Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2479 insertions, 0 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
new file mode 100644
index 000000000..1d9a71a0c
--- /dev/null
+++ b/fs/eventpoll.c
@@ -0,0 +1,2479 @@
+// 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) epnested_mutex (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().
+ * The epnested_mutex is 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 "epnested_mutex" (together with "ep->lock") to have it working,
+ * but having "ep->mtx" will make the interface more scalable.
+ * Events that require holding "epnested_mutex" 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;
+
+/* Wait structure used by the poll hooks */
+struct eppoll_entry {
+	/* List header used to link this structure to the "struct epitem" */
+	struct eppoll_entry *next;
+
+	/* 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;
+};
+
+/*
+ * 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;
+
+	/*
+	 * Protected by file->f_lock, true for to-be-released epitem already
+	 * removed from the "struct file" items list; together with
+	 * eventpoll->refcount orchestrates "struct eventpoll" disposal
+	 */
+	bool dying;
+
+	/* List containing poll wait queues */
+	struct eppoll_entry *pwqlist;
+
+	/* The "container" of this item */
+	struct eventpoll *ep;
+
+	/* List header used to link this item to the "struct file" items list */
+	struct hlist_node 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;
+	struct hlist_head refs;
+
+	/*
+	 * usage count, used together with epitem->dying to
+	 * orchestrate the disposal of this struct
+	 */
+	refcount_t refcount;
+
+#ifdef CONFIG_NET_RX_BUSY_POLL
+	/* used to track busy poll napi_id */
+	unsigned int napi_id;
+#endif
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	/* tracks wakeup nests for lockdep validation */
+	u8 nests;
+#endif
+};
+
+/* Wrapper struct used by poll queueing */
+struct ep_pqueue {
+	poll_table pt;
+	struct epitem *epi;
+};
+
+/*
+ * Configuration options available inside /proc/sys/fs/epoll/
+ */
+/* Maximum number of epoll watched descriptors, per user */
+static long max_user_watches __read_mostly;
+
+/* Used for cycles detection */
+static DEFINE_MUTEX(epnested_mutex);
+
+static u64 loop_check_gen = 0;
+
+/* Used to check for epoll file descriptor inclusion loops */
+static struct eventpoll *inserting_into;
+
+/* 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 epnested_mutex.
+ */
+struct epitems_head {
+	struct hlist_head epitems;
+	struct epitems_head *next;
+};
+static struct epitems_head *tfile_check_list = EP_UNACTIVE_PTR;
+
+static struct kmem_cache *ephead_cache __read_mostly;
+
+static inline void free_ephead(struct epitems_head *head)
+{
+	if (head)
+		kmem_cache_free(ephead_cache, head);
+}
+
+static void list_file(struct file *file)
+{
+	struct epitems_head *head;
+
+	head = container_of(file->f_ep, struct epitems_head, epitems);
+	if (!head->next) {
+		head->next = tfile_check_list;
+		tfile_check_list = head;
+	}
+}
+
+static void unlist_file(struct epitems_head *head)
+{
+	struct epitems_head *to_free = head;
+	struct hlist_node *p = rcu_dereference(hlist_first_rcu(&head->epitems));
+	if (p) {
+		struct epitem *epi= container_of(p, struct epitem, fllink);
+		spin_lock(&epi->ffd.file->f_lock);
+		if (!hlist_empty(&head->epitems))
+			to_free = NULL;
+		head->next = NULL;
+		spin_unlock(&epi->ffd.file->f_lock);
+	}
+	free_ephead(to_free);
+}
+
+#ifdef CONFIG_SYSCTL
+
+#include <linux/sysctl.h>
+
+static long long_zero;
+static long long_max = LONG_MAX;
+
+static 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,
+	},
+	{ }
+};
+
+static void __init epoll_sysctls_init(void)
+{
+	register_sysctl("fs/epoll", epoll_table);
+}
+#else
+#define epoll_sysctls_init() do { } while (0)
+#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;
+}
+
+/**
+ * ep_events_available - Checks if ready events might be available.
+ *
+ * @ep: Pointer to the eventpoll context.
+ *
+ * Return: 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 bool 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, false,
+			       BUSY_POLL_BUDGET);
+		if (ep_events_available(ep))
+			return true;
+		/*
+		 * Busy poll timed out.  Drop NAPI ID for now, we can add
+		 * it back in when we have moved a socket with a valid NAPI
+		 * ID onto the ready list.
+		 */
+		ep->napi_id = 0;
+		return false;
+	}
+	return false;
+}
+
+/*
+ * 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;
+
+	if (!net_busy_loop_on())
+		return;
+
+	sock = sock_from_file(epi->ffd.file);
+	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 bool ep_busy_loop(struct eventpoll *ep, int nonblock)
+{
+	return false;
+}
+
+static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
+{
+}
+
+#endif /* CONFIG_NET_RX_BUSY_POLL */
+
+/*
+ * 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_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 void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
+			     unsigned pollflags)
+{
+	struct eventpoll *ep_src;
+	unsigned long flags;
+	u8 nests = 0;
+
+	/*
+	 * To set the subclass or nesting level for spin_lock_irqsave_nested()
+	 * it might be natural to create a per-cpu nest count. However, since
+	 * we can recurse on ep->poll_wait.lock, and a non-raw spinlock can
+	 * schedule() in the -rt kernel, the per-cpu variable are no longer
+	 * protected. Thus, we are introducing a per eventpoll nest field.
+	 * If we are not being call from ep_poll_callback(), epi is NULL and
+	 * we are at the first level of nesting, 0. Otherwise, we are being
+	 * called from ep_poll_callback() and if a previous wakeup source is
+	 * not an epoll file itself, we are at depth 1 since the wakeup source
+	 * is depth 0. If the wakeup source is a previous epoll file in the
+	 * wakeup chain then we use its nests value and record ours as
+	 * nests + 1. The previous epoll file nests value is stable since its
+	 * already holding its own poll_wait.lock.
