// SPDX-License-Identifier: GPL-2.0-or-later

  /*

   *  fs/eventpoll.c (Efficient event retrieval implementation)
   *  Copyright (C) 2001,...,2009	 Davide Libenzi

   *

   *  Davide Libenzi <davidel@xmailserver.org>

   */

  #include <linux/init.h>
  #include <linux/kernel.h>

  #include <linux/sched/signal.h>

  #include <linux/fs.h>
  #include <linux/file.h>
  #include <linux/signal.h>
  #include <linux/errno.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/poll.h>

  #include <linux/string.h>
  #include <linux/list.h>
  #include <linux/hash.h>
  #include <linux/spinlock.h>
  #include <linux/syscalls.h>

  #include <linux/rbtree.h>
  #include <linux/wait.h>
  #include <linux/eventpoll.h>
  #include <linux/mount.h>
  #include <linux/bitops.h>

  #include <linux/mutex.h>

  #include <linux/anon_inodes.h>

  #include <linux/device.h>

  #include <linux/uaccess.h>

  #include <asm/io.h>
  #include <asm/mman.h>

  #include <linux/atomic.h>

  #include <linux/proc_fs.h>
  #include <linux/seq_file.h>

  #include <linux/compat.h>

  #include <linux/rculist.h>

  #include <net/busy_poll.h>

  /*
   * LOCKING:
   * There are three level of locking required by epoll :
   *

   * 1) epmutex (mutex)

   * 2) ep->mtx (mutex)

   * 3) ep->lock (rwlock)

   *
   * The acquire order is the one listed above, from 1 to 3.

   * We need a rwlock (ep->lock) because we manipulate objects

   * from inside the poll callback, that might be triggered from
   * a wake_up() that in turn might be called from IRQ context.
   * So we can't sleep inside the poll callback and hence we need
   * a spinlock. During the event transfer loop (from kernel to
   * user space) we could end up sleeping due a copy_to_user(), so
   * we need a lock that will allow us to sleep. This lock is a

   * mutex (ep->mtx). It is acquired during the event transfer loop,
   * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
   * Then we also need a global mutex to serialize eventpoll_release_file()
   * and ep_free().
   * This mutex is acquired by ep_free() during the epoll file

   * cleanup path and it is also acquired by eventpoll_release_file()
   * if a file has been pushed inside an epoll set and it is then

   * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).

   * It is also acquired when inserting an epoll fd onto another epoll
   * fd. We do this so that we walk the epoll tree and ensure that this
   * insertion does not create a cycle of epoll file descriptors, which
   * could lead to deadlock. We need a global mutex to prevent two
   * simultaneous inserts (A into B and B into A) from racing and
   * constructing a cycle without either insert observing that it is
   * going to.

   * It is necessary to acquire multiple "ep->mtx"es at once in the
   * case when one epoll fd is added to another. In this case, we
   * always acquire the locks in the order of nesting (i.e. after
   * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
   * before e2->mtx). Since we disallow cycles of epoll file
   * descriptors, this ensures that the mutexes are well-ordered. In
   * order to communicate this nesting to lockdep, when walking a tree
   * of epoll file descriptors, we use the current recursion depth as
   * the lockdep subkey.

   * It is possible to drop the "ep->mtx" and to use the global

   * mutex "epmutex" (together with "ep->lock") to have it working,

   * but having "ep->mtx" will make the interface more scalable.

   * Events that require holding "epmutex" are very rare, while for

   * normal operations the epoll private "ep->mtx" will guarantee
   * a better scalability.

   */

  /* Epoll private bits inside the event mask */

  #define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | EPOLLEXCLUSIVE)

  #define EPOLLINOUT_BITS (EPOLLIN | EPOLLOUT)

  #define EPOLLEXCLUSIVE_OK_BITS (EPOLLINOUT_BITS | EPOLLERR | EPOLLHUP | \

  				EPOLLWAKEUP | EPOLLET | EPOLLEXCLUSIVE)

  /* Maximum number of nesting allowed inside epoll sets */
  #define EP_MAX_NESTS 4

  #define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))

  #define EP_UNACTIVE_PTR ((void *) -1L)

  #define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))

  struct epoll_filefd {
  	struct file *file;
  	int fd;

  } __packed;

  
  /*

   * Structure used to track possible nested calls, for too deep recursions
   * and loop cycles.

   */

  struct nested_call_node {

  	struct list_head llink;

  	void *cookie;

  	void *ctx;

  };
  
  /*

   * This structure is used as collector for nested calls, to check for
   * maximum recursion dept and loop cycles.

   */

  struct nested_calls {
  	struct list_head tasks_call_list;

  	spinlock_t lock;
  };
  
  /*

   * Each file descriptor added to the eventpoll interface will
   * have an entry of this type linked to the "rbr" RB tree.

   * Avoid increasing the size of this struct, there can be many thousands
   * of these on a server and we do not want this to take another cache line.

   */
  struct epitem {

  	union {
  		/* RB tree node links this structure to the eventpoll RB tree */
  		struct rb_node rbn;
  		/* Used to free the struct epitem */
  		struct rcu_head rcu;
  	};

  
  	/* List header used to link this structure to the eventpoll ready list */
  	struct list_head rdllink;

  	/*
  	 * Works together "struct eventpoll"->ovflist in keeping the
  	 * single linked chain of items.
  	 */
  	struct epitem *next;

  	/* The file descriptor information this item refers to */
  	struct epoll_filefd ffd;
  
  	/* Number of active wait queue attached to poll operations */
  	int nwait;
  
  	/* List containing poll wait queues */
  	struct list_head pwqlist;
  
  	/* The "container" of this item */
  	struct eventpoll *ep;

  	/* List header used to link this item to the "struct file" items list */
  	struct list_head fllink;

  	/* wakeup_source used when EPOLLWAKEUP is set */

  	struct wakeup_source __rcu *ws;

  	/* The structure that describe the interested events and the source fd */
  	struct epoll_event event;

  };
  
  /*

   * This structure is stored inside the "private_data" member of the file

   * structure and represents the main data structure for the eventpoll

   * interface.
   */
  struct eventpoll {

  	/*

  	 * This mutex is used to ensure that files are not removed
  	 * while epoll is using them. This is held during the event
  	 * collection loop, the file cleanup path, the epoll file exit
  	 * code and the ctl operations.

