/*
 *  Copyright (C) 2008 Red Hat, Inc., Eric Paris <eparis@redhat.com>
 *
 *  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 guarenteed 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:
 *
 * entry->lock
 * group->mark_lock
 * inode->i_lock
 *
 * entry->lock protects 2 things, entry->group and entry->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 mark_entries list anchored inside a given group
 * and each entry 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_mark_entries list anchored inside a
 * given inode and each entry 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_by_entry)
 * - 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_mark_entries 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/module.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/writeback.h> /* for inode_lock */

#include <asm/atomic.h>

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

void fsnotify_get_mark(struct fsnotify_mark_entry *entry)
{
        atomic_inc(&entry->refcnt);
}

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

/*
 * Recalculate the mask of events relevant to a given inode locked.
 */
static void fsnotify_recalc_inode_mask_locked(struct inode *inode)
{
        struct fsnotify_mark_entry *entry;
        struct hlist_node *pos;
        __u32 new_mask = 0;

        assert_spin_locked(&inode->i_lock);

        hlist_for_each_entry(entry, pos, &inode->i_fsnotify_mark_entries, i_list)
                new_mask |= entry->mask;
        inode->i_fsnotify_mask = new_mask;
}

/*
 * Recalculate the inode->i_fsnotify_mask, or the mask of all FS_* event types
 * any notifier is interested in hearing for this inode.
 */
void fsnotify_recalc_inode_mask(struct inode *inode)
{
        spin_lock(&inode->i_lock);
        fsnotify_recalc_inode_mask_locked(inode);
        spin_unlock(&inode->i_lock);

        __fsnotify_update_child_dentry_flags(inode);
}

/*
 * 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 entry->lock
 */
void fsnotify_destroy_mark_by_entry(struct fsnotify_mark_entry *entry)
{
        struct fsnotify_group *group;
        struct inode *inode;

        spin_lock(&entry->lock);

        group = entry->group;
        inode = entry->inode;

        BUG_ON(group && !inode);
        BUG_ON(!group && inode);

        /* if !group something else already marked this to die */
        if (!group) {
                spin_unlock(&entry->lock);
                return;
        }

        /* 1 from caller and 1 for being on i_list/g_list */
        BUG_ON(atomic_read(&entry->refcnt) < 2);

        spin_lock(&group->mark_lock);
        spin_lock(&inode->i_lock);

        hlist_del_init(&entry->i_list);
        entry->inode = NULL;

        list_del_init(&entry->g_list);
        entry->group = NULL;

        fsnotify_put_mark(entry); /* for i_list and g_list */

        /*
         * this mark is now off the inode->i_fsnotify_mark_entries list and we
         * hold the inode->i_lock, so this is the perfect time to update the
         * inode->i_fsnotify_mask
         */
        fsnotify_recalc_inode_mask_locked(inode);

        spin_unlock(&inode->i_lock);
        spin_unlock(&group->mark_lock);
        spin_unlock(&entry->lock);

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


        iput(inode);

        /*
         * 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);
}

/*
 * Given a group, destroy all of the marks associated with that group.
 */
void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
{
        struct fsnotify_mark_entry *lentry, *entry;
        LIST_HEAD(free_list);

        spin_lock(&group->mark_lock);
        list_for_each_entry_safe(entry, lentry, &group->mark_entries, g_list) {
                list_add(&entry->free_g_list, &free_list);
                list_del_init(&entry->g_list);
                fsnotify_get_mark(entry);
        }
        spin_unlock(&group->mark_lock);

        list_for_each_entry_safe(entry, lentry, &free_list, free_g_list) {
                fsnotify_destroy_mark_by_entry(entry);
                fsnotify_put_mark(entry);
        }
}

/*
 * Given an inode, destroy all of the marks associated with that inode.
 */
void fsnotify_clear_marks_by_inode(struct inode *inode)
{
        struct fsnotify_mark_entry *entry, *lentry;
        struct hlist_node *pos, *n;
        LIST_HEAD(free_list);

        spin_lock(&inode->i_lock);
        hlist_for_each_entry_safe(entry, pos, n, &inode->i_fsnotify_mark_entries, i_list) {
                list_add(&entry->free_i_list, &free_list);
                hlist_del_init(&entry->i_list);
                fsnotify_get_mark(entry);
        }
        spin_unlock(&inode->i_lock);

        list_for_each_entry_safe(entry, lentry, &free_list, free_i_list) {
                fsnotify_destroy_mark_by_entry(entry);
                fsnotify_put_mark(entry);
        }
}