+	 */
+	if (epi) {
+		if ((is_file_epoll(epi->ffd.file))) {
+			ep_src = epi->ffd.file->private_data;
+			nests = ep_src->nests;
+		} else {
+			nests = 1;
+		}
+	}
+	spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);
+	ep->nests = nests + 1;
+	wake_up_locked_poll(&ep->poll_wait, EPOLLIN | pollflags);
+	ep->nests = 0;
+	spin_unlock_irqrestore(&ep->poll_wait.lock, flags);
+}
+
+#else
+
+static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
+			     __poll_t pollflags)
+{
+	wake_up_poll(&ep->poll_wait, EPOLLIN | pollflags);
+}
+
+#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.
+ */
+static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
+{
+	struct eppoll_entry **p = &epi->pwqlist;
+	struct eppoll_entry *pwq;
+
+	while ((pwq = *p) != NULL) {
+		*p = pwq->next;
+		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->mutex needs to be held because we could be hit by
+ * eventpoll_release_file() and epoll_ctl().
+ */
+static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist)
+{
+	/*
+	 * 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.
+	 */
+	lockdep_assert_irqs_enabled();
+	write_lock_irq(&ep->lock);
+	list_splice_init(&ep->rdllist, txlist);
+	WRITE_ONCE(ep->ovflist, NULL);
+	write_unlock_irq(&ep->lock);
+}
+
+static void ep_done_scan(struct eventpoll *ep,
+			 struct list_head *txlist)
+{
+	struct epitem *epi, *nepi;
+
+	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)) {
+		if (waitqueue_active(&ep->wq))
+			wake_up(&ep->wq);
+	}
+
+	write_unlock_irq(&ep->lock);
+}
+
+static void epi_rcu_free(struct rcu_head *head)
+{
+	struct epitem *epi = container_of(head, struct epitem, rcu);
+	kmem_cache_free(epi_cache, epi);
+}
+
+static void ep_get(struct eventpoll *ep)
+{
+	refcount_inc(&ep->refcount);
+}
+
+/*
+ * Returns true if the event poll can be disposed
+ */
+static bool ep_refcount_dec_and_test(struct eventpoll *ep)
+{
+	if (!refcount_dec_and_test(&ep->refcount))
+		return false;
+
+	WARN_ON_ONCE(!RB_EMPTY_ROOT(&ep->rbr.rb_root));
+	return true;
+}
+
+static void ep_free(struct eventpoll *ep)
+{
+	mutex_destroy(&ep->mtx);
+	free_uid(ep->user);
+	wakeup_source_unregister(ep->ws);
+	kfree(ep);
+}
+
+/*
+ * Removes a "struct epitem" from the eventpoll RB tree and deallocates
+ * all the associated resources. Must be called with "mtx" held.
+ * If the dying flag is set, do the removal only if force is true.
+ * This prevents ep_clear_and_put() from dropping all the ep references
+ * while running concurrently with eventpoll_release_file().
+ * Returns true if the eventpoll can be disposed.
+ */
+static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
+{
+	struct file *file = epi->ffd.file;
+	struct epitems_head *to_free;
+	struct hlist_head *head;
+
+	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);
+	if (epi->dying && !force) {
+		spin_unlock(&file->f_lock);
+		return false;
+	}
+
+	to_free = NULL;
+	head = file->f_ep;
+	if (head->first == &epi->fllink && !epi->fllink.next) {
+		file->f_ep = NULL;
+		if (!is_file_epoll(file)) {
+			struct epitems_head *v;
+			v = container_of(head, struct epitems_head, epitems);
+			if (!smp_load_acquire(&v->next))
+				to_free = v;
+		}
+	}
+	hlist_del_rcu(&epi->fllink);
+	spin_unlock(&file->f_lock);
+	free_ephead(to_free);
+
+	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);
+
+	percpu_counter_dec(&ep->user->epoll_watches);
+	return ep_refcount_dec_and_test(ep);
+}
+
+/*
+ * ep_remove variant for callers owing an additional reference to the ep
+ */
+static void ep_remove_safe(struct eventpoll *ep, struct epitem *epi)
+{
+	WARN_ON_ONCE(__ep_remove(ep, epi, false));
+}
+
+static void ep_clear_and_put(struct eventpoll *ep)
+{
+	struct rb_node *rbp, *next;
+	struct epitem *epi;
+	bool dispose;
+
+	/* We need to release all tasks waiting for these file */
+	if (waitqueue_active(&ep->poll_wait))
+		ep_poll_safewake(ep, NULL, 0);
+
+	mutex_lock(&ep->mtx);
+
+	/*
+	 * 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 and try to free each "struct epitem".
+	 * Note that ep_remove_safe() will not remove the epitem in case of a
+	 * racing eventpoll_release_file(); the latter will do the removal.
+	 * At this point we are sure no poll callbacks will be lingering around.
+	 * Since we still own a reference to the eventpoll struct, the loop can't
+	 * dispose it.
+	 */
+	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = next) {
+		next = rb_next(rbp);
+		epi = rb_entry(rbp, struct epitem, rbn);
+		ep_remove_safe(ep, epi);
+		cond_resched();
+	}
+
+	dispose = ep_refcount_dec_and_test(ep);
+	mutex_unlock(&ep->mtx);
+
+	if (dispose)
+		ep_free(ep);
+}
+
+static int ep_eventpoll_release(struct inode *inode, struct file *file)
+{
+	struct eventpoll *ep = file->private_data;
+
+	if (ep)
+		ep_clear_and_put(ep);
+
+	return 0;
+}
+
+static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt, int depth);
+
+static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int depth)
+{
+	struct eventpoll *ep = file->private_data;
+	LIST_HEAD(txlist);
+	struct epitem *epi, *tmp;
+	poll_table pt;
+	__poll_t res = 0;
+
+	init_poll_funcptr(&pt, NULL);
+
+	/* 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.