  	 */

  	struct mutex mtx;

  
  	/* Wait queue used by sys_epoll_wait() */
  	wait_queue_head_t wq;
  
  	/* Wait queue used by file->poll() */
  	wait_queue_head_t poll_wait;
  
  	/* List of ready file descriptors */
  	struct list_head rdllist;

  	/* Lock which protects rdllist and ovflist */
  	rwlock_t lock;

  	/* RB tree root used to store monitored fd structs */

  	struct rb_root_cached rbr;

  
  	/*
  	 * This is a single linked list that chains all the "struct epitem" that

  	 * happened while transferring ready events to userspace w/out

  	 * holding ->lock.

  	 */
  	struct epitem *ovflist;

  	/* wakeup_source used when ep_scan_ready_list is running */
  	struct wakeup_source *ws;

  	/* The user that created the eventpoll descriptor */
  	struct user_struct *user;

  
  	struct file *file;
  
  	/* used to optimize loop detection check */

  	u64 gen;

  
  #ifdef CONFIG_NET_RX_BUSY_POLL
  	/* used to track busy poll napi_id */
  	unsigned int napi_id;
  #endif

  };
  
  /* Wait structure used by the poll hooks */
  struct eppoll_entry {
  	/* List header used to link this structure to the "struct epitem" */
  	struct list_head llink;
  
  	/* The "base" pointer is set to the container "struct epitem" */

  	struct epitem *base;

  
  	/*
  	 * Wait queue item that will be linked to the target file wait
  	 * queue head.
  	 */

  	wait_queue_entry_t wait;

  
  	/* The wait queue head that linked the "wait" wait queue item */
  	wait_queue_head_t *whead;
  };

  /* Wrapper struct used by poll queueing */
  struct ep_pqueue {
  	poll_table pt;
  	struct epitem *epi;
  };

  /* Used by the ep_send_events() function as callback private data */
  struct ep_send_events_data {
  	int maxevents;
  	struct epoll_event __user *events;

  	int res;

  };

  /*

   * Configuration options available inside /proc/sys/fs/epoll/
   */

  /* Maximum number of epoll watched descriptors, per user */

  static long max_user_watches __read_mostly;

  
  /*

   * This mutex is used to serialize ep_free() and eventpoll_release_file().

   */

  static DEFINE_MUTEX(epmutex);

  static u64 loop_check_gen = 0;

  /* Used to check for epoll file descriptor inclusion loops */
  static struct nested_calls poll_loop_ncalls;

  /* Slab cache used to allocate "struct epitem" */

  static struct kmem_cache *epi_cache __read_mostly;

  
  /* Slab cache used to allocate "struct eppoll_entry" */

  static struct kmem_cache *pwq_cache __read_mostly;

  /*
   * List of files with newly added links, where we may need to limit the number
   * of emanating paths. Protected by the epmutex.
   */
  static LIST_HEAD(tfile_check_list);

  #ifdef CONFIG_SYSCTL
  
  #include <linux/sysctl.h>

  static long long_zero;

  static long long_max = LONG_MAX;

  struct ctl_table epoll_table[] = {

  	{

  		.procname	= "max_user_watches",
  		.data		= &max_user_watches,

  		.maxlen		= sizeof(max_user_watches),

  		.mode		= 0644,

  		.proc_handler	= proc_doulongvec_minmax,

  		.extra1		= &long_zero,

  		.extra2		= &long_max,

  	},

  	{ }

  };
  #endif /* CONFIG_SYSCTL */

  static const struct file_operations eventpoll_fops;
  
  static inline int is_file_epoll(struct file *f)
  {
  	return f->f_op == &eventpoll_fops;
  }

  /* Setup the structure that is used as key for the RB tree */

  static inline void ep_set_ffd(struct epoll_filefd *ffd,
  			      struct file *file, int fd)
  {
  	ffd->file = file;
  	ffd->fd = fd;
  }

  /* Compare RB tree keys */

  static inline int ep_cmp_ffd(struct epoll_filefd *p1,
  			     struct epoll_filefd *p2)
  {
  	return (p1->file > p2->file ? +1:
  	        (p1->file < p2->file ? -1 : p1->fd - p2->fd));
  }

  /* Tells us if the item is currently linked */

  static inline int ep_is_linked(struct epitem *epi)

  {

  	return !list_empty(&epi->rdllink);

  }

  static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_entry_t *p)

  {
  	return container_of(p, struct eppoll_entry, wait);
  }

  /* Get the "struct epitem" from a wait queue pointer */

  static inline struct epitem *ep_item_from_wait(wait_queue_entry_t *p)

  {
  	return container_of(p, struct eppoll_entry, wait)->base;
  }
  
  /* Get the "struct epitem" from an epoll queue wrapper */

  static inline struct epitem *ep_item_from_epqueue(poll_table *p)

  {
  	return container_of(p, struct ep_pqueue, pt)->epi;
  }
  
  /* Tells if the epoll_ctl(2) operation needs an event copy from userspace */

  static inline int ep_op_has_event(int op)

  {

  	return op != EPOLL_CTL_DEL;

  }

  /* Initialize the poll safe wake up structure */

  static void ep_nested_calls_init(struct nested_calls *ncalls)

  {

  	INIT_LIST_HEAD(&ncalls->tasks_call_list);
  	spin_lock_init(&ncalls->lock);

  }

  /**

   * ep_events_available - Checks if ready events might be available.
   *
   * @ep: Pointer to the eventpoll context.
   *
   * Returns: Returns a value different than zero if ready events are available,
   *          or zero otherwise.
   */
  static inline int ep_events_available(struct eventpoll *ep)
  {

  	return !list_empty_careful(&ep->rdllist) ||
  		READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;

  }

  #ifdef CONFIG_NET_RX_BUSY_POLL
  static bool ep_busy_loop_end(void *p, unsigned long start_time)
  {
  	struct eventpoll *ep = p;
  
  	return ep_events_available(ep) || busy_loop_timeout(start_time);
  }

  
  /*
   * Busy poll if globally on and supporting sockets found && no events,
   * busy loop will return if need_resched or ep_events_available.
   *
   * we must do our busy polling with irqs enabled
   */
  static void ep_busy_loop(struct eventpoll *ep, int nonblock)
  {

  	unsigned int napi_id = READ_ONCE(ep->napi_id);
  
  	if ((napi_id >= MIN_NAPI_ID) && net_busy_loop_on())
  		napi_busy_loop(napi_id, nonblock ? NULL : ep_busy_loop_end, ep);

