garbage.c

Go to the documentation of this file.

/*
 * NET3:    Garbage Collector For AF_UNIX sockets
 *
 * Garbage Collector:
 *  Copyright (C) Barak A. Pearlmutter.
 *  Released under the GPL version 2 or later.
 *
 * Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
 * If it doesn't work blame me, it worked when Barak sent it.
 *
 * Assumptions:
 *
 *  - object w/ a bit
 *  - free list
 *
 * Current optimizations:
 *
 *  - explicit stack instead of recursion
 *  - tail recurse on first born instead of immediate push/pop
 *  - we gather the stuff that should not be killed into tree
 *    and stack is just a path from root to the current pointer.
 *
 *  Future optimizations:
 *
 *  - don't just push entire root set; process in place
 *
 *  This program is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public License
 *  as published by the Free Software Foundation; either version
 *  2 of the License, or (at your option) any later version.
 *
 *  Fixes:
 *  Alan Cox    07 Sept 1997    Vmalloc internal stack as needed.
 *                  Cope with changing max_files.
 *  Al Viro     11 Oct 1998
 *      Graph may have cycles. That is, we can send the descriptor
 *      of foo to bar and vice versa. Current code chokes on that.
 *      Fix: move SCM_RIGHTS ones into the separate list and then
 *      skb_free() them all instead of doing explicit fput's.
 *      Another problem: since fput() may block somebody may
 *      create a new unix_socket when we are in the middle of sweep
 *      phase. Fix: revert the logic wrt MARKED. Mark everything
 *      upon the beginning and unmark non-junk ones.
 *
 *      [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
 *      sent to connect()'ed but still not accept()'ed sockets.
 *      Fixed. Old code had slightly different problem here:
 *      extra fput() in situation when we passed the descriptor via
 *      such socket and closed it (descriptor). That would happen on
 *      each unix_gc() until the accept(). Since the struct file in
 *      question would go to the free list and might be reused...
 *      That might be the reason of random oopses on filp_close()
 *      in unrelated processes.
 *
 *  AV      28 Feb 1999
 *      Kill the explicit allocation of stack. Now we keep the tree
 *      with root in dummy + pointer (gc_current) to one of the nodes.
 *      Stack is represented as path from gc_current to dummy. Unmark
 *      now means "add to tree". Push == "make it a son of gc_current".
 *      Pop == "move gc_current to parent". We keep only pointers to
 *      parents (->gc_tree).
 *  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>
#include <linux/string.h>
#include <linux/socket.h>
#include <linux/un.h>
#include <linux/net.h>
#include <linux/fs.h>
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <linux/file.h>
#include <linux/proc_fs.h>
#include <linux/mutex.h>
#include <linux/wait.h>

#include <net/sock.h>
#include <net/af_unix.h>
#include <net/scm.h>
#include <net/tcp_states.h>

/* Internal data structures and random procedures: */

static LIST_HEAD(gc_inflight_list);
static LIST_HEAD(gc_candidates);
static DEFINE_SPINLOCK(unix_gc_lock);
static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);

unsigned int unix_tot_inflight;


struct sock *unix_get_socket(struct file *filp)
{
    struct sock *u_sock = NULL;
    struct inode *inode = filp->f_path.dentry->d_inode;

    /*
     *  Socket ?
     */
    if (S_ISSOCK(inode->i_mode) && !(filp->f_mode & FMODE_PATH)) {
        struct socket *sock = SOCKET_I(inode);
        struct sock *s = sock->sk;

        /*
         *  PF_UNIX ?
         */
        if (s && sock->ops && sock->ops->family == PF_UNIX)
            u_sock = s;
    }
    return u_sock;
}

/*
 *  Keep the number of times in flight count for the file
 *  descriptor if it is for an AF_UNIX socket.
 */

void unix_inflight(struct file *fp)
{
    struct sock *s = unix_get_socket(fp);
    if (s) {
        struct unix_sock *u = unix_sk(s);
        spin_lock(&unix_gc_lock);
        if (atomic_long_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));
        }
        unix_tot_inflight++;
        spin_unlock(&unix_gc_lock);
    }
}

void unix_notinflight(struct file *fp)
{
    struct sock *s = unix_get_socket(fp);
    if (s) {
        struct unix_sock *u = unix_sk(s);
        spin_lock(&unix_gc_lock);
        BUG_ON(list_empty(&u->link));
        if (atomic_long_dec_and_test(&u->inflight))
            list_del_init(&u->link);
        unix_tot_inflight--;
        spin_unlock(&unix_gc_lock);
    }
}

static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
              struct sk_buff_head *hitlist)
{
    struct sk_buff *skb;
    struct sk_buff *next;

    spin_lock(&x->sk_receive_queue.lock);
    skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
        /*
         *  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) {
                    struct unix_sock *u = unix_sk(sk);

