1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* 3 * Copyright (C) 2008 Red Hat, Inc., Eric Paris <eparis@redhat.com> 4 */ 5 6 /* 7 * Basic idea behind the notification queue: An fsnotify group (like inotify) 8 * sends the userspace notification about events asynchronously some time after 9 * the event happened. When inotify gets an event it will need to add that 10 * event to the group notify queue. Since a single event might need to be on 11 * multiple group's notification queues we can't add the event directly to each 12 * queue and instead add a small "event_holder" to each queue. This event_holder 13 * has a pointer back to the original event. Since the majority of events are 14 * going to end up on one, and only one, notification queue we embed one 15 * event_holder into each event. This means we have a single allocation instead 16 * of always needing two. If the embedded event_holder is already in use by 17 * another group a new event_holder (from fsnotify_event_holder_cachep) will be 18 * allocated and used. 19 */ 20 21 #include <linux/fs.h> 22 #include <linux/init.h> 23 #include <linux/kernel.h> 24 #include <linux/list.h> 25 #include <linux/module.h> 26 #include <linux/mount.h> 27 #include <linux/mutex.h> 28 #include <linux/namei.h> 29 #include <linux/path.h> 30 #include <linux/slab.h> 31 #include <linux/spinlock.h> 32 33 #include <linux/atomic.h> 34 35 #include <linux/fsnotify_backend.h> 36 #include "fsnotify.h" 37 38 static atomic_t fsnotify_sync_cookie = ATOMIC_INIT(0); 39 40 /** 41 * fsnotify_get_cookie - return a unique cookie for use in synchronizing events. 42 * Called from fsnotify_move, which is inlined into filesystem modules. 43 */ 44 u32 fsnotify_get_cookie(void) 45 { 46 return atomic_inc_return(&fsnotify_sync_cookie); 47 } 48 EXPORT_SYMBOL_GPL(fsnotify_get_cookie); 49 50 /* return true if the notify queue is empty, false otherwise */ 51 bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group) 52 { 53 assert_spin_locked(&group->notification_lock); 54 return list_empty(&group->notification_list) ? true : false; 55 } 56 57 void fsnotify_destroy_event(struct fsnotify_group *group, 58 struct fsnotify_event *event) 59 { 60 /* Overflow events are per-group and we don't want to free them */ 61 if (!event || event == group->overflow_event) 62 return; 63 /* 64 * If the event is still queued, we have a problem... Do an unreliable 65 * lockless check first to avoid locking in the common case. The 66 * locking may be necessary for permission events which got removed 67 * from the list by a different CPU than the one freeing the event. 68 */ 69 if (!list_empty(&event->list)) { 70 spin_lock(&group->notification_lock); 71 WARN_ON(!list_empty(&event->list)); 72 spin_unlock(&group->notification_lock); 73 } 74 group->ops->free_event(event); 75 } 76 77 /* 78 * Add an event to the group notification queue. The group can later pull this 79 * event off the queue to deal with. The function returns 0 if the event was 80 * added to the queue, 1 if the event was merged with some other queued event, 81 * 2 if the event was not queued - either the queue of events has overflown 82 * or the group is shutting down. 83 */ 84 int fsnotify_add_event(struct fsnotify_group *group, 85 struct fsnotify_event *event, 86 int (*merge)(struct list_head *, 87 struct fsnotify_event *)) 88 { 89 int ret = 0; 90 struct list_head *list = &group->notification_list; 91 92 pr_debug("%s: group=%p event=%p\n", __func__, group, event); 93 94 spin_lock(&group->notification_lock); 95 96 if (group->shutdown) { 97 spin_unlock(&group->notification_lock); 98 return 2; 99 } 100 101 if (event == group->overflow_event || 102 group->q_len >= group->max_events) { 103 ret = 2; 104 /* Queue overflow event only if it isn't already queued */ 105 if (!list_empty(&group->overflow_event->list)) { 106 spin_unlock(&group->notification_lock); 107 return ret; 108 } 109 event = group->overflow_event; 110 goto queue; 111 } 112 113 if (!list_empty(list) && merge) { 114 ret = merge(list, event); 115 if (ret) { 116 spin_unlock(&group->notification_lock); 117 return ret; 118 } 119 } 120 121 queue: 122 group->q_len++; 123 list_add_tail(&event->list, list); 124 spin_unlock(&group->notification_lock); 125 126 wake_up(&group->notification_waitq); 127 kill_fasync(&group->fsn_fa, SIGIO, POLL_IN); 128 return ret; 129 } 130 131 void fsnotify_remove_queued_event(struct fsnotify_group *group, 132 struct fsnotify_event *event) 133 { 134 assert_spin_locked(&group->notification_lock); 135 /* 136 * We need to init list head for the case of overflow event so that 137 * check in fsnotify_add_event() works 138 */ 139 list_del_init(&event->list); 140 group->q_len--; 141 } 142 143 /* 144 * Remove and return the first event from the notification list. It is the 145 * responsibility of the caller to destroy the obtained event 146 */ 147 struct fsnotify_event *fsnotify_remove_first_event(struct fsnotify_group *group) 148 { 149 struct fsnotify_event *event; 150 151 assert_spin_locked(&group->notification_lock); 152 153 pr_debug("%s: group=%p\n", __func__, group); 154 155 event = list_first_entry(&group->notification_list, 156 struct fsnotify_event, list); 157 fsnotify_remove_queued_event(group, event); 158 return event; 159 } 160 161 /* 162 * This will not remove the event, that must be done with 163 * fsnotify_remove_first_event() 164 */ 165 struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group) 166 { 167 assert_spin_locked(&group->notification_lock); 168 169 return list_first_entry(&group->notification_list, 170 struct fsnotify_event, list); 171 } 172 173 /* 174 * Called when a group is being torn down to clean up any outstanding 175 * event notifications. 176 */ 177 void fsnotify_flush_notify(struct fsnotify_group *group) 178 { 179 struct fsnotify_event *event; 180 181 spin_lock(&group->notification_lock); 182 while (!fsnotify_notify_queue_is_empty(group)) { 183 event = fsnotify_remove_first_event(group); 184 spin_unlock(&group->notification_lock); 185 fsnotify_destroy_event(group, event); 186 spin_lock(&group->notification_lock); 187 } 188 spin_unlock(&group->notification_lock); 189 } 190