eventpoll.c 59 KB
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/*
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 *  fs/eventpoll.c (Efficient event retrieval implementation)
 *  Copyright (C) 2001,...,2009	 Davide Libenzi
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 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  Davide Libenzi <davidel@xmailserver.org>
 *
 */

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/sched.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>
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#include <linux/mutex.h>
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#include <linux/anon_inodes.h>
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#include <linux/device.h>
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#include <asm/uaccess.h>
#include <asm/io.h>
#include <asm/mman.h>
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#include <linux/atomic.h>
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#include <linux/proc_fs.h>
#include <linux/seq_file.h>
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#include <linux/compat.h>
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#include <linux/rculist.h>
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/*
 * LOCKING:
 * There are three level of locking required by epoll :
 *
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 * 1) epmutex (mutex)
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 * 2) ep->mtx (mutex)
 * 3) ep->lock (spinlock)
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 *
 * The acquire order is the one listed above, from 1 to 3.
 * We need a spinlock (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
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 * 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
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 * 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
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 * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).
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 * 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.
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 * 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.
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 * 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.
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 * Events that require holding "epmutex" are very rare, while for
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 * normal operations the epoll private "ep->mtx" will guarantee
 * a better scalability.
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 */

/* Epoll private bits inside the event mask */
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#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
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/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4
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#define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))

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

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

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struct epoll_filefd {
	struct file *file;
	int fd;
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} __packed;
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/*
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 * Structure used to track possible nested calls, for too deep recursions
 * and loop cycles.
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 */
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struct nested_call_node {
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	struct list_head llink;
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	void *cookie;
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	void *ctx;
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};

/*
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 * This structure is used as collector for nested calls, to check for
 * maximum recursion dept and loop cycles.
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 */
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struct nested_calls {
	struct list_head tasks_call_list;
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	spinlock_t lock;
};

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/*
 * Each file descriptor added to the eventpoll interface will
 * have an entry of this type linked to the "rbr" RB tree.
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 * 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.
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 */
struct epitem {
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	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;
	};
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	/* List header used to link this structure to the eventpoll ready list */
	struct list_head rdllink;

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

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	/* 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;

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	/* wakeup_source used when EPOLLWAKEUP is set */
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	struct wakeup_source __rcu *ws;
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	/* The structure that describe the interested events and the source fd */
	struct epoll_event event;
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};

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/*
 * This structure is stored inside the "private_data" member of the file
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 * structure and represents the main data structure for the eventpoll
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 * interface.
 */
struct eventpoll {
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	/* Protect the access to this structure */
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	spinlock_t lock;
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	/*
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	 * 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.
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	 */
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	struct mutex mtx;
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	/* 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;

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	/* RB tree root used to store monitored fd structs */
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	struct rb_root rbr;
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	/*
	 * This is a single linked list that chains all the "struct epitem" that
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	 * happened while transferring ready events to userspace w/out
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	 * holding ->lock.
	 */
	struct epitem *ovflist;
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	/* wakeup_source used when ep_scan_ready_list is running */
	struct wakeup_source *ws;

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

	/* used to optimize loop detection check */
	int visited;
	struct list_head visited_list_link;
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};

/* 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" */
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	struct epitem *base;
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	/*
	 * Wait queue item that will be linked to the target file wait
	 * queue head.
	 */
	wait_queue_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;
};

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

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/*
 * Configuration options available inside /proc/sys/fs/epoll/
 */
/* Maximum number of epoll watched descriptors, per user */
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static long max_user_watches __read_mostly;
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/*
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 * This mutex is used to serialize ep_free() and eventpoll_release_file().
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 */
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static DEFINE_MUTEX(epmutex);
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/* Used to check for epoll file descriptor inclusion loops */
static struct nested_calls poll_loop_ncalls;

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/* Used for safe wake up implementation */
static struct nested_calls poll_safewake_ncalls;

/* Used to call file's f_op->poll() under the nested calls boundaries */
static struct nested_calls poll_readywalk_ncalls;
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/* Slab cache used to allocate "struct epitem" */
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static struct kmem_cache *epi_cache __read_mostly;
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/* Slab cache used to allocate "struct eppoll_entry" */
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static struct kmem_cache *pwq_cache __read_mostly;
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/* Visited nodes during ep_loop_check(), so we can unset them when we finish */
static LIST_HEAD(visited_list);