+	 */
+	mutex_lock_nested(&ep->mtx, depth);
+	ep_start_scan(ep, &txlist);
+	list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
+		if (ep_item_poll(epi, &pt, depth + 1)) {
+			res = EPOLLIN | EPOLLRDNORM;
+			break;
+		} 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);
+		}
+	}
+	ep_done_scan(ep, &txlist);
+	mutex_unlock(&ep->mtx);
+	return res;
+}
+
+/*
+ * 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 file *file = epi->ffd.file;
+	__poll_t res;
+
+	pt->_key = epi->event.events;
+	if (!is_file_epoll(file))
+		res = vfs_poll(file, pt);
+	else
+		res = __ep_eventpoll_poll(file, pt, depth);
+	return res & epi->event.events;
+}
+
+static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
+{
+	return __ep_eventpoll_poll(file, wait, 0);
+}
+
+#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\n",
+			   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;
+	bool dispose;
+
+	/*
+	 * Use the 'dying' flag to prevent a concurrent ep_clear_and_put() from
+	 * touching the epitems list before eventpoll_release_file() can access
+	 * the ep->mtx.
+	 */
+again:
+	spin_lock(&file->f_lock);
+	if (file->f_ep && file->f_ep->first) {
+		epi = hlist_entry(file->f_ep->first, struct epitem, fllink);
+		epi->dying = true;
+		spin_unlock(&file->f_lock);
+
+		/*
+		 * ep access is safe as we still own a reference to the ep
+		 * struct
+		 */
+		ep = epi->ep;
+		mutex_lock(&ep->mtx);
+		dispose = __ep_remove(ep, epi, true);
+		mutex_unlock(&ep->mtx);
+
+		if (dispose)
+			ep_free(ep);
+		goto again;
+	}
+	spin_unlock(&file->f_lock);
+}
+
+static int ep_alloc(struct eventpoll **pep)
+{
+	struct eventpoll *ep;
+
+	ep = kzalloc(sizeof(*ep), GFP_KERNEL);
+	if (unlikely(!ep))
+		return -ENOMEM;
+
+	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 = get_current_user();
+	refcount_set(&ep->refcount, 1);
+
+	*pep = ep;
+
+	return 0;
+}
+
+/*
+ * 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_KCMP
+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_KCMP */
+
+/*
+ * 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 or to the head, otherwise
+ *        concurrent access will corrupt the list.
+ *
+ * Return: %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 (!try_cmpxchg(&new->next, &new, head))
+		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.
+ *
+ * Return: %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 contend with concurrent
+ * events from another file descriptor, 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, epi, pollflags & EPOLL_URING_WAKE);
+
+	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_clear_and_put() 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 ep_pqueue *epq = container_of(pt, struct ep_pqueue, pt);
+	struct epitem *epi = epq->epi;
+	struct eppoll_entry *pwq;
+
+	if (unlikely(!epi))	// an earlier allocation has failed
+		return;
+
+	pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL);
+	if (unlikely(!pwq)) {
+		epq->epi = NULL;
+		return;
+	}
+
+	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);
+	pwq->next = epi->pwqlist;
+	epi->pwqlist = pwq;
+}
+
+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 epnested_mutex.
+ */
+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(struct hlist_head *refs, int depth)
+{
+	int error = 0;
+	struct epitem *epi;
+
+	if (depth > EP_MAX_NESTS) /* too deep nesting */
+		return -1;
+
+	/* CTL_DEL can remove links here, but that can't increase our count */
+	hlist_for_each_entry_rcu(epi, refs, fllink) {
+		struct hlist_head *refs = &epi->ep->refs;
+		if (hlist_empty(refs))
+			error = path_count_inc(depth);
+		else
+			error = reverse_path_check_proc(refs, depth + 1);
+		if (error != 0)
+			break;
+	}
+	return error;
+}
+
+/**
+ * reverse_path_check - The tfile_check_list is list of epitem_head, 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.
+ *
+ * Return: %zero if the proposed links don't create too many paths,
+ *	    %-1 otherwise.
+ */
+static int reverse_path_check(void)
+{
+	struct epitems_head *p;
+
+	for (p = tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {
+		int error;
+		path_count_init();
+		rcu_read_lock();
+		error = reverse_path_check_proc(&p->epitems, 0);
+		rcu_read_unlock();
+		if (error)
+			return error;
+	}
+	return 0;
+}
+
+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);
+}
+
+static int attach_epitem(struct file *file, struct epitem *epi)
+{
+	struct epitems_head *to_free = NULL;
+	struct hlist_head *head = NULL;
+	struct eventpoll *ep = NULL;
+
+	if (is_file_epoll(file))
+		ep = file->private_data;
+
+	if (ep) {
+		head = &ep->refs;
+	} else if (!READ_ONCE(file->f_ep)) {
+allocate:
+		to_free = kmem_cache_zalloc(ephead_cache, GFP_KERNEL);
+		if (!to_free)
+			return -ENOMEM;
+		head = &to_free->epitems;
+	}
+	spin_lock(&file->f_lock);
+	if (!file->f_ep) {
+		if (unlikely(!head)) {
+			spin_unlock(&file->f_lock);
+			goto allocate;
+		}
+		file->f_ep = head;
+		to_free = NULL;
+	}
+	hlist_add_head_rcu(&epi->fllink, file->f_ep);
+	spin_unlock(&file->f_lock);
+	free_ephead(to_free);
+	return 0;
+}
+
+/*
+ * 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;
+	struct epitem *epi;
+	struct ep_pqueue epq;
+	struct eventpoll *tep = NULL;
+
+	if (is_file_epoll(tfile))
+		tep = tfile->private_data;
+
+	lockdep_assert_irqs_enabled();
+
+	if (unlikely(percpu_counter_compare(&ep->user->epoll_watches,
+					    max_user_watches) >= 0))
+		return -ENOSPC;
+	percpu_counter_inc(&ep->user->epoll_watches);
+
+	if (!(epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL))) {
+		percpu_counter_dec(&ep->user->epoll_watches);
+		return -ENOMEM;
+	}
+
+	/* Item initialization follow here ... */
+	INIT_LIST_HEAD(&epi->rdllink);
+	epi->ep = ep;
+	ep_set_ffd(&epi->ffd, tfile, fd);
+	epi->event = *event;
+	epi->next = EP_UNACTIVE_PTR;
+
+	if (tep)
+		mutex_lock_nested(&tep->mtx, 1);
+	/* Add the current item to the list of active epoll hook for this file */
+	if (unlikely(attach_epitem(tfile, epi) < 0)) {
+		if (tep)
+			mutex_unlock(&tep->mtx);
+		kmem_cache_free(epi_cache, epi);
+		percpu_counter_dec(&ep->user->epoll_watches);
+		return -ENOMEM;
+	}
+
+	if (full_check && !tep)
+		list_file(tfile);
+
+	/*
+	 * 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);
+	if (tep)
+		mutex_unlock(&tep->mtx);
+
+	/*
+	 * ep_remove_safe() calls in the later error paths can't lead to
+	 * ep_free() as the ep file itself still holds an ep reference.
