/* * 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/kthread.h> #include <linux/module.h> #include <linux/mutex.h> #include <linux/slab.h> #include <linux/spinlock.h> #include <linux/srcu.h> #include <linux/writeback.h> /* for inode_lock */ #include <asm/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; /* 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) { 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); /* * 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);