  }
  
  static inline void ep_reset_busy_poll_napi_id(struct eventpoll *ep)
  {

  	if (ep->napi_id)
  		ep->napi_id = 0;

  }
  
  /*
   * Set epoll busy poll NAPI ID from sk.
   */
  static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
  {

  	struct eventpoll *ep;
  	unsigned int napi_id;
  	struct socket *sock;
  	struct sock *sk;
  	int err;
  
  	if (!net_busy_loop_on())
  		return;
  
  	sock = sock_from_file(epi->ffd.file, &err);
  	if (!sock)
  		return;
  
  	sk = sock->sk;
  	if (!sk)
  		return;
  
  	napi_id = READ_ONCE(sk->sk_napi_id);
  	ep = epi->ep;
  
  	/* Non-NAPI IDs can be rejected
  	 *	or
  	 * Nothing to do if we already have this ID
  	 */
  	if (napi_id < MIN_NAPI_ID || napi_id == ep->napi_id)
  		return;
  
  	/* record NAPI ID for use in next busy poll */
  	ep->napi_id = napi_id;

  }

  #else
  
  static inline void ep_busy_loop(struct eventpoll *ep, int nonblock)
  {
  }
  
  static inline void ep_reset_busy_poll_napi_id(struct eventpoll *ep)
  {
  }
  
  static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
  {
  }
  
  #endif /* CONFIG_NET_RX_BUSY_POLL */

  /**

   * ep_call_nested - Perform a bound (possibly) nested call, by checking
   *                  that the recursion limit is not exceeded, and that
   *                  the same nested call (by the meaning of same cookie) is
   *                  no re-entered.
   *
   * @ncalls: Pointer to the nested_calls structure to be used for this call.

   * @nproc: Nested call core function pointer.
   * @priv: Opaque data to be passed to the @nproc callback.
   * @cookie: Cookie to be used to identify this nested call.

   * @ctx: This instance context.

   *
   * Returns: Returns the code returned by the @nproc callback, or -1 if
   *          the maximum recursion limit has been exceeded.

   */

  static int ep_call_nested(struct nested_calls *ncalls,

  			  int (*nproc)(void *, void *, int), void *priv,

  			  void *cookie, void *ctx)

  {

  	int error, call_nests = 0;

  	unsigned long flags;

  	struct list_head *lsthead = &ncalls->tasks_call_list;
  	struct nested_call_node *tncur;
  	struct nested_call_node tnode;

  	spin_lock_irqsave(&ncalls->lock, flags);

  	/*
  	 * Try to see if the current task is already inside this wakeup call.
  	 * We use a list here, since the population inside this set is always
  	 * very much limited.
  	 */

  	list_for_each_entry(tncur, lsthead, llink) {

  		if (tncur->ctx == ctx &&

  		    (tncur->cookie == cookie || ++call_nests > EP_MAX_NESTS)) {

  			/*
  			 * Ops ... loop detected or maximum nest level reached.
  			 * We abort this wake by breaking the cycle itself.
  			 */

  			error = -1;
  			goto out_unlock;

  		}
  	}

  	/* Add the current task and cookie to the list */

  	tnode.ctx = ctx;

  	tnode.cookie = cookie;

  	list_add(&tnode.llink, lsthead);

  	spin_unlock_irqrestore(&ncalls->lock, flags);

  	/* Call the nested function */
  	error = (*nproc)(priv, cookie, call_nests);

  
  	/* Remove the current task from the list */

  	spin_lock_irqsave(&ncalls->lock, flags);

  	list_del(&tnode.llink);

  out_unlock:

  	spin_unlock_irqrestore(&ncalls->lock, flags);
  
  	return error;
  }

  /*
   * As described in commit 0ccf831cb lockdep: annotate epoll
   * the use of wait queues used by epoll is done in a very controlled
   * manner. Wake ups can nest inside each other, but are never done
   * with the same locking. For example:
   *
   *   dfd = socket(...);
   *   efd1 = epoll_create();
   *   efd2 = epoll_create();
   *   epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
   *   epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
   *
   * When a packet arrives to the device underneath "dfd", the net code will
   * issue a wake_up() on its poll wake list. Epoll (efd1) has installed a
   * callback wakeup entry on that queue, and the wake_up() performed by the
   * "dfd" net code will end up in ep_poll_callback(). At this point epoll
   * (efd1) notices that it may have some event ready, so it needs to wake up
   * the waiters on its poll wait list (efd2). So it calls ep_poll_safewake()
   * that ends up in another wake_up(), after having checked about the
   * recursion constraints. That are, no more than EP_MAX_POLLWAKE_NESTS, to
   * avoid stack blasting.
   *
   * When CONFIG_DEBUG_LOCK_ALLOC is enabled, make sure lockdep can handle
   * this special case of epoll.
   */

  #ifdef CONFIG_DEBUG_LOCK_ALLOC

  
  static struct nested_calls poll_safewake_ncalls;
  
  static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)

  {
  	unsigned long flags;

  	wait_queue_head_t *wqueue = (wait_queue_head_t *)cookie;

  	spin_lock_irqsave_nested(&wqueue->lock, flags, call_nests + 1);

  	wake_up_locked_poll(wqueue, EPOLLIN);

  	spin_unlock_irqrestore(&wqueue->lock, flags);

  	return 0;
  }

  static void ep_poll_safewake(wait_queue_head_t *wq)
  {

  	int this_cpu = get_cpu();

  	ep_call_nested(&poll_safewake_ncalls,

  		       ep_poll_wakeup_proc, NULL, wq, (void *) (long) this_cpu);
  
  	put_cpu();

  }

  #else
  
  static void ep_poll_safewake(wait_queue_head_t *wq)
  {

  	wake_up_poll(wq, EPOLLIN);

  }
  
  #endif

  static void ep_remove_wait_queue(struct eppoll_entry *pwq)
  {
  	wait_queue_head_t *whead;
  
  	rcu_read_lock();

  	/*
  	 * If it is cleared by POLLFREE, it should be rcu-safe.
  	 * If we read NULL we need a barrier paired with
  	 * smp_store_release() in ep_poll_callback(), otherwise
  	 * we rely on whead->lock.
  	 */
  	whead = smp_load_acquire(&pwq->whead);

  	if (whead)
  		remove_wait_queue(whead, &pwq->wait);
  	rcu_read_unlock();
  }

  /*

   * This function unregisters poll callbacks from the associated file
   * descriptor.  Must be called with "mtx" held (or "epmutex" if called from
   * ep_free).