+	 */
+	ep_get(ep);
+
+	/* now check if we've created too many backpaths */
+	if (unlikely(full_check && reverse_path_check())) {
+		ep_remove_safe(ep, epi);
+		return -EINVAL;
+	}
+
+	if (epi->event.events & EPOLLWAKEUP) {
+		error = ep_create_wakeup_source(epi);
+		if (error) {
+			ep_remove_safe(ep, epi);
+			return error;
+		}
+	}
+
+	/* 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.
+	 */
+	if (unlikely(!epq.epi)) {
+		ep_remove_safe(ep, epi);
+		return -ENOMEM;
+	}
+
+	/* 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);
+
+	/* We have to call this outside the lock */
+	if (pwake)
+		ep_poll_safewake(ep, NULL, 0);
+
+	return 0;
+}
+
+/*
+ * Modify the interest event mask by dropping an event if the new mask
+ * has a match in the current file status. Must be called with "mtx" held.
+ */
+static int ep_modify(struct eventpoll *ep, struct epitem *epi,
+		     const struct epoll_event *event)
+{
+	int pwake = 0;
+	poll_table pt;
+
+	lockdep_assert_irqs_enabled();
+
+	init_poll_funcptr(&pt, NULL);
+
+	/*
+	 * Set the new event interest mask before calling f_op->poll();
+	 * otherwise we might miss an event that happens between the
+	 * f_op->poll() call and the new event set registering.
+	 */
+	epi->event.events = event->events; /* need barrier below */
+	epi->event.data = event->data; /* protected by mtx */
+	if (epi->event.events & EPOLLWAKEUP) {
+		if (!ep_has_wakeup_source(epi))
+			ep_create_wakeup_source(epi);
+	} else if (ep_has_wakeup_source(epi)) {
+		ep_destroy_wakeup_source(epi);
+	}
+
+	/*
+	 * The following barrier has two effects:
+	 *
+	 * 1) Flush epi changes above to other CPUs.  This ensures
+	 *    we do not miss events from ep_poll_callback if an
+	 *    event occurs immediately after we call f_op->poll().
+	 *    We need this because we did not take ep->lock while
+	 *    changing epi above (but ep_poll_callback does take
+	 *    ep->lock).
+	 *
+	 * 2) We also need to ensure we do not miss _past_ events
+	 *    when calling f_op->poll().  This barrier also
+	 *    pairs with the barrier in wq_has_sleeper (see
+	 *    comments for wq_has_sleeper).
+	 *
+	 * This barrier will now guarantee ep_poll_callback or f_op->poll
+	 * (or both) will notice the readiness of an item.
+	 */
+	smp_mb();
+
+	/*
+	 * Get current event bits. We can safely use the file* here because
+	 * its usage count has been increased by the caller of this function.
+	 * If the item is "hot" and it is not registered inside the ready
+	 * list, push it inside.
+	 */
+	if (ep_item_poll(epi, &pt, 1)) {
+		write_lock_irq(&ep->lock);
+		if (!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);
+	}
+
+	/* We have to call this outside the lock */
+	if (pwake)
+		ep_poll_safewake(ep, NULL, 0);
+
+	return 0;
+}
+
+static int ep_send_events(struct eventpoll *ep,
+			  struct epoll_event __user *events, int maxevents)
+{
+	struct epitem *epi, *tmp;
+	LIST_HEAD(txlist);
+	poll_table pt;
+	int res = 0;
+
+	/*
+	 * Always short-circuit for fatal signals to allow threads to make a
+	 * timely exit without the chance of finding more events available and
+	 * fetching repeatedly.
+	 */
+	if (fatal_signal_pending(current))
+		return -EINTR;
+
+	init_poll_funcptr(&pt, NULL);
+
+	mutex_lock(&ep->mtx);
+	ep_start_scan(ep, &txlist);
+
+	/*
+	 * We can loop without lock because we are passed a task private list.
+	 * Items cannot vanish during the loop we are holding ep->mtx.
+	 */
+	list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
+		struct wakeup_source *ws;
+		__poll_t revents;
+
+		if (res >= maxevents)
+			break;
+
+		/*
+		 * Activate ep->ws before deactivating epi->ws to prevent
+		 * triggering auto-suspend here (in case we reactive epi->ws
+		 * below).
+		 *
+		 * This could be rearranged to delay the deactivation of epi->ws
+		 * instead, but then epi->ws would temporarily be out of sync
+		 * with ep_is_linked().
+		 */
+		ws = ep_wakeup_source(epi);
+		if (ws) {
+			if (ws->active)
+				__pm_stay_awake(ep->ws);
+			__pm_relax(ws);
+		}
+
+		list_del_init(&epi->rdllink);
+
+		/*
+		 * If the event mask intersect the caller-requested one,
+		 * deliver the event to userspace. Again, we are holding ep->mtx,
+		 * so no operations coming from userspace can change the item.
