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 /* 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 }