   */

  static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)

  {

  	struct list_head *lsthead = &epi->pwqlist;
  	struct eppoll_entry *pwq;

  	while (!list_empty(lsthead)) {
  		pwq = list_first_entry(lsthead, struct eppoll_entry, llink);

  		list_del(&pwq->llink);

  		ep_remove_wait_queue(pwq);

  		kmem_cache_free(pwq_cache, pwq);

  	}

  }

  /* call only when ep->mtx is held */
  static inline struct wakeup_source *ep_wakeup_source(struct epitem *epi)
  {
  	return rcu_dereference_check(epi->ws, lockdep_is_held(&epi->ep->mtx));
  }
  
  /* call only when ep->mtx is held */
  static inline void ep_pm_stay_awake(struct epitem *epi)
  {
  	struct wakeup_source *ws = ep_wakeup_source(epi);
  
  	if (ws)
  		__pm_stay_awake(ws);
  }
  
  static inline bool ep_has_wakeup_source(struct epitem *epi)
  {
  	return rcu_access_pointer(epi->ws) ? true : false;
  }
  
  /* call when ep->mtx cannot be held (ep_poll_callback) */
  static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
  {
  	struct wakeup_source *ws;
  
  	rcu_read_lock();
  	ws = rcu_dereference(epi->ws);
  	if (ws)
  		__pm_stay_awake(ws);
  	rcu_read_unlock();
  }

  /**
   * ep_scan_ready_list - Scans the ready list in a way that makes possible for
   *                      the scan code, to call f_op->poll(). Also allows for
   *                      O(NumReady) performance.
   *
   * @ep: Pointer to the epoll private data structure.
   * @sproc: Pointer to the scan callback.
   * @priv: Private opaque data passed to the @sproc callback.

   * @depth: The current depth of recursive f_op->poll calls.

   * @ep_locked: caller already holds ep->mtx

   *
   * Returns: The same integer error code returned by the @sproc callback.
   */

  static __poll_t ep_scan_ready_list(struct eventpoll *ep,
  			      __poll_t (*sproc)(struct eventpoll *,

  					   struct list_head *, void *),

  			      void *priv, int depth, bool ep_locked)

  {

  	__poll_t res;
  	int pwake = 0;

  	struct epitem *epi, *nepi;

  	LIST_HEAD(txlist);

  	lockdep_assert_irqs_enabled();

  	/*
  	 * We need to lock this because we could be hit by

  	 * eventpoll_release_file() and epoll_ctl().

  	 */

  
  	if (!ep_locked)
  		mutex_lock_nested(&ep->mtx, depth);

  
  	/*
  	 * Steal the ready list, and re-init the original one to the
  	 * empty list. Also, set ep->ovflist to NULL so that events
  	 * happening while looping w/out locks, are not lost. We cannot
  	 * have the poll callback to queue directly on ep->rdllist,
  	 * because we want the "sproc" callback to be able to do it
  	 * in a lockless way.
  	 */

  	write_lock_irq(&ep->lock);

  	list_splice_init(&ep->rdllist, &txlist);

  	WRITE_ONCE(ep->ovflist, NULL);

  	write_unlock_irq(&ep->lock);

  
  	/*
  	 * Now call the callback function.
  	 */

  	res = (*sproc)(ep, &txlist, priv);

  	write_lock_irq(&ep->lock);

  	/*
  	 * During the time we spent inside the "sproc" callback, some
  	 * other events might have been queued by the poll callback.
  	 * We re-insert them inside the main ready-list here.
  	 */

  	for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;

  	     nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
  		/*
  		 * We need to check if the item is already in the list.
  		 * During the "sproc" callback execution time, items are
  		 * queued into ->ovflist but the "txlist" might already
  		 * contain them, and the list_splice() below takes care of them.
  		 */

  		if (!ep_is_linked(epi)) {

  			/*
  			 * ->ovflist is LIFO, so we have to reverse it in order
  			 * to keep in FIFO.
  			 */
  			list_add(&epi->rdllink, &ep->rdllist);

  			ep_pm_stay_awake(epi);

  		}

  	}
  	/*
  	 * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
  	 * releasing the lock, events will be queued in the normal way inside
  	 * ep->rdllist.
  	 */

  	WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);

  
  	/*
  	 * Quickly re-inject items left on "txlist".
  	 */
  	list_splice(&txlist, &ep->rdllist);

  	__pm_relax(ep->ws);

  
  	if (!list_empty(&ep->rdllist)) {
  		/*

  		 * Wake up (if active) both the eventpoll wait list and
  		 * the ->poll() wait list (delayed after we release the lock).

  		 */
  		if (waitqueue_active(&ep->wq))

  			wake_up(&ep->wq);

  		if (waitqueue_active(&ep->poll_wait))
  			pwake++;
  	}

  	write_unlock_irq(&ep->lock);

  	if (!ep_locked)
  		mutex_unlock(&ep->mtx);

  
  	/* We have to call this outside the lock */
  	if (pwake)
  		ep_poll_safewake(&ep->poll_wait);

  	return res;

  }

  static void epi_rcu_free(struct rcu_head *head)
  {
  	struct epitem *epi = container_of(head, struct epitem, rcu);
  	kmem_cache_free(epi_cache, epi);
  }

  /*

   * Removes a "struct epitem" from the eventpoll RB tree and deallocates

   * all the associated resources. Must be called with "mtx" held.

   */
  static int ep_remove(struct eventpoll *ep, struct epitem *epi)
  {

  	struct file *file = epi->ffd.file;

  	lockdep_assert_irqs_enabled();

  	/*

  	 * Removes poll wait queue hooks.

  	 */

  	ep_unregister_pollwait(ep, epi);

  	/* Remove the current item from the list of epoll hooks */

  	spin_lock(&file->f_lock);

  	list_del_rcu(&epi->fllink);

  	spin_unlock(&file->f_lock);

  	rb_erase_cached(&epi->rbn, &ep->rbr);

  	write_lock_irq(&ep->lock);

  	if (ep_is_linked(epi))

  		list_del_init(&epi->rdllink);

  	write_unlock_irq(&ep->lock);

  	wakeup_source_unregister(ep_wakeup_source(epi));

  	/*
  	 * At this point it is safe to free the eventpoll item. Use the union
  	 * field epi->rcu, since we are trying to minimize the size of
  	 * 'struct epitem'. The 'rbn' field is no longer in use. Protected by
  	 * ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
  	 * use of the rbn field.
  	 */
  	call_rcu(&epi->rcu, epi_rcu_free);

  	atomic_long_dec(&ep->user->epoll_watches);

  	return 0;

  }

  static void ep_free(struct eventpoll *ep)

  {

  	struct rb_node *rbp;
  	struct epitem *epi;

  	/* We need to release all tasks waiting for these file */
  	if (waitqueue_active(&ep->poll_wait))

  		ep_poll_safewake(&ep->poll_wait);

  	/*
  	 * We need to lock this because we could be hit by
  	 * eventpoll_release_file() while we're freeing the "struct eventpoll".