+		 */
+		revents = ep_item_poll(epi, &pt, 1);
+		if (!revents)
+			continue;
+
+		events = epoll_put_uevent(revents, epi->event.data, events);
+		if (!events) {
+			list_add(&epi->rdllink, &txlist);
+			ep_pm_stay_awake(epi);
+			if (!res)
+				res = -EFAULT;
+			break;
+		}
+		res++;
+		if (epi->event.events & EPOLLONESHOT)
+			epi->event.events &= EP_PRIVATE_BITS;
+		else if (!(epi->event.events & EPOLLET)) {
+			/*
+			 * If this file has been added with Level
+			 * Trigger mode, we need to insert back inside
+			 * the ready list, so that the next call to
+			 * epoll_wait() will check again the events
+			 * availability. At this point, no one can insert
+			 * into ep->rdllist besides us. The epoll_ctl()
+			 * callers are locked out by
+			 * ep_scan_ready_list() holding "mtx" and the
+			 * poll callback will queue them in ep->ovflist.
+			 */
+			list_add_tail(&epi->rdllink, &ep->rdllist);
+			ep_pm_stay_awake(epi);
+		}
+	}
+	ep_done_scan(ep, &txlist);
+	mutex_unlock(&ep->mtx);
+
+	return res;
+}
+
+static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms)
+{
+	struct timespec64 now;
+
+	if (ms < 0)
+		return NULL;
+
+	if (!ms) {
+		to->tv_sec = 0;
+		to->tv_nsec = 0;
+		return to;
+	}
+
+	to->tv_sec = ms / MSEC_PER_SEC;
+	to->tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC);
+
+	ktime_get_ts64(&now);
+	*to = timespec64_add_safe(now, *to);
+	return to;
+}
+
+/*
+ * autoremove_wake_function, but remove even on failure to wake up, because we
+ * know that default_wake_function/ttwu will only fail if the thread is already
+ * woken, and in that case the ep_poll loop will remove the entry anyways, not
+ * try to reuse it.
+ */
+static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
+				       unsigned int mode, int sync, void *key)
+{
+	int ret = default_wake_function(wq_entry, mode, sync, key);
+
+	/*
+	 * Pairs with list_empty_careful in ep_poll, and ensures future loop
+	 * iterations see the cause of this wakeup.
+	 */
+	list_del_init_careful(&wq_entry->entry);
+	return ret;
+}
+
+/**
+ * ep_poll - Retrieves ready events, and delivers them to the caller-supplied
+ *           event buffer.
+ *
+ * @ep: Pointer to the eventpoll context.
+ * @events: Pointer to the userspace buffer where the ready events should be
+ *          stored.
+ * @maxevents: Size (in terms of number of events) of the caller event buffer.
+ * @timeout: Maximum timeout for the ready events fetch operation, in
+ *           timespec. If the timeout is zero, the function will not block,
+ *           while if the @timeout ptr is NULL, the function will block
+ *           until at least one event has been retrieved (or an error
+ *           occurred).
+ *
+ * Return: the number of ready events which have been fetched, or an
+ *          error code, in case of error.
+ */
+static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
+		   int maxevents, struct timespec64 *timeout)
+{
+	int res, eavail, timed_out = 0;
+	u64 slack = 0;
+	wait_queue_entry_t wait;
+	ktime_t expires, *to = NULL;
+
+	lockdep_assert_irqs_enabled();
+
+	if (timeout && (timeout->tv_sec | timeout->tv_nsec)) {
+		slack = select_estimate_accuracy(timeout);
+		to = &expires;
+		*to = timespec64_to_ktime(*timeout);
+	} else if (timeout) {
+		/*
+		 * Avoid the unnecessary trip to the wait queue loop, if the
+		 * caller specified a non blocking operation.
+		 */
+		timed_out = 1;
+	}
+
+	/*
+	 * This call is racy: We may or may not see events that are being added
+	 * to the ready list under the lock (e.g., in IRQ callbacks). For cases
+	 * with a non-zero timeout, this thread will check the ready list under
+	 * lock and will add to the wait queue.  For cases with a zero
+	 * timeout, the user by definition should not care and will have to
+	 * recheck again.
+	 */
+	eavail = ep_events_available(ep);
+
+	while (1) {
+		if (eavail) {
+			/*
+			 * Try to transfer events to user space. In case we get
+			 * 0 events and there's still timeout left over, we go
+			 * trying again in search of more luck.
+			 */
+			res = ep_send_events(ep, events, maxevents);
+			if (res)
+				return res;
+		}
+
+		if (timed_out)
+			return 0;
+
+		eavail = ep_busy_loop(ep, timed_out);
+		if (eavail)
+			continue;
+
+		if (signal_pending(current))
+			return -EINTR;
+
+		/*
+		 * Internally init_wait() uses autoremove_wake_function(),
+		 * thus wait entry is removed from the wait queue on each
+		 * wakeup. Why it is important? In case of several waiters
+		 * each new wakeup will hit the next waiter, giving it the
+		 * chance to harvest new event. Otherwise wakeup can be
+		 * lost. This is also good performance-wise, because on
+		 * normal wakeup path no need to call __remove_wait_queue()
+		 * explicitly, thus ep->lock is not taken, which halts the
+		 * event delivery.
+		 *
+		 * In fact, we now use an even more aggressive function that
+		 * unconditionally removes, because we don't reuse the wait
+		 * entry between loop iterations. This lets us also avoid the
+		 * performance issue if a process is killed, causing all of its
+		 * threads to wake up without being removed normally.
+		 */
+		init_wait(&wait);
+		wait.func = ep_autoremove_wake_function;
+
+		write_lock_irq(&ep->lock);
+		/*
+		 * Barrierless variant, waitqueue_active() is called under
+		 * the same lock on wakeup ep_poll_callback() side, so it
+		 * is safe to avoid an explicit barrier.
+		 */
+		__set_current_state(TASK_INTERRUPTIBLE);
+
+		/*
+		 * Do the final check under the lock. ep_scan_ready_list()
+		 * plays with two lists (->rdllist and ->ovflist) and there
+		 * is always a race when both lists are empty for short
+		 * period of time although events are pending, so lock is
+		 * important.