/*
 * 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);

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#ifdef CONFIG_SYSCTL

#include <linux/sysctl.h>

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static long zero;
static long long_max = LONG_MAX;
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struct ctl_table epoll_table[] = {
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	{
		.procname	= "max_user_watches",
		.data		= &max_user_watches,
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		.maxlen		= sizeof(max_user_watches),
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		.mode		= 0644,
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		.proc_handler	= proc_doulongvec_minmax,
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		.extra1		= &zero,
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		.extra2		= &long_max,
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	},
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	{ }
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};
#endif /* CONFIG_SYSCTL */

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static const struct file_operations eventpoll_fops;

static inline int is_file_epoll(struct file *f)
{
	return f->f_op == &eventpoll_fops;
}
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/* Setup the structure that is used as key for the RB tree */
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static inline void ep_set_ffd(struct epoll_filefd *ffd,
			      struct file *file, int fd)
{
	ffd->file = file;
	ffd->fd = fd;
}

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/* Compare RB tree keys */
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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 list_head *p)
{
	return !list_empty(p);
}

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static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p)
{
	return container_of(p, struct eppoll_entry, wait);
}

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/* Get the "struct epitem" from a wait queue pointer */
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static inline struct epitem *ep_item_from_wait(wait_queue_t *p)
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{
	return container_of(p, struct eppoll_entry, wait)->base;
}

/* Get the "struct epitem" from an epoll queue wrapper */
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static inline struct epitem *ep_item_from_epqueue(poll_table *p)
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{
	return container_of(p, struct ep_pqueue, pt)->epi;
}

/* Tells if the epoll_ctl(2) operation needs an event copy from userspace */
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static inline int ep_op_has_event(int op)
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{
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	return op != EPOLL_CTL_DEL;
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}

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/* Initialize the poll safe wake up structure */
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static void ep_nested_calls_init(struct nested_calls *ncalls)
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{
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	INIT_LIST_HEAD(&ncalls->tasks_call_list);
	spin_lock_init(&ncalls->lock);
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}

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/**
 * 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(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
}

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/**
 * 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.
 * @max_nests: Maximum number of allowed nesting calls.
 * @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.
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 * @ctx: This instance context.
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 *
 * Returns: Returns the code returned by the @nproc callback, or -1 if
 *          the maximum recursion limit has been exceeded.
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 */
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static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
			  int (*nproc)(void *, void *, int), void *priv,
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			  void *cookie, void *ctx)
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{
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	int error, call_nests = 0;
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	unsigned long flags;
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	struct list_head *lsthead = &ncalls->tasks_call_list;
	struct nested_call_node *tncur;
	struct nested_call_node tnode;
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	spin_lock_irqsave(&ncalls->lock, flags);
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	/*
	 * 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.
	 */
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	list_for_each_entry(tncur, lsthead, llink) {
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		if (tncur->ctx == ctx &&
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		    (tncur->cookie == cookie || ++call_nests > max_nests)) {
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			/*
			 * Ops ... loop detected or maximum nest level reached.
			 * We abort this wake by breaking the cycle itself.
			 */
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			error = -1;
			goto out_unlock;
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		}
	}

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	/* Add the current task and cookie to the list */
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	tnode.ctx = ctx;
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	tnode.cookie = cookie;
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	list_add(&tnode.llink, lsthead);

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	spin_unlock_irqrestore(&ncalls->lock, flags);
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	/* Call the nested function */
	error = (*nproc)(priv, cookie, call_nests);
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	/* Remove the current task from the list */
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	spin_lock_irqsave(&ncalls->lock, flags);
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	list_del(&tnode.llink);
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out_unlock:
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	spin_unlock_irqrestore(&ncalls->lock, flags);

	return error;
}

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/*
 * 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.
 */
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
				     unsigned long events, int subclass)
{
	unsigned long flags;

	spin_lock_irqsave_nested(&wqueue->lock, flags, subclass);
	wake_up_locked_poll(wqueue, events);
	spin_unlock_irqrestore(&wqueue->lock, flags);
}
#else
static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
				     unsigned long events, int subclass)
{
	wake_up_poll(wqueue, events);
}
#endif

