-        /*
-         *      Here we are. Hitlist is filled. Die.
-         */
+        /* All candidates should have been detached by now. */
+        BUG_ON(!list_empty(&gc_candidates));
+        gc_in_progress = false;
 
-        __skb_queue_purge(&hitlist);
-        mutex_unlock(&unix_gc_sem);
+ out:
+        spin_unlock(&unix_gc_lock);
 }