+		 */
+		eavail = ep_events_available(ep);
+		if (!eavail)
+			__add_wait_queue_exclusive(&ep->wq, &wait);
+
+		write_unlock_irq(&ep->lock);
+
+		if (!eavail)
+			timed_out = !schedule_hrtimeout_range(to, slack,
+							      HRTIMER_MODE_ABS);
+		__set_current_state(TASK_RUNNING);
+
+		/*
+		 * We were woken up, thus go and try to harvest some events.
+		 * If timed out and still on the wait queue, recheck eavail
+		 * carefully under lock, below.
+		 */
+		eavail = 1;
+
+		if (!list_empty_careful(&wait.entry)) {
+			write_lock_irq(&ep->lock);
+			/*
+			 * If the thread timed out and is not on the wait queue,
+			 * it means that the thread was woken up after its
+			 * timeout expired before it could reacquire the lock.
+			 * Thus, when wait.entry is empty, it needs to harvest
+			 * events.
+			 */
+			if (timed_out)
+				eavail = list_empty(&wait.entry);
+			__remove_wait_queue(&ep->wq, &wait);
+			write_unlock_irq(&ep->lock);
+		}
+	}
+}
+
+/**
+ * ep_loop_check_proc - verify that adding an epoll file inside another
+ *                      epoll structure does not violate the constraints, in
+ *                      terms of closed loops, or too deep chains (which can
+ *                      result in excessive stack usage).
+ *
+ * @ep: the &struct eventpoll to be currently checked.
+ * @depth: Current depth of the path being checked.
+ *
+ * Return: %zero if adding the epoll @file inside current epoll
+ *          structure @ep does not violate the constraints, or %-1 otherwise.
+ */
+static int ep_loop_check_proc(struct eventpoll *ep, int depth)
+{
+	int error = 0;
+	struct rb_node *rbp;
+	struct epitem *epi;
+
+	mutex_lock_nested(&ep->mtx, depth + 1);
+	ep->gen = loop_check_gen;
+	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
+		epi = rb_entry(rbp, struct epitem, rbn);
+		if (unlikely(is_file_epoll(epi->ffd.file))) {
+			struct eventpoll *ep_tovisit;
+			ep_tovisit = epi->ffd.file->private_data;
+			if (ep_tovisit->gen == loop_check_gen)
+				continue;
+			if (ep_tovisit == inserting_into || depth > EP_MAX_NESTS)
+				error = -1;
+			else
+				error = ep_loop_check_proc(ep_tovisit, depth + 1);
+			if (error != 0)
+				break;
+		} else {
+			/*
+			 * If we've reached a file that is not associated with
+			 * an ep, then we need to check if the newly added
+			 * links are going to add too many wakeup paths. We do
+			 * this by adding it to the tfile_check_list, if it's
+			 * not already there, and calling reverse_path_check()
+			 * during ep_insert().
+			 */
+			list_file(epi->ffd.file);
+		}
+	}
+	mutex_unlock(&ep->mtx);
+
+	return error;
+}
+
+/**
+ * ep_loop_check - Performs a check to verify that adding an epoll file (@to)
+ *                 into another epoll file (represented by @ep) does not create
+ *                 closed loops or too deep chains.
+ *
+ * @ep: Pointer to the epoll we are inserting into.
+ * @to: Pointer to the epoll to be inserted.
+ *
+ * Return: %zero if adding the epoll @to inside the epoll @from
+ * does not violate the constraints, or %-1 otherwise.
+ */
+static int ep_loop_check(struct eventpoll *ep, struct eventpoll *to)
+{
+	inserting_into = ep;
+	return ep_loop_check_proc(to, 0);
+}
+
+static void clear_tfile_check_list(void)
+{
+	rcu_read_lock();
+	while (tfile_check_list != EP_UNACTIVE_PTR) {
+		struct epitems_head *head = tfile_check_list;
+		tfile_check_list = head->next;
+		unlist_file(head);
+	}
+	rcu_read_unlock();
+}
+
+/*
+ * Open an eventpoll file descriptor.
+ */
+static int do_epoll_create(int flags)
+{
+	int error, fd;
+	struct eventpoll *ep = NULL;
+	struct file *file;
+
+	/* Check the EPOLL_* constant for consistency.  */
+	BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
+
+	if (flags & ~EPOLL_CLOEXEC)
+		return -EINVAL;
+	/*
+	 * Create the internal data structure ("struct eventpoll").
+	 */
+	error = ep_alloc(&ep);
+	if (error < 0)
+		return error;
+	/*
+	 * Creates all the items needed to setup an eventpoll file. That is,
+	 * a file structure and a free file descriptor.
+	 */
+	fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
+	if (fd < 0) {
+		error = fd;
+		goto out_free_ep;
+	}
+	file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
+				 O_RDWR | (flags & O_CLOEXEC));
+	if (IS_ERR(file)) {
+		error = PTR_ERR(file);
+		goto out_free_fd;
+	}
+	ep->file = file;
+	fd_install(fd, file);
+	return fd;
+
+out_free_fd:
+	put_unused_fd(fd);
+out_free_ep:
+	ep_clear_and_put(ep);
+	return error;
+}
+
+SYSCALL_DEFINE1(epoll_create1, int, flags)
+{
+	return do_epoll_create(flags);
+}
+
+SYSCALL_DEFINE1(epoll_create, int, size)
+{
+	if (size <= 0)
+		return -EINVAL;
+
+	return do_epoll_create(0);
+}
+
+#ifdef CONFIG_PM_SLEEP
+static inline void ep_take_care_of_epollwakeup(struct epoll_event *epev)
+{
+	if ((epev->events & EPOLLWAKEUP) && !capable(CAP_BLOCK_SUSPEND))
+		epev->events &= ~EPOLLWAKEUP;
+}
+#else
+static inline void ep_take_care_of_epollwakeup(struct epoll_event *epev)
+{
+	epev->events &= ~EPOLLWAKEUP;
+}
+#endif
+
+static inline int epoll_mutex_lock(struct mutex *mutex, int depth,
+				   bool nonblock)
+{
+	if (!nonblock) {
+		mutex_lock_nested(mutex, depth);
+		return 0;
+	}
+	if (mutex_trylock(mutex))
+		return 0;
+	return -EAGAIN;
+}
+
+int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
+		 bool nonblock)
+{
+	int error;
+	int full_check = 0;
+	struct fd f, tf;
+	struct eventpoll *ep;
+	struct epitem *epi;
+	struct eventpoll *tep = NULL;
+
+	error = -EBADF;
+	f = fdget(epfd);
+	if (!f.file)
+		goto error_return;
+
+	/* Get the "struct file *" for the target file */
+	tf = fdget(fd);
+	if (!tf.file)
+		goto error_fput;
+
+	/* The target file descriptor must support poll */
+	error = -EPERM;
+	if (!file_can_poll(tf.file))
+		goto error_tgt_fput;
+
+	/* Check if EPOLLWAKEUP is allowed */
+	if (ep_op_has_event(op))
+		ep_take_care_of_epollwakeup(epds);
+
+	/*
+	 * We have to check that the file structure underneath the file descriptor
+	 * the user passed to us _is_ an eventpoll file. And also we do not permit
+	 * adding an epoll file descriptor inside itself.