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static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)
{
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	ep_wake_up_nested((wait_queue_head_t *) cookie, POLLIN,
			  1 + call_nests);
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	return 0;
}

/*
 * Perform a safe wake up of the poll wait list. The problem is that
 * with the new callback'd wake up system, it is possible that the
 * poll callback is reentered from inside the call to wake_up() done
 * on the poll wait queue head. The rule is that we cannot reenter the
 * wake up code from the same task more than EP_MAX_NESTS times,
 * and we cannot reenter the same wait queue head at all. This will
 * enable to have a hierarchy of epoll file descriptor of no more than
 * EP_MAX_NESTS deep.
 */
static void ep_poll_safewake(wait_queue_head_t *wq)
{
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	int this_cpu = get_cpu();

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	ep_call_nested(&poll_safewake_ncalls, EP_MAX_NESTS,
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		       ep_poll_wakeup_proc, NULL, wq, (void *) (long) this_cpu);

	put_cpu();
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}

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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 */
	whead = rcu_dereference(pwq->whead);
	if (whead)
		remove_wait_queue(whead, &pwq->wait);
	rcu_read_unlock();
}

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/*
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 * This function unregisters poll callbacks from the associated file
 * descriptor.  Must be called with "mtx" held (or "epmutex" if called from
 * ep_free).
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 */
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static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
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{
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	struct list_head *lsthead = &epi->pwqlist;
	struct eppoll_entry *pwq;
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	while (!list_empty(lsthead)) {
		pwq = list_first_entry(lsthead, struct eppoll_entry, llink);
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		list_del(&pwq->llink);
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		ep_remove_wait_queue(pwq);
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		kmem_cache_free(pwq_cache, pwq);
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	}
}

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/* 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();
}

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/**
 * 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.
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 * @depth: The current depth of recursive f_op->poll calls.
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 * @ep_locked: caller already holds ep->mtx
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 *
 * Returns: The same integer error code returned by the @sproc callback.
 */
static int ep_scan_ready_list(struct eventpoll *ep,
			      int (*sproc)(struct eventpoll *,
					   struct list_head *, void *),
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			      void *priv, int depth, bool ep_locked)
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{
	int error, pwake = 0;
	unsigned long flags;
	struct epitem *epi, *nepi;
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	LIST_HEAD(txlist);
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	/*
	 * We need to lock this because we could be hit by
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	 * eventpoll_release_file() and epoll_ctl().
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	 */
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	if (!ep_locked)
		mutex_lock_nested(&ep->mtx, depth);
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	/*
	 * 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.
	 */
	spin_lock_irqsave(&ep->lock, flags);
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	list_splice_init(&ep->rdllist, &txlist);
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	ep->ovflist = NULL;
	spin_unlock_irqrestore(&ep->lock, flags);

	/*
	 * Now call the callback function.
	 */
	error = (*sproc)(ep, &txlist, priv);

	spin_lock_irqsave(&ep->lock, flags);
	/*
	 * 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 = 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.
		 */
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		if (!ep_is_linked(&epi->rdllink)) {
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			list_add_tail(&epi->rdllink, &ep->rdllist);
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			ep_pm_stay_awake(epi);
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		}
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	}
	/*
	 * 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.
	 */
	ep->ovflist = EP_UNACTIVE_PTR;

	/*
	 * Quickly re-inject items left on "txlist".
	 */
	list_splice(&txlist, &ep->rdllist);
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	__pm_relax(ep->ws);
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	if (!list_empty(&ep->rdllist)) {
		/*
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		 * Wake up (if active) both the eventpoll wait list and
		 * the ->poll() wait list (delayed after we release the lock).
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		 */
		if (waitqueue_active(&ep->wq))
			wake_up_locked(&ep->wq);
		if (waitqueue_active(&ep->poll_wait))
			pwake++;
	}
	spin_unlock_irqrestore(&ep->lock, flags);

