/*
* 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:
*
* 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/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 *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;
spin_lock(&mark->lock);
group = mark->group;
inode = mark->i.inode;
BUG_ON(group && !inode);
BUG_ON(!group && inode);
/* if !group something else already marked this to die */
if (!group) {
spin_unlock(&mark->lock);
return;
}
/* 1 from caller and 1 for being on i_list/g_list */
BUG_ON(atomic_read(&mark->refcnt) < 2);
spin_lock(&group->mark_lock);
if (mark->flags & FSNOTIFY_MARK_FLAG_INODE)
fsnotify_destroy_inode_mark(mark);
else
BUG();
list_del_init(&mark->g_list);
mark->group = NULL;
fsnotify_put_mark(mark); /* for i_list and g_list */
spin_unlock(&group->mark_lock);
spin_unlock(&mark->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 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...)
*/
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);
}
/*
* 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(mnt);
BUG_ON(inode && mnt);
BUG_ON(!inode && !mnt);
/*
* if this group isn't being testing for inode type events we need
* to start testing
*/
if (inode && unlikely(list_empty(&group->inode_group_list)))
fsnotify_add_inode_group(group);
else if (mnt && unlikely(list_empty(&group->vfsmount_group_list)))
fsnotify_add_vfsmount_group(group);
/*
* LOCKING ORDER!!!!
* mark->lock
* group->mark_lock
* inode->i_lock
*/
spin_lock(&mark->lock);
spin_lock(&group->mark_lock);
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 {
BUG();
}
spin_unlock(&group->mark_lock);
spin_unlock(&mark->lock);
if (inode)
__fsnotify_update_child_dentry_flags(inode);
return ret;
err:
mark->group = NULL;
list_del_init(&mark->g_list);
atomic_dec(&group->num_marks);
fsnotify_put_mark(mark);
spin_unlock(&group->mark_lock);
spin_unlock(&mark->lock);
return ret;
}
/*
* 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 *lmark, *mark;
LIST_HEAD(free_list);
spin_lock(&group->mark_lock);
list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
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);
}
}
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))
{
spin_lock_init(&mark->lock);
atomic_set(&mark->refcnt, 1);
INIT_HLIST_NODE(&mark->i.i_list);
mark->group = NULL;
mark->mask = 0;
mark->i.inode = NULL;
mark->free_mark = free_mark;
}