/*
 * given a group and inode, find the mark associated with that combination.
 * if found take a reference to that mark and return it, else return NULL
 */
struct fsnotify_mark_entry *fsnotify_find_mark_entry(struct fsnotify_group *group,
                                                     struct inode *inode)
{
        struct fsnotify_mark_entry *entry;
        struct hlist_node *pos;

        assert_spin_locked(&inode->i_lock);

        hlist_for_each_entry(entry, pos, &inode->i_fsnotify_mark_entries, i_list) {
                if (entry->group == group) {
                        fsnotify_get_mark(entry);
                        return entry;
                }
        }
        return NULL;
}

/*
 * Nothing fancy, just initialize lists and locks and counters.
 */
void fsnotify_init_mark(struct fsnotify_mark_entry *entry,
                        void (*free_mark)(struct fsnotify_mark_entry *entry))

{
        spin_lock_init(&entry->lock);
        atomic_set(&entry->refcnt, 1);
        INIT_HLIST_NODE(&entry->i_list);
        entry->group = NULL;
        entry->mask = 0;
        entry->inode = NULL;
        entry->free_mark = free_mark;
}

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

        inode = igrab(inode);
        if (unlikely(!inode))
                return -EINVAL;

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

        lentry = fsnotify_find_mark_entry(group, inode);
        if (!lentry) {
                entry->group = group;
                entry->inode = inode;

                hlist_add_head(&entry->i_list, &inode->i_fsnotify_mark_entries);
                list_add(&entry->g_list, &group->mark_entries);

                fsnotify_get_mark(entry); /* for i_list and g_list */

                atomic_inc(&group->num_marks);

                fsnotify_recalc_inode_mask_locked(inode);
        }

        spin_unlock(&inode->i_lock);
        spin_unlock(&group->mark_lock);
        spin_unlock(&entry->lock);

        if (lentry) {
                ret = -EEXIST;
                iput(inode);
                fsnotify_put_mark(lentry);
        } else {
                __fsnotify_update_child_dentry_flags(inode);
        }

        return ret;
}

/**
 * fsnotify_unmount_inodes - an sb is unmounting.  handle any watched inodes.
 * @list: list of inodes being unmounted (sb->s_inodes)
 *
 * Called with inode_lock held, protecting the unmounting super block's list
 * of inodes, and with iprune_mutex held, keeping shrink_icache_memory() at bay.
 * We temporarily drop inode_lock, however, and CAN block.
 */
void fsnotify_unmount_inodes(struct list_head *list)
{
        struct inode *inode, *next_i, *need_iput = NULL;

        list_for_each_entry_safe(inode, next_i, list, i_sb_list) {
                struct inode *need_iput_tmp;

                /*
                 * We cannot __iget() an inode in state I_CLEAR, I_FREEING,
                 * I_WILL_FREE, or I_NEW which is fine because by that point
                 * the inode cannot have any associated watches.
                 */
                if (inode->i_state & (I_CLEAR|I_FREEING|I_WILL_FREE|I_NEW))
                        continue;

                /*
                 * If i_count is zero, the inode cannot have any watches and
                 * doing an __iget/iput with MS_ACTIVE clear would actually
                 * evict all inodes with zero i_count from icache which is
                 * unnecessarily violent and may in fact be illegal to do.
                 */
                if (!atomic_read(&inode->i_count))
                        continue;

                need_iput_tmp = need_iput;
                need_iput = NULL;

                /* In case fsnotify_inode_delete() drops a reference. */
                if (inode != need_iput_tmp)
                        __iget(inode);
                else
                        need_iput_tmp = NULL;

                /* In case the dropping of a reference would nuke next_i. */
                if ((&next_i->i_sb_list != list) &&
                    atomic_read(&next_i->i_count) &&
                    !(next_i->i_state & (I_CLEAR | I_FREEING | I_WILL_FREE))) {
                        __iget(next_i);
                        need_iput = next_i;
                }

                /*
                 * We can safely drop inode_lock here because we hold
                 * references on both inode and next_i.  Also no new inodes
                 * will be added since the umount has begun.  Finally,
                 * iprune_mutex keeps shrink_icache_memory() away.
                 */
                spin_unlock(&inode_lock);

                if (need_iput_tmp)
                        iput(need_iput_tmp);

                /* for each watch, send FS_UNMOUNT and then remove it */
                fsnotify(inode, FS_UNMOUNT, inode, FSNOTIFY_EVENT_INODE, NULL, 0);

                fsnotify_inode_delete(inode);

                iput(inode);

                spin_lock(&inode_lock);
        }
}