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

	return error;
}

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

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/*
 * Removes a "struct epitem" from the eventpoll RB tree and deallocates
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 * all the associated resources. Must be called with "mtx" held.
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 */
static int ep_remove(struct eventpoll *ep, struct epitem *epi)
{
	unsigned long flags;
	struct file *file = epi->ffd.file;
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	/*
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	 * Removes poll wait queue hooks. We _have_ to do this without holding
	 * the "ep->lock" otherwise a deadlock might occur. This because of the
	 * sequence of the lock acquisition. Here we do "ep->lock" then the wait
	 * queue head lock when unregistering the wait queue. The wakeup callback
	 * will run by holding the wait queue head lock and will call our callback
	 * that will try to get "ep->lock".
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	 */
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	ep_unregister_pollwait(ep, epi);
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	/* Remove the current item from the list of epoll hooks */
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	spin_lock(&file->f_lock);
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	list_del_rcu(&epi->fllink);
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	spin_unlock(&file->f_lock);
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	rb_erase(&epi->rbn, &ep->rbr);
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	spin_lock_irqsave(&ep->lock, flags);
	if (ep_is_linked(&epi->rdllink))
		list_del_init(&epi->rdllink);
	spin_unlock_irqrestore(&ep->lock, flags);
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	wakeup_source_unregister(ep_wakeup_source(epi));
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	/*
	 * 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);
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	atomic_long_dec(&ep->user->epoll_watches);
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	return 0;
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}

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static void ep_free(struct eventpoll *ep)
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{
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	struct rb_node *rbp;
	struct epitem *epi;
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	/* We need to release all tasks waiting for these file */
	if (waitqueue_active(&ep->poll_wait))
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		ep_poll_safewake(&ep->poll_wait);
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	/*
	 * We need to lock this because we could be hit by
	 * eventpoll_release_file() while we're freeing the "struct eventpoll".
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	 * We do not need to hold "ep->mtx" here because the epoll file
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	 * 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
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	 * holding "epmutex" is sufficient here.
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	 */
	mutex_lock(&epmutex);
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	/*
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	 * Walks through the whole tree by unregistering poll callbacks.
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	 */
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	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
		epi = rb_entry(rbp, struct epitem, rbn);

		ep_unregister_pollwait(ep, epi);
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		cond_resched();
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	}
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	/*
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	 * 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
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	 * holding "epmutex" we can be sure that no file cleanup code will hit
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	 * us during this operation. So we can avoid the lock on "ep->lock".
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	 * We do not need to lock ep->mtx, either, we only do it to prevent
	 * a lockdep warning.
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	 */
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	mutex_lock(&ep->mtx);
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	while ((rbp = rb_first(&ep->rbr)) != NULL) {
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		epi = rb_entry(rbp, struct epitem, rbn);
		ep_remove(ep, epi);
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		cond_resched();
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	}
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	mutex_unlock(&ep->mtx);
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	mutex_unlock(&epmutex);
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	mutex_destroy(&ep->mtx);
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	free_uid(ep->user);
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	wakeup_source_unregister(ep->ws);
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	kfree(ep);
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}
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static int ep_eventpoll_release(struct inode *inode, struct file *file)
{
	struct eventpoll *ep = file->private_data;
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	if (ep)
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		ep_free(ep);

	return 0;
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}

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static inline unsigned int ep_item_poll(struct epitem *epi, poll_table *pt)
{
	pt->_key = epi->event.events;

	return epi->ffd.file->f_op->poll(epi->ffd.file, pt) & epi->event.events;
}

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static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
			       void *priv)
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{
	struct epitem *epi, *tmp;
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	poll_table pt;
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	init_poll_funcptr(&pt, NULL);
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	list_for_each_entry_safe(epi, tmp, head, rdllink) {
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		if (ep_item_poll(epi, &pt))
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			return POLLIN | POLLRDNORM;
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		else {
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			/*
			 * 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.
			 */
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			__pm_relax(ep_wakeup_source(epi));
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			list_del_init(&epi->rdllink);
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		}
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	}

	return 0;
}

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static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
				 poll_table *pt);

struct readyevents_arg {
	struct eventpoll *ep;
	bool locked;
};

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static int ep_poll_readyevents_proc(void *priv, void *cookie, int call_nests)
{
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	struct readyevents_arg *arg = priv;

	return ep_scan_ready_list(arg->ep, ep_read_events_proc, NULL,
				  call_nests + 1, arg->locked);
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}

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static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
{
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	int pollflags;
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	struct eventpoll *ep = file->private_data;
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	struct readyevents_arg arg;

