mark.c

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/*
 *  Copyright (C) 2008 Red Hat, Inc., Eric Paris <[email protected]>
 *
 *  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, or (at your option)
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; see the file COPYING.  If not, write to
 *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 */

/*
 * fsnotify inode mark locking/lifetime/and refcnting
 *
 * REFCNT:
 * The mark->refcnt tells how many "things" in the kernel currently are
 * referencing this object.  The object typically will live inside the kernel
 * with a refcnt of 2, one for each list it is on (i_list, g_list).  Any task
 * which can find this object holding the appropriete locks, can take a reference
 * and the object itself is guaranteed to survive until the reference is dropped.
 *
 * LOCKING:
 * There are 3 spinlocks involved with fsnotify inode marks and they MUST
 * be taken in order as follows:
 *
 * mark->lock
 * group->mark_lock
 * inode->i_lock
 *
 * mark->lock protects 2 things, mark->group and mark->inode.  You must hold
 * that lock to dereference either of these things (they could be NULL even with
 * the lock)
 *
 * group->mark_lock protects the marks_list anchored inside a given group
 * and each mark is hooked via the g_list.  It also sorta protects the
 * free_g_list, which when used is anchored by a private list on the stack of the
 * task which held the group->mark_lock.
 *
 * inode->i_lock protects the i_fsnotify_marks list anchored inside a
 * given inode and each mark is hooked via the i_list. (and sorta the
 * free_i_list)
 *
 *
 * LIFETIME:
 * Inode marks survive between when they are added to an inode and when their
 * refcnt==0.
 *
 * The inode mark can be cleared for a number of different reasons including:
 * - The inode is unlinked for the last time.  (fsnotify_inode_remove)
 * - The inode is being evicted from cache. (fsnotify_inode_delete)
 * - The fs the inode is on is unmounted.  (fsnotify_inode_delete/fsnotify_unmount_inodes)
 * - Something explicitly requests that it be removed.  (fsnotify_destroy_mark)
 * - The fsnotify_group associated with the mark is going away and all such marks
 *   need to be cleaned up. (fsnotify_clear_marks_by_group)
 *
 * Worst case we are given an inode and need to clean up all the marks on that
 * inode.  We take i_lock and walk the i_fsnotify_marks safely.  For each
 * mark on the list we take a reference (so the mark can't disappear under us).
 * We remove that mark form the inode's list of marks and we add this mark to a
 * private list anchored on the stack using i_free_list;  At this point we no
 * longer fear anything finding the mark using the inode's list of marks.
 *
 * We can safely and locklessly run the private list on the stack of everything
 * we just unattached from the original inode.  For each mark on the private list
 * we grab the mark-> and can thus dereference mark->group and mark->inode.  If
 * we see the group and inode are not NULL we take those locks.  Now holding all
 * 3 locks we can completely remove the mark from other tasks finding it in the
 * future.  Remember, 10 things might already be referencing this mark, but they
 * better be holding a ref.  We drop our reference we took before we unhooked it
 * from the inode.  When the ref hits 0 we can free the mark.
 *
 * Very similarly for freeing by group, except we use free_g_list.
 *
 * This has the very interesting property of being able to run concurrently with
 * any (or all) other directions.
 */

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/srcu.h>

#include <linux/atomic.h>

#include <linux/fsnotify_backend.h>
#include "fsnotify.h"

struct srcu_struct fsnotify_mark_srcu;
static DEFINE_SPINLOCK(destroy_lock);
static LIST_HEAD(destroy_list);
static DECLARE_WAIT_QUEUE_HEAD(destroy_waitq);

void fsnotify_get_mark(struct fsnotify_mark *mark)
{
    atomic_inc(&mark->refcnt);
}

void fsnotify_put_mark(struct fsnotify_mark *mark)
{
    if (atomic_dec_and_test(&mark->refcnt))
        mark->free_mark(mark);
}

/*
 * Any time a mark is getting freed we end up here.
 * The caller had better be holding a reference to this mark so we don't actually
 * do the final put under the mark->lock
 */
void fsnotify_destroy_mark(struct fsnotify_mark *mark)
{
    struct fsnotify_group *group;
    struct inode *inode = NULL;

    spin_lock(&mark->lock);

    group = mark->group;

    /* something else already called this function on this mark */
    if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
        spin_unlock(&mark->lock);
        return;
    }

    mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;

    spin_lock(&group->mark_lock);

    if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
        inode = mark->i.inode;
        fsnotify_destroy_inode_mark(mark);
    } else if (mark->flags & FSNOTIFY_MARK_FLAG_VFSMOUNT)
        fsnotify_destroy_vfsmount_mark(mark);
    else
        BUG();

    list_del_init(&mark->g_list);

    spin_unlock(&group->mark_lock);
    spin_unlock(&mark->lock);

    spin_lock(&destroy_lock);
    list_add(&mark->destroy_list, &destroy_list);
    spin_unlock(&destroy_lock);
    wake_up(&destroy_waitq);

    /*
     * Some groups like to know that marks are being freed.  This is a
     * callback to the group function to let it know that this mark
     * is being freed.
     */
    if (group->ops->freeing_mark)
        group->ops->freeing_mark(mark, group);

    /*
     * __fsnotify_update_child_dentry_flags(inode);
     *
     * I really want to call that, but we can't, we have no idea if the inode
     * still exists the second we drop the mark->lock.
     *
     * The next time an event arrive to this inode from one of it's children
     * __fsnotify_parent will see that the inode doesn't care about it's
     * children and will update all of these flags then.  So really this
     * is just a lazy update (and could be a perf win...)
     */

    if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
        iput(inode);