  	 * We do not need to hold "ep->mtx" here because the epoll file

  	 * is on the way to be removed and no one has references to it
  	 * anymore. The only hit might come from eventpoll_release_file() but

  	 * holding "epmutex" is sufficient here.

  	 */
  	mutex_lock(&epmutex);

  
  	/*

  	 * Walks through the whole tree by unregistering poll callbacks.

  	 */

  	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {

  		epi = rb_entry(rbp, struct epitem, rbn);
  
  		ep_unregister_pollwait(ep, epi);

  		cond_resched();

  	}

  
  	/*

  	 * Walks through the whole tree by freeing each "struct epitem". At this
  	 * point we are sure no poll callbacks will be lingering around, and also by

  	 * holding "epmutex" we can be sure that no file cleanup code will hit

  	 * us during this operation. So we can avoid the lock on "ep->lock".

  	 * We do not need to lock ep->mtx, either, we only do it to prevent
  	 * a lockdep warning.

  	 */

  	mutex_lock(&ep->mtx);

  	while ((rbp = rb_first_cached(&ep->rbr)) != NULL) {

  		epi = rb_entry(rbp, struct epitem, rbn);
  		ep_remove(ep, epi);

  		cond_resched();

  	}

  	mutex_unlock(&ep->mtx);

  	mutex_unlock(&epmutex);

  	mutex_destroy(&ep->mtx);

  	free_uid(ep->user);

  	wakeup_source_unregister(ep->ws);

  	kfree(ep);

  }

  static int ep_eventpoll_release(struct inode *inode, struct file *file)
  {
  	struct eventpoll *ep = file->private_data;

  	if (ep)

  		ep_free(ep);

  	return 0;

  }

  static __poll_t ep_read_events_proc(struct eventpoll *ep, struct list_head *head,

  			       void *priv);
  static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
  				 poll_table *pt);
  
  /*
   * Differs from ep_eventpoll_poll() in that internal callers already have
   * the ep->mtx so we need to start from depth=1, such that mutex_lock_nested()
   * is correctly annotated.
   */

  static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,

  				 int depth)

  {

  	struct eventpoll *ep;
  	bool locked;

  	pt->_key = epi->event.events;

  	if (!is_file_epoll(epi->ffd.file))

  		return vfs_poll(epi->ffd.file, pt) & epi->event.events;

  	ep = epi->ffd.file->private_data;
  	poll_wait(epi->ffd.file, &ep->poll_wait, pt);
  	locked = pt && (pt->_qproc == ep_ptable_queue_proc);

  	return ep_scan_ready_list(epi->ffd.file->private_data,
  				  ep_read_events_proc, &depth, depth,
  				  locked) & epi->event.events;

  }

  static __poll_t ep_read_events_proc(struct eventpoll *ep, struct list_head *head,

  			       void *priv)

  {
  	struct epitem *epi, *tmp;

  	poll_table pt;

  	int depth = *(int *)priv;

  	init_poll_funcptr(&pt, NULL);

  	depth++;

  	list_for_each_entry_safe(epi, tmp, head, rdllink) {

  		if (ep_item_poll(epi, &pt, depth)) {

  			return EPOLLIN | EPOLLRDNORM;

  		} else {

  			/*
  			 * Item has been dropped into the ready list by the poll
  			 * callback, but it's not actually ready, as far as
  			 * caller requested events goes. We can remove it here.
  			 */

  			__pm_relax(ep_wakeup_source(epi));

  			list_del_init(&epi->rdllink);

  		}

  	}
  
  	return 0;
  }

  static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)

  {
  	struct eventpoll *ep = file->private_data;
  	int depth = 0;

  	/* Insert inside our poll wait queue */
  	poll_wait(file, &ep->poll_wait, wait);

  	/*
  	 * Proceed to find out if wanted events are really available inside

  	 * the ready list.

  	 */

  	return ep_scan_ready_list(ep, ep_read_events_proc,
  				  &depth, depth, false);

  }

  #ifdef CONFIG_PROC_FS

  static void ep_show_fdinfo(struct seq_file *m, struct file *f)

  {
  	struct eventpoll *ep = f->private_data;
  	struct rb_node *rbp;

  
  	mutex_lock(&ep->mtx);

  	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {

  		struct epitem *epi = rb_entry(rbp, struct epitem, rbn);

  		struct inode *inode = file_inode(epi->ffd.file);

  		seq_printf(m, "tfd: %8d events: %8x data: %16llx "
  			   " pos:%lli ino:%lx sdev:%x
  ",

  			   epi->ffd.fd, epi->event.events,

  			   (long long)epi->event.data,
  			   (long long)epi->ffd.file->f_pos,
  			   inode->i_ino, inode->i_sb->s_dev);

  		if (seq_has_overflowed(m))

  			break;
  	}
  	mutex_unlock(&ep->mtx);

  }
  #endif

  /* File callbacks that implement the eventpoll file behaviour */
  static const struct file_operations eventpoll_fops = {

  #ifdef CONFIG_PROC_FS
  	.show_fdinfo	= ep_show_fdinfo,
  #endif

  	.release	= ep_eventpoll_release,

  	.poll		= ep_eventpoll_poll,

  	.llseek		= noop_llseek,

  };

  /*

   * This is called from eventpoll_release() to unlink files from the eventpoll
   * interface. We need to have this facility to cleanup correctly files that are
   * closed without being removed from the eventpoll interface.

   */

  void eventpoll_release_file(struct file *file)

  {

  	struct eventpoll *ep;

  	struct epitem *epi, *next;

  
  	/*

  	 * We don't want to get "file->f_lock" because it is not

  	 * necessary. It is not necessary because we're in the "struct file"

  	 * cleanup path, and this means that no one is using this file anymore.

  	 * So, for example, epoll_ctl() cannot hit here since if we reach this

  	 * point, the file counter already went to zero and fget() would fail.