+	 */
+	error = -EINVAL;
+	if (f.file == tf.file || !is_file_epoll(f.file))
+		goto error_tgt_fput;
+
+	/*
+	 * epoll adds to the wakeup queue at EPOLL_CTL_ADD time only,
+	 * so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.
+	 * Also, we do not currently supported nested exclusive wakeups.
+	 */
+	if (ep_op_has_event(op) && (epds->events & EPOLLEXCLUSIVE)) {
+		if (op == EPOLL_CTL_MOD)
+			goto error_tgt_fput;
+		if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||
+				(epds->events & ~EPOLLEXCLUSIVE_OK_BITS)))
+			goto error_tgt_fput;
+	}
+
+	/*
+	 * At this point it is safe to assume that the "private_data" contains
+	 * our own data structure.
+	 */
+	ep = f.file->private_data;
+
+	/*
+	 * When we insert an epoll file descriptor inside another epoll file
+	 * descriptor, there is the chance of creating closed loops, which are
+	 * better be handled here, than in more critical paths. While we are
+	 * checking for loops we also determine the list of files reachable
+	 * and hang them on the tfile_check_list, so we can check that we
+	 * haven't created too many possible wakeup paths.
+	 *
+	 * We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
+	 * the epoll file descriptor is attaching directly to a wakeup source,
+	 * unless the epoll file descriptor is nested. The purpose of taking the
+	 * 'epnested_mutex' on add is to prevent complex toplogies such as loops and
+	 * deep wakeup paths from forming in parallel through multiple
+	 * EPOLL_CTL_ADD operations.
+	 */
+	error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
+	if (error)
+		goto error_tgt_fput;
+	if (op == EPOLL_CTL_ADD) {
+		if (READ_ONCE(f.file->f_ep) || ep->gen == loop_check_gen ||
+		    is_file_epoll(tf.file)) {
+			mutex_unlock(&ep->mtx);
+			error = epoll_mutex_lock(&epnested_mutex, 0, nonblock);
+			if (error)
+				goto error_tgt_fput;
+			loop_check_gen++;
+			full_check = 1;
+			if (is_file_epoll(tf.file)) {
+				tep = tf.file->private_data;
+				error = -ELOOP;
+				if (ep_loop_check(ep, tep) != 0)
+					goto error_tgt_fput;
+			}
+			error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
+			if (error)
+				goto error_tgt_fput;
+		}
+	}
+
+	/*
+	 * Try to lookup the file inside our RB tree. Since we grabbed "mtx"
+	 * above, we can be sure to be able to use the item looked up by
+	 * ep_find() till we release the mutex.
+	 */
+	epi = ep_find(ep, tf.file, fd);
+
+	error = -EINVAL;
+	switch (op) {
+	case EPOLL_CTL_ADD:
+		if (!epi) {
+			epds->events |= EPOLLERR | EPOLLHUP;
+			error = ep_insert(ep, epds, tf.file, fd, full_check);
+		} else
+			error = -EEXIST;
+		break;
+	case EPOLL_CTL_DEL:
+		if (epi) {
+			/*
+			 * The eventpoll itself is still alive: the refcount
+			 * can't go to zero here.
+			 */
+			ep_remove_safe(ep, epi);
+			error = 0;
+		} else {
+			error = -ENOENT;
+		}
+		break;
+	case EPOLL_CTL_MOD:
+		if (epi) {
+			if (!(epi->event.events & EPOLLEXCLUSIVE)) {
+				epds->events |= EPOLLERR | EPOLLHUP;
+				error = ep_modify(ep, epi, epds);
+			}
+		} else
+			error = -ENOENT;
+		break;
+	}
+	mutex_unlock(&ep->mtx);
+
+error_tgt_fput:
+	if (full_check) {
+		clear_tfile_check_list();
+		loop_check_gen++;
+		mutex_unlock(&epnested_mutex);
+	}
+
+	fdput(tf);
+error_fput:
+	fdput(f);
+error_return:
+
+	return error;
+}
+
+/*
+ * The following function implements the controller interface for
+ * the eventpoll file that enables the insertion/removal/change of
+ * file descriptors inside the interest set.
+ */
+SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
+		struct epoll_event __user *, event)
+{
+	struct epoll_event epds;
+
+	if (ep_op_has_event(op) &&
+	    copy_from_user(&epds, event, sizeof(struct epoll_event)))
+		return -EFAULT;
+
+	return do_epoll_ctl(epfd, op, fd, &epds, false);
+}
+
+/*
+ * Implement the event wait interface for the eventpoll file. It is the kernel
+ * part of the user space epoll_wait(2).