    /*
     * We don't necessarily have a ref on mark from caller so the above iput
     * may have already destroyed it.  Don't touch from now on.
     */

    /*
     * it's possible that this group tried to destroy itself, but this
     * this mark was simultaneously being freed by inode.  If that's the
     * case, we finish freeing the group here.
     */
    if (unlikely(atomic_dec_and_test(&group->num_marks)))
        fsnotify_final_destroy_group(group);
}

void fsnotify_set_mark_mask_locked(struct fsnotify_mark *mark, __u32 mask)
{
    assert_spin_locked(&mark->lock);

    mark->mask = mask;

    if (mark->flags & FSNOTIFY_MARK_FLAG_INODE)
        fsnotify_set_inode_mark_mask_locked(mark, mask);
}

void fsnotify_set_mark_ignored_mask_locked(struct fsnotify_mark *mark, __u32 mask)
{
    assert_spin_locked(&mark->lock);

    mark->ignored_mask = mask;
}

/*
 * Attach an initialized mark to a given group and fs object.
 * These marks may be used for the fsnotify backend to determine which
 * event types should be delivered to which group.
 */
int fsnotify_add_mark(struct fsnotify_mark *mark,
              struct fsnotify_group *group, struct inode *inode,
              struct vfsmount *mnt, int allow_dups)
{
    int ret = 0;

    BUG_ON(inode && mnt);
    BUG_ON(!inode && !mnt);

    /*
     * LOCKING ORDER!!!!
     * mark->lock
     * group->mark_lock
     * inode->i_lock
     */
    spin_lock(&mark->lock);
    spin_lock(&group->mark_lock);

    mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE;

    mark->group = group;
    list_add(&mark->g_list, &group->marks_list);
    atomic_inc(&group->num_marks);
    fsnotify_get_mark(mark); /* for i_list and g_list */

    if (inode) {
        ret = fsnotify_add_inode_mark(mark, group, inode, allow_dups);
        if (ret)
            goto err;
    } else if (mnt) {
        ret = fsnotify_add_vfsmount_mark(mark, group, mnt, allow_dups);
        if (ret)
            goto err;
    } else {
        BUG();
    }

    spin_unlock(&group->mark_lock);

    /* this will pin the object if appropriate */
    fsnotify_set_mark_mask_locked(mark, mark->mask);

    spin_unlock(&mark->lock);

    if (inode)
        __fsnotify_update_child_dentry_flags(inode);

    return ret;
err:
    mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
    list_del_init(&mark->g_list);
    mark->group = NULL;
    atomic_dec(&group->num_marks);

    spin_unlock(&group->mark_lock);
    spin_unlock(&mark->lock);

    spin_lock(&destroy_lock);
    list_add(&mark->destroy_list, &destroy_list);
    spin_unlock(&destroy_lock);
    wake_up(&destroy_waitq);

    return ret;
}

/*
 * clear any marks in a group in which mark->flags & flags is true
 */
void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
                     unsigned int flags)
{
    struct fsnotify_mark *lmark, *mark;
    LIST_HEAD(free_list);

    spin_lock(&group->mark_lock);
    list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
        if (mark->flags & flags) {
            list_add(&mark->free_g_list, &free_list);
            list_del_init(&mark->g_list);
            fsnotify_get_mark(mark);
        }
    }
    spin_unlock(&group->mark_lock);

    list_for_each_entry_safe(mark, lmark, &free_list, free_g_list) {
        fsnotify_destroy_mark(mark);
        fsnotify_put_mark(mark);
    }
}

/*
 * Given a group, destroy all of the marks associated with that group.
 */
void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
{
    fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1);
}

void fsnotify_duplicate_mark(struct fsnotify_mark *new, struct fsnotify_mark *old)
{
    assert_spin_locked(&old->lock);
    new->i.inode = old->i.inode;
    new->m.mnt = old->m.mnt;
    new->group = old->group;
    new->mask = old->mask;
    new->free_mark = old->free_mark;
}

/*
 * Nothing fancy, just initialize lists and locks and counters.
 */
void fsnotify_init_mark(struct fsnotify_mark *mark,
            void (*free_mark)(struct fsnotify_mark *mark))
{
    memset(mark, 0, sizeof(*mark));
    spin_lock_init(&mark->lock);
    atomic_set(&mark->refcnt, 1);
    mark->free_mark = free_mark;
}

static int fsnotify_mark_destroy(void *ignored)
{
    struct fsnotify_mark *mark, *next;
    LIST_HEAD(private_destroy_list);

    for (;;) {
        spin_lock(&destroy_lock);
        /* exchange the list head */
        list_replace_init(&destroy_list, &private_destroy_list);
        spin_unlock(&destroy_lock);

        synchronize_srcu(&fsnotify_mark_srcu);

        list_for_each_entry_safe(mark, next, &private_destroy_list, destroy_list) {
            list_del_init(&mark->destroy_list);
            fsnotify_put_mark(mark);
        }

        wait_event_interruptible(destroy_waitq, !list_empty(&destroy_list));
    }

    return 0;
}

static int __init fsnotify_mark_init(void)
{
    struct task_struct *thread;

    thread = kthread_run(fsnotify_mark_destroy, NULL,
                 "fsnotify_mark");
    if (IS_ERR(thread))
        panic("unable to start fsnotify mark destruction thread.");

    return 0;
}
device_initcall(fsnotify_mark_init);