  	 * The only hit might come from ep_free() but by holding the mutex

  	 * will correctly serialize the operation. We do need to acquire

  	 * "ep->mtx" after "epmutex" because ep_remove() requires it when called

  	 * from anywhere but ep_free().

  	 *
  	 * Besides, ep_remove() acquires the lock, so we can't hold it here.

  	 */

  	mutex_lock(&epmutex);

  	list_for_each_entry_safe(epi, next, &file->f_ep_links, fllink) {

  		ep = epi->ep;

  		mutex_lock_nested(&ep->mtx, 0);

  		ep_remove(ep, epi);

  		mutex_unlock(&ep->mtx);

  	}

  	mutex_unlock(&epmutex);

  }

  static int ep_alloc(struct eventpoll **pep)

  {

  	int error;
  	struct user_struct *user;
  	struct eventpoll *ep;

  	user = get_current_user();

  	error = -ENOMEM;
  	ep = kzalloc(sizeof(*ep), GFP_KERNEL);
  	if (unlikely(!ep))
  		goto free_uid;

  	mutex_init(&ep->mtx);

  	rwlock_init(&ep->lock);

  	init_waitqueue_head(&ep->wq);
  	init_waitqueue_head(&ep->poll_wait);
  	INIT_LIST_HEAD(&ep->rdllist);

  	ep->rbr = RB_ROOT_CACHED;

  	ep->ovflist = EP_UNACTIVE_PTR;

  	ep->user = user;

  	*pep = ep;

  	return 0;

  
  free_uid:
  	free_uid(user);
  	return error;

  }

  /*

   * Search the file inside the eventpoll tree. The RB tree operations
   * are protected by the "mtx" mutex, and ep_find() must be called with
   * "mtx" held.

   */
  static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
  {
  	int kcmp;

  	struct rb_node *rbp;
  	struct epitem *epi, *epir = NULL;
  	struct epoll_filefd ffd;

  	ep_set_ffd(&ffd, file, fd);

  	for (rbp = ep->rbr.rb_root.rb_node; rbp; ) {

  		epi = rb_entry(rbp, struct epitem, rbn);

  		kcmp = ep_cmp_ffd(&ffd, &epi->ffd);

  		if (kcmp > 0)
  			rbp = rbp->rb_right;
  		else if (kcmp < 0)
  			rbp = rbp->rb_left;
  		else {

  			epir = epi;
  			break;
  		}
  	}

  	return epir;
  }

  #ifdef CONFIG_CHECKPOINT_RESTORE

  static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long toff)
  {
  	struct rb_node *rbp;
  	struct epitem *epi;

  	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {

  		epi = rb_entry(rbp, struct epitem, rbn);
  		if (epi->ffd.fd == tfd) {
  			if (toff == 0)
  				return epi;
  			else
  				toff--;
  		}
  		cond_resched();
  	}
  
  	return NULL;
  }
  
  struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
  				     unsigned long toff)
  {
  	struct file *file_raw;
  	struct eventpoll *ep;
  	struct epitem *epi;
  
  	if (!is_file_epoll(file))
  		return ERR_PTR(-EINVAL);
  
  	ep = file->private_data;
  
  	mutex_lock(&ep->mtx);
  	epi = ep_find_tfd(ep, tfd, toff);
  	if (epi)
  		file_raw = epi->ffd.file;
  	else
  		file_raw = ERR_PTR(-ENOENT);
  	mutex_unlock(&ep->mtx);
  
  	return file_raw;
  }

  #endif /* CONFIG_CHECKPOINT_RESTORE */

  /**
   * Adds a new entry to the tail of the list in a lockless way, i.e.
   * multiple CPUs are allowed to call this function concurrently.
   *
   * Beware: it is necessary to prevent any other modifications of the
   *         existing list until all changes are completed, in other words
   *         concurrent list_add_tail_lockless() calls should be protected
   *         with a read lock, where write lock acts as a barrier which
   *         makes sure all list_add_tail_lockless() calls are fully
   *         completed.
   *
   *        Also an element can be locklessly added to the list only in one
   *        direction i.e. either to the tail either to the head, otherwise
   *        concurrent access will corrupt the list.
   *
   * Returns %false if element has been already added to the list, %true
   * otherwise.
   */
  static inline bool list_add_tail_lockless(struct list_head *new,
  					  struct list_head *head)
  {
  	struct list_head *prev;
  
  	/*
  	 * This is simple 'new->next = head' operation, but cmpxchg()
  	 * is used in order to detect that same element has been just
  	 * added to the list from another CPU: the winner observes
  	 * new->next == new.
  	 */
  	if (cmpxchg(&new->next, new, head) != new)
  		return false;
  
  	/*
  	 * Initially ->next of a new element must be updated with the head
  	 * (we are inserting to the tail) and only then pointers are atomically
  	 * exchanged.  XCHG guarantees memory ordering, thus ->next should be
  	 * updated before pointers are actually swapped and pointers are
  	 * swapped before prev->next is updated.
  	 */
  
  	prev = xchg(&head->prev, new);
  
  	/*
  	 * It is safe to modify prev->next and new->prev, because a new element
  	 * is added only to the tail and new->next is updated before XCHG.
  	 */
  
  	prev->next = new;
  	new->prev = prev;
  
  	return true;
  }
  
  /**
   * Chains a new epi entry to the tail of the ep->ovflist in a lockless way,
   * i.e. multiple CPUs are allowed to call this function concurrently.
   *
   * Returns %false if epi element has been already chained, %true otherwise.
   */
  static inline bool chain_epi_lockless(struct epitem *epi)
  {
  	struct eventpoll *ep = epi->ep;

  	/* Fast preliminary check */
  	if (epi->next != EP_UNACTIVE_PTR)
  		return false;

  	/* Check that the same epi has not been just chained from another CPU */
  	if (cmpxchg(&epi->next, EP_UNACTIVE_PTR, NULL) != EP_UNACTIVE_PTR)
  		return false;
  
  	/* Atomically exchange tail */
  	epi->next = xchg(&ep->ovflist, epi);
  
  	return true;
  }

  /*

   * This is the callback that is passed to the wait queue wakeup

   * mechanism. It is called by the stored file descriptors when they

   * have events to report.