+ */
+static int do_epoll_wait(int epfd, struct epoll_event __user *events,
+			 int maxevents, struct timespec64 *to)
+{
+	int error;
+	struct fd f;
+	struct eventpoll *ep;
+
+	/* The maximum number of event must be greater than zero */
+	if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
+		return -EINVAL;
+
+	/* Verify that the area passed by the user is writeable */
+	if (!access_ok(events, maxevents * sizeof(struct epoll_event)))
+		return -EFAULT;
+
+	/* Get the "struct file *" for the eventpoll file */
+	f = fdget(epfd);
+	if (!f.file)
+		return -EBADF;
+
+	/*
+	 * We have to check that the file structure underneath the fd
+	 * the user passed to us _is_ an eventpoll file.
+	 */
+	error = -EINVAL;
+	if (!is_file_epoll(f.file))
+		goto error_fput;
+
+	/*
+	 * At this point it is safe to assume that the "private_data" contains
+	 * our own data structure.
+	 */
+	ep = f.file->private_data;
+
+	/* Time to fish for events ... */
+	error = ep_poll(ep, events, maxevents, to);
+
+error_fput:
+	fdput(f);
+	return error;
+}
+
+SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
+		int, maxevents, int, timeout)
+{
+	struct timespec64 to;
+
+	return do_epoll_wait(epfd, events, maxevents,
+			     ep_timeout_to_timespec(&to, timeout));
+}
+
+/*
+ * Implement the event wait interface for the eventpoll file. It is the kernel
+ * part of the user space epoll_pwait(2).
+ */
+static int do_epoll_pwait(int epfd, struct epoll_event __user *events,
+			  int maxevents, struct timespec64 *to,
+			  const sigset_t __user *sigmask, size_t sigsetsize)
+{
+	int error;
+
+	/*
+	 * If the caller wants a certain signal mask to be set during the wait,
+	 * we apply it here.
+	 */
+	error = set_user_sigmask(sigmask, sigsetsize);
+	if (error)
+		return error;
+
+	error = do_epoll_wait(epfd, events, maxevents, to);
+
+	restore_saved_sigmask_unless(error == -EINTR);
+
+	return error;
+}
+
+SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
+		int, maxevents, int, timeout, const sigset_t __user *, sigmask,
+		size_t, sigsetsize)
+{
+	struct timespec64 to;
+
+	return do_epoll_pwait(epfd, events, maxevents,
+			      ep_timeout_to_timespec(&to, timeout),
+			      sigmask, sigsetsize);
+}
+
+SYSCALL_DEFINE6(epoll_pwait2, int, epfd, struct epoll_event __user *, events,
+		int, maxevents, const struct __kernel_timespec __user *, timeout,
+		const sigset_t __user *, sigmask, size_t, sigsetsize)
+{
+	struct timespec64 ts, *to = NULL;
+
+	if (timeout) {
+		if (get_timespec64(&ts, timeout))
+			return -EFAULT;
+		to = &ts;
+		if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
+			return -EINVAL;
+	}
+
+	return do_epoll_pwait(epfd, events, maxevents, to,
+			      sigmask, sigsetsize);
+}
+
+#ifdef CONFIG_COMPAT
+static int do_compat_epoll_pwait(int epfd, struct epoll_event __user *events,
+				 int maxevents, struct timespec64 *timeout,
+				 const compat_sigset_t __user *sigmask,
+				 compat_size_t sigsetsize)
+{
+	long err;
+
+	/*
+	 * If the caller wants a certain signal mask to be set during the wait,
+	 * we apply it here.
+	 */
+	err = set_compat_user_sigmask(sigmask, sigsetsize);
+	if (err)
+		return err;
+
+	err = do_epoll_wait(epfd, events, maxevents, timeout);
+
+	restore_saved_sigmask_unless(err == -EINTR);
+
+	return err;
+}
+
+COMPAT_SYSCALL_DEFINE6(epoll_pwait, int, epfd,
+		       struct epoll_event __user *, events,
+		       int, maxevents, int, timeout,
+		       const compat_sigset_t __user *, sigmask,
+		       compat_size_t, sigsetsize)
+{
+	struct timespec64 to;
+
+	return do_compat_epoll_pwait(epfd, events, maxevents,
+				     ep_timeout_to_timespec(&to, timeout),
+				     sigmask, sigsetsize);
+}
+
+COMPAT_SYSCALL_DEFINE6(epoll_pwait2, int, epfd,
+		       struct epoll_event __user *, events,
+		       int, maxevents,
+		       const struct __kernel_timespec __user *, timeout,
+		       const compat_sigset_t __user *, sigmask,
+		       compat_size_t, sigsetsize)
+{
+	struct timespec64 ts, *to = NULL;
+
+	if (timeout) {
+		if (get_timespec64(&ts, timeout))
+			return -EFAULT;
+		to = &ts;
+		if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
+			return -EINVAL;
+	}
+
+	return do_compat_epoll_pwait(epfd, events, maxevents, to,
+				     sigmask, sigsetsize);
+}
+
+#endif
+
+static int __init eventpoll_init(void)
+{
+	struct sysinfo si;
+
+	si_meminfo(&si);
+	/*
+	 * Allows top 4% of lomem to be allocated for epoll watches (per user).
+	 */
+	max_user_watches = (((si.totalram - si.totalhigh) / 25) << PAGE_SHIFT) /
+		EP_ITEM_COST;
+	BUG_ON(max_user_watches < 0);
+
+	/*
+	 * We can have many thousands of epitems, so prevent this from
+	 * using an extra cache line on 64-bit (and smaller) CPUs
+	 */
+	BUILD_BUG_ON(sizeof(void *) <= 8 && sizeof(struct epitem) > 128);
+
+	/* Allocates slab cache used to allocate "struct epitem" items */
+	epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
+			0, SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL);
+
+	/* Allocates slab cache used to allocate "struct eppoll_entry" */
+	pwq_cache = kmem_cache_create("eventpoll_pwq",
+		sizeof(struct eppoll_entry), 0, SLAB_PANIC|SLAB_ACCOUNT, NULL);
+	epoll_sysctls_init();
+
+	ephead_cache = kmem_cache_create("ep_head",
+		sizeof(struct epitems_head), 0, SLAB_PANIC|SLAB_ACCOUNT, NULL);
+
+	return 0;
+}
+fs_initcall(eventpoll_init);