fs/notify/notification.c

   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 void fsnotify_destroy_event(struct fsnotify_group *group,
  51                             struct fsnotify_event *event)
  52 {
  53         /* Overflow events are per-group and we don't want to free them */
  54         if (!event || event == group->overflow_event)
  55                 return;
  56         /*
  57          * If the event is still queued, we have a problem... Do an unreliable
  58          * lockless check first to avoid locking in the common case. The
  59          * locking may be necessary for permission events which got removed
  60          * from the list by a different CPU than the one freeing the event.
  61          */
  62         if (!list_empty(&event->list)) {
  63                 spin_lock(&group->notification_lock);
  64                 WARN_ON(!list_empty(&event->list));
  65                 spin_unlock(&group->notification_lock);
  66         }
  67         group->ops->free_event(group, event);
  68 }
  69
  70 /*
  71  * Try to add an event to the notification queue.
  72  * The group can later pull this event off the queue to deal with.
  73  * The group can use the @merge hook to merge the event with a queued event.
  74  * The group can use the @insert hook to insert the event into hash table.
  75  * The function returns:
  76  * 0 if the event was added to a queue
  77  * 1 if the event was merged with some other queued event
  78  * 2 if the event was not queued - either the queue of events has overflown
  79  *   or the group is shutting down.
  80  */
  81 int fsnotify_insert_event(struct fsnotify_group *group,
  82                           struct fsnotify_event *event,
  83                           int (*merge)(struct fsnotify_group *,
  84                                        struct fsnotify_event *),
  85                           void (*insert)(struct fsnotify_group *,
  86                                          struct fsnotify_event *))
  87 {
  88         int ret = 0;
  89         struct list_head *list = &group->notification_list;
  90
  91         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
  92
  93         spin_lock(&group->notification_lock);
  94
  95         if (group->shutdown) {
  96                 spin_unlock(&group->notification_lock);
  97                 return 2;
  98         }
  99
 100         if (event == group->overflow_event ||
 101             group->q_len >= group->max_events) {
 102                 ret = 2;
 103                 /* Queue overflow event only if it isn't already queued */
 104                 if (!list_empty(&group->overflow_event->list)) {
 105                         spin_unlock(&group->notification_lock);
 106                         return ret;
 107                 }
 108                 event = group->overflow_event;
 109                 goto queue;
 110         }
 111
 112         if (!list_empty(list) && merge) {
 113                 ret = merge(group, event);
 114                 if (ret) {
 115                         spin_unlock(&group->notification_lock);
 116                         return ret;
 117                 }
 118         }
 119
 120 queue:
 121         group->q_len++;
 122         list_add_tail(&event->list, list);
 123         if (insert)
 124                 insert(group, event);
 125         spin_unlock(&group->notification_lock);
 126
 127         wake_up(&group->notification_waitq);
 128         kill_fasync(&group->fsn_fa, SIGIO, POLL_IN);
 129         return ret;
 130 }
 131
 132 void fsnotify_remove_queued_event(struct fsnotify_group *group,
 133                                   struct fsnotify_event *event)
 134 {
 135         assert_spin_locked(&group->notification_lock);
 136         /*
 137          * We need to init list head for the case of overflow event so that
 138          * check in fsnotify_add_event() works
 139          */
 140         list_del_init(&event->list);
 141         group->q_len--;
 142 }
 143
 144 /*
 145  * Return the first event on the notification list without removing it.
 146  * Returns NULL if the list is empty.
 147  */
 148 struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group)
 149 {
 150         assert_spin_locked(&group->notification_lock);
 151
 152         if (fsnotify_notify_queue_is_empty(group))
 153                 return NULL;
 154
 155         return list_first_entry(&group->notification_list,
 156                                 struct fsnotify_event, list);
 157 }
 158
 159 /*
 160  * Remove and return the first event from the notification list.  It is the
 161  * responsibility of the caller to destroy the obtained event
 162  */
 163 struct fsnotify_event *fsnotify_remove_first_event(struct fsnotify_group *group)
 164 {
 165         struct fsnotify_event *event = fsnotify_peek_first_event(group);
 166
 167         if (!event)
 168                 return NULL;
 169
 170         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 171
 172         fsnotify_remove_queued_event(group, event);
 173
 174         return event;
 175 }
 176
 177 /*
 178  * Called when a group is being torn down to clean up any outstanding
 179  * event notifications.
 180  */
 181 void fsnotify_flush_notify(struct fsnotify_group *group)
 182 {
 183         struct fsnotify_event *event;
 184
 185         spin_lock(&group->notification_lock);
 186         while (!fsnotify_notify_queue_is_empty(group)) {
 187                 event = fsnotify_remove_first_event(group);
 188                 spin_unlock(&group->notification_lock);
 189                 fsnotify_destroy_event(group, event);
 190                 spin_lock(&group->notification_lock);
 191         }
 192         spin_unlock(&group->notification_lock);
 193 }