   *
   * This callback takes a read lock in order not to content with concurrent
   * events from another file descriptors, thus all modifications to ->rdllist
   * or ->ovflist are lockless.  Read lock is paired with the write lock from
   * ep_scan_ready_list(), which stops all list modifications and guarantees
   * that lists state is seen correctly.
   *
   * Another thing worth to mention is that ep_poll_callback() can be called
   * concurrently for the same @epi from different CPUs if poll table was inited
   * with several wait queues entries.  Plural wakeup from different CPUs of a
   * single wait queue is serialized by wq.lock, but the case when multiple wait
   * queues are used should be detected accordingly.  This is detected using
   * cmpxchg() operation.

   */

  static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)

  {

  	int pwake = 0;

  	struct epitem *epi = ep_item_from_wait(wait);
  	struct eventpoll *ep = epi->ep;

  	__poll_t pollflags = key_to_poll(key);

  	unsigned long flags;

  	int ewake = 0;

  	read_lock_irqsave(&ep->lock, flags);

  	ep_set_busy_poll_napi_id(epi);

  	/*
  	 * If the event mask does not contain any poll(2) event, we consider the
  	 * descriptor to be disabled. This condition is likely the effect of the
  	 * EPOLLONESHOT bit that disables the descriptor when an event is received,
  	 * until the next EPOLL_CTL_MOD will be issued.
  	 */
  	if (!(epi->event.events & ~EP_PRIVATE_BITS))

  		goto out_unlock;
  
  	/*

  	 * Check the events coming with the callback. At this stage, not
  	 * every device reports the events in the "key" parameter of the
  	 * callback. We need to be able to handle both cases here, hence the
  	 * test for "key" != NULL before the event match test.
  	 */

  	if (pollflags && !(pollflags & epi->event.events))

  		goto out_unlock;
  
  	/*

  	 * If we are transferring events to userspace, we can hold no locks

  	 * (because we're accessing user memory, and because of linux f_op->poll()

  	 * semantics). All the events that happen during that period of time are

  	 * chained in ep->ovflist and requeued later on.
  	 */

  	if (READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR) {

  		if (chain_epi_lockless(epi))
  			ep_pm_stay_awake_rcu(epi);
  	} else if (!ep_is_linked(epi)) {
  		/* In the usual case, add event to ready list. */
  		if (list_add_tail_lockless(&epi->rdllink, &ep->rdllist))

  			ep_pm_stay_awake_rcu(epi);

  	}

  	/*
  	 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
  	 * wait list.
  	 */

  	if (waitqueue_active(&ep->wq)) {

  		if ((epi->event.events & EPOLLEXCLUSIVE) &&

  					!(pollflags & POLLFREE)) {
  			switch (pollflags & EPOLLINOUT_BITS) {

  			case EPOLLIN:
  				if (epi->event.events & EPOLLIN)

  					ewake = 1;
  				break;

  			case EPOLLOUT:
  				if (epi->event.events & EPOLLOUT)

  					ewake = 1;
  				break;
  			case 0:
  				ewake = 1;
  				break;
  			}
  		}

  		wake_up(&ep->wq);

  	}

  	if (waitqueue_active(&ep->poll_wait))
  		pwake++;

  out_unlock:

  	read_unlock_irqrestore(&ep->lock, flags);

  	/* We have to call this outside the lock */
  	if (pwake)

  		ep_poll_safewake(&ep->poll_wait);

  	if (!(epi->event.events & EPOLLEXCLUSIVE))
  		ewake = 1;

  	if (pollflags & POLLFREE) {

  		/*
  		 * If we race with ep_remove_wait_queue() it can miss
  		 * ->whead = NULL and do another remove_wait_queue() after
  		 * us, so we can't use __remove_wait_queue().
  		 */
  		list_del_init(&wait->entry);
  		/*
  		 * ->whead != NULL protects us from the race with ep_free()
  		 * or ep_remove(), ep_remove_wait_queue() takes whead->lock
  		 * held by the caller. Once we nullify it, nothing protects
  		 * ep/epi or even wait.
  		 */
  		smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);
  	}

  	return ewake;

  }

  
  /*
   * This is the callback that is used to add our wait queue to the
   * target file wakeup lists.
   */
  static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
  				 poll_table *pt)
  {

  	struct epitem *epi = ep_item_from_epqueue(pt);

  	struct eppoll_entry *pwq;

  	if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {

  		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
  		pwq->whead = whead;
  		pwq->base = epi;

  		if (epi->event.events & EPOLLEXCLUSIVE)
  			add_wait_queue_exclusive(whead, &pwq->wait);
  		else
  			add_wait_queue(whead, &pwq->wait);

  		list_add_tail(&pwq->llink, &epi->pwqlist);
  		epi->nwait++;

  	} else {

  		/* We have to signal that an error occurred */
  		epi->nwait = -1;

  	}

  }

  static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
  {
  	int kcmp;

  	struct rb_node **p = &ep->rbr.rb_root.rb_node, *parent = NULL;

  	struct epitem *epic;

  	bool leftmost = true;

  
  	while (*p) {
  		parent = *p;
  		epic = rb_entry(parent, struct epitem, rbn);

  		kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);

  		if (kcmp > 0) {

  			p = &parent->rb_right;

  			leftmost = false;
  		} else

  			p = &parent->rb_left;
  	}
  	rb_link_node(&epi->rbn, parent, p);

  	rb_insert_color_cached(&epi->rbn, &ep->rbr, leftmost);

  }

  #define PATH_ARR_SIZE 5
  /*
   * These are the number paths of length 1 to 5, that we are allowing to emanate
   * from a single file of interest. For example, we allow 1000 paths of length
   * 1, to emanate from each file of interest. This essentially represents the
   * potential wakeup paths, which need to be limited in order to avoid massive
   * uncontrolled wakeup storms. The common use case should be a single ep which
   * is connected to n file sources. In this case each file source has 1 path
   * of length 1. Thus, the numbers below should be more than sufficient. These
   * path limits are enforced during an EPOLL_CTL_ADD operation, since a modify
   * and delete can't add additional paths. Protected by the epmutex.
   */
  static const int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };
  static int path_count[PATH_ARR_SIZE];
  
  static int path_count_inc(int nests)
  {

  	/* Allow an arbitrary number of depth 1 paths */
  	if (nests == 0)
  		return 0;

  	if (++path_count[nests] > path_limits[nests])
  		return -1;
  	return 0;
  }
  
  static void path_count_init(void)
  {
  	int i;
  
  	for (i = 0; i < PATH_ARR_SIZE; i++)
  		path_count[i] = 0;
  }
  
  static int reverse_path_check_proc(void *priv, void *cookie, int call_nests)
  {
  	int error = 0;
  	struct file *file = priv;
  	struct file *child_file;
  	struct epitem *epi;