                    /*
                     * Ignore non-candidates, they could
                     * have been added to the queues after
                     * starting the garbage collection
                     */
                    if (u->gc_candidate) {
                        hit = true;
                        func(u);
                    }
                }
            }
            if (hit && hitlist != NULL) {
                __skb_unlink(skb, &x->sk_receive_queue);
                __skb_queue_tail(hitlist, skb);
            }
        }
    }
    spin_unlock(&x->sk_receive_queue.lock);
}

static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
              struct sk_buff_head *hitlist)
{
    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);
        skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
            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 dec_inflight(struct unix_sock *usk)
{
    atomic_long_dec(&usk->inflight);
}

static void inc_inflight(struct unix_sock *usk)
{
    atomic_long_inc(&usk->inflight);
}

static void inc_inflight_move_tail(struct unix_sock *u)
{
    atomic_long_inc(&u->inflight);
    /*
     * If this still might be part of a cycle, move it to the end
     * of the list, so that it's checked even if it was already
     * passed over
     */
    if (u->gc_maybe_cycle)
        list_move_tail(&u->link, &gc_candidates);
}

static bool gc_in_progress = false;
#define UNIX_INFLIGHT_TRIGGER_GC 16000

void wait_for_unix_gc(void)
{
    /*
     * If number of inflight sockets is insane,
     * force a garbage collect right now.
     */
    if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
        unix_gc();
    wait_event(unix_gc_wait, gc_in_progress == false);
}

/* The external entry point: unix_gc() */
void unix_gc(void)
{
    struct unix_sock *u;
    struct unix_sock *next;
    struct sk_buff_head hitlist;
    struct list_head cursor;
    LIST_HEAD(not_cycle_list);

    spin_lock(&unix_gc_lock);

    /* Avoid a recursive GC. */
    if (gc_in_progress)
        goto out;

    gc_in_progress = true;
    /*
     * 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.  Since there are no possible receivers, all
     * buffers currently on the candidates' queues stay there
     * during the garbage collection.
     *
     * We also know that no new candidate can be added onto the
     * receive queues.  Other, non candidate sockets _can_ be
     * added to queue, so we must make sure only to touch
     * candidates.
     */
    list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
        long total_refs;
        long inflight_refs;

        total_refs = file_count(u->sk.sk_socket->file);
        inflight_refs = atomic_long_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;
            u->gc_maybe_cycle = 1;
        }
    }

    /*
     * Now remove all internal in-flight reference to children of
     * the candidates.
     */
    list_for_each_entry(u, &gc_candidates, link)
        scan_children(&u->sk, dec_inflight, NULL);

    /*
     * 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);

        /* Move cursor to after the current position. */
        list_move(&cursor, &u->link);

        if (atomic_long_read(&u->inflight) > 0) {
            list_move_tail(&u->link, &not_cycle_list);
            u->gc_maybe_cycle = 0;
            scan_children(&u->sk, inc_inflight_move_tail, NULL);
        }
    }
    list_del(&cursor);

    /*
     * not_cycle_list contains those sockets which do not make up a
     * cycle.  Restore these to the inflight list.
     */
    while (!list_empty(&not_cycle_list)) {
        u = list_entry(not_cycle_list.next, struct unix_sock, link);
        u->gc_candidate = 0;
        list_move_tail(&u->link, &gc_inflight_list);
    }

    /*
     * 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);

    spin_unlock(&unix_gc_lock);

    /* Here we are. Hitlist is filled. Die. */
    __skb_queue_purge(&hitlist);

    spin_lock(&unix_gc_lock);

    /* All candidates should have been detached by now. */
    BUG_ON(!list_empty(&gc_candidates));
    gc_in_progress = false;
    wake_up(&unix_gc_wait);

 out:
    spin_unlock(&unix_gc_lock);
}