  	/* CTL_DEL can remove links here, but that can't increase our count */
  	rcu_read_lock();
  	list_for_each_entry_rcu(epi, &file->f_ep_links, fllink) {

  		child_file = epi->ep->file;
  		if (is_file_epoll(child_file)) {
  			if (list_empty(&child_file->f_ep_links)) {
  				if (path_count_inc(call_nests)) {
  					error = -1;
  					break;
  				}
  			} else {
  				error = ep_call_nested(&poll_loop_ncalls,

  							reverse_path_check_proc,
  							child_file, child_file,
  							current);
  			}
  			if (error != 0)
  				break;
  		} else {
  			printk(KERN_ERR "reverse_path_check_proc: "
  				"file is not an ep!
  ");
  		}
  	}

  	rcu_read_unlock();

  	return error;
  }
  
  /**
   * reverse_path_check - The tfile_check_list is list of file *, which have
   *                      links that are proposed to be newly added. We need to
   *                      make sure that those added links don't add too many
   *                      paths such that we will spend all our time waking up
   *                      eventpoll objects.
   *
   * Returns: Returns zero if the proposed links don't create too many paths,
   *	    -1 otherwise.
   */
  static int reverse_path_check(void)
  {

  	int error = 0;
  	struct file *current_file;
  
  	/* let's call this for all tfiles */
  	list_for_each_entry(current_file, &tfile_check_list, f_tfile_llink) {

  		path_count_init();

  		error = ep_call_nested(&poll_loop_ncalls,

  					reverse_path_check_proc, current_file,
  					current_file, current);
  		if (error)
  			break;
  	}
  	return error;
  }

  static int ep_create_wakeup_source(struct epitem *epi)
  {

  	struct name_snapshot n;

  	struct wakeup_source *ws;

  
  	if (!epi->ep->ws) {

  		epi->ep->ws = wakeup_source_register(NULL, "eventpoll");

  		if (!epi->ep->ws)
  			return -ENOMEM;
  	}

  	take_dentry_name_snapshot(&n, epi->ffd.file->f_path.dentry);
  	ws = wakeup_source_register(NULL, n.name.name);
  	release_dentry_name_snapshot(&n);

  
  	if (!ws)

  		return -ENOMEM;

  	rcu_assign_pointer(epi->ws, ws);

  
  	return 0;
  }

  /* rare code path, only used when EPOLL_CTL_MOD removes a wakeup source */
  static noinline void ep_destroy_wakeup_source(struct epitem *epi)

  {

  	struct wakeup_source *ws = ep_wakeup_source(epi);

  	RCU_INIT_POINTER(epi->ws, NULL);

  
  	/*
  	 * wait for ep_pm_stay_awake_rcu to finish, synchronize_rcu is
  	 * used internally by wakeup_source_remove, too (called by
  	 * wakeup_source_unregister), so we cannot use call_rcu
  	 */
  	synchronize_rcu();
  	wakeup_source_unregister(ws);

  }

  /*
   * Must be called with "mtx" held.
   */

  static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,

  		     struct file *tfile, int fd, int full_check)

  {

  	int error, pwake = 0;
  	__poll_t revents;

  	long user_watches;

  	struct epitem *epi;
  	struct ep_pqueue epq;

  	lockdep_assert_irqs_enabled();

  	user_watches = atomic_long_read(&ep->user->epoll_watches);
  	if (unlikely(user_watches >= max_user_watches))

  		return -ENOSPC;

  	if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))

  		return -ENOMEM;

  
  	/* Item initialization follow here ... */

  	INIT_LIST_HEAD(&epi->rdllink);
  	INIT_LIST_HEAD(&epi->fllink);

  	INIT_LIST_HEAD(&epi->pwqlist);
  	epi->ep = ep;

  	ep_set_ffd(&epi->ffd, tfile, fd);

  	epi->event = *event;

  	epi->nwait = 0;

  	epi->next = EP_UNACTIVE_PTR;

  	if (epi->event.events & EPOLLWAKEUP) {
  		error = ep_create_wakeup_source(epi);
  		if (error)
  			goto error_create_wakeup_source;
  	} else {

  		RCU_INIT_POINTER(epi->ws, NULL);

  	}

  	/* Add the current item to the list of active epoll hook for this file */
  	spin_lock(&tfile->f_lock);
  	list_add_tail_rcu(&epi->fllink, &tfile->f_ep_links);
  	spin_unlock(&tfile->f_lock);
  
  	/*
  	 * Add the current item to the RB tree. All RB tree operations are
  	 * protected by "mtx", and ep_insert() is called with "mtx" held.
  	 */
  	ep_rbtree_insert(ep, epi);
  
  	/* now check if we've created too many backpaths */
  	error = -EINVAL;
  	if (full_check && reverse_path_check())
  		goto error_remove_epi;

  	/* Initialize the poll table using the queue callback */
  	epq.epi = epi;
  	init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
  
  	/*
  	 * Attach the item to the poll hooks and get current event bits.
  	 * We can safely use the file* here because its usage count has

  	 * been increased by the caller of this function. Note that after
  	 * this operation completes, the poll callback can start hitting
  	 * the new item.

  	 */

  	revents = ep_item_poll(epi, &epq.pt, 1);

  
  	/*
  	 * We have to check if something went wrong during the poll wait queue
  	 * install process. Namely an allocation for a wait queue failed due
  	 * high memory pressure.
  	 */

  	error = -ENOMEM;

  	if (epi->nwait < 0)

  		goto error_unregister;

  	/* We have to drop the new item inside our item list to keep track of it */

  	write_lock_irq(&ep->lock);

  	/* record NAPI ID of new item if present */
  	ep_set_busy_poll_napi_id(epi);

  	/* If the file is already "ready" we drop it inside the ready list */

  	if (revents && !ep_is_linked(epi)) {

  		list_add_tail(&epi->rdllink, &ep->rdllist);

  		ep_pm_stay_awake(epi);

  
  		/* Notify waiting tasks that events are available */
  		if (waitqueue_active(&ep->wq))

  			wake_up(&ep->wq);

  		if (waitqueue_active(&ep->poll_wait))
  			pwake++;
  	}

  	write_unlock_irq(&ep->lock);

  	atomic_long_inc(&ep->user->epoll_watches);