	/*
	 * During ep_insert() we already hold the ep->mtx for the tfile.
	 * Prevent re-aquisition.
	 */
	arg.locked = wait && (wait->_qproc == ep_ptable_queue_proc);
	arg.ep = ep;
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	/* Insert inside our poll wait queue */
	poll_wait(file, &ep->poll_wait, wait);

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	/*
	 * Proceed to find out if wanted events are really available inside
	 * the ready list. This need to be done under ep_call_nested()
	 * supervision, since the call to f_op->poll() done on listed files
	 * could re-enter here.
	 */
	pollflags = ep_call_nested(&poll_readywalk_ncalls, EP_MAX_NESTS,
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				   ep_poll_readyevents_proc, &arg, ep, current);
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	return pollflags != -1 ? pollflags : 0;
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}

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#ifdef CONFIG_PROC_FS
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static void ep_show_fdinfo(struct seq_file *m, struct file *f)
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{
	struct eventpoll *ep = f->private_data;
	struct rb_node *rbp;

	mutex_lock(&ep->mtx);
	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
		struct epitem *epi = rb_entry(rbp, struct epitem, rbn);

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		seq_printf(m, "tfd: %8d events: %8x data: %16llx\n",
			   epi->ffd.fd, epi->event.events,
			   (long long)epi->event.data);
		if (seq_has_overflowed(m))
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			break;
	}
	mutex_unlock(&ep->mtx);
}
#endif

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/* File callbacks that implement the eventpoll file behaviour */
static const struct file_operations eventpoll_fops = {
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#ifdef CONFIG_PROC_FS
	.show_fdinfo	= ep_show_fdinfo,
#endif
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	.release	= ep_eventpoll_release,
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	.poll		= ep_eventpoll_poll,
	.llseek		= noop_llseek,
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};

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/*
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 * 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.
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 */
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void eventpoll_release_file(struct file *file)
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{
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	struct eventpoll *ep;
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	struct epitem *epi, *next;
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	/*
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	 * We don't want to get "file->f_lock" because it is not
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	 * necessary. It is not necessary because we're in the "struct file"
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	 * cleanup path, and this means that no one is using this file anymore.
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	 * So, for example, epoll_ctl() cannot hit here since if we reach this
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	 * point, the file counter already went to zero and fget() would fail.
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	 * The only hit might come from ep_free() but by holding the mutex
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	 * will correctly serialize the operation. We do need to acquire
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	 * "ep->mtx" after "epmutex" because ep_remove() requires it when called
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	 * from anywhere but ep_free().
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	 *
	 * Besides, ep_remove() acquires the lock, so we can't hold it here.
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	 */
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	mutex_lock(&epmutex);
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	list_for_each_entry_safe(epi, next, &file->f_ep_links, fllink) {
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		ep = epi->ep;
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		mutex_lock_nested(&ep->mtx, 0);
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		ep_remove(ep, epi);
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		mutex_unlock(&ep->mtx);
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	}
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	mutex_unlock(&epmutex);
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}

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static int ep_alloc(struct eventpoll **pep)
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{
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	int error;
	struct user_struct *user;
	struct eventpoll *ep;
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	user = get_current_user();
	error = -ENOMEM;
	ep = kzalloc(sizeof(*ep), GFP_KERNEL);
	if (unlikely(!ep))
		goto free_uid;
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947
	spin_lock_init(&ep->lock);
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	mutex_init(&ep->mtx);
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	init_waitqueue_head(&ep->wq);
	init_waitqueue_head(&ep->poll_wait);
	INIT_LIST_HEAD(&ep->rdllist);
	ep->rbr = RB_ROOT;
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	ep->ovflist = EP_UNACTIVE_PTR;
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	ep->user = user;
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	*pep = ep;
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	return 0;
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free_uid:
	free_uid(user);
	return error;
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}

/*
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 * 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.
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 */
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;

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	ep_set_ffd(&ffd, file, fd);
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	for (rbp = ep->rbr.rb_node; rbp; ) {
		epi = rb_entry(rbp, struct epitem, rbn);
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		kcmp = ep_cmp_ffd(&ffd, &epi->ffd);
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		if (kcmp > 0)
			rbp = rbp->rb_right;
		else if (kcmp < 0)
			rbp = rbp->rb_left;
		else {
			epir = epi;
			break;
		}
	}

	return epir;
}

/*
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 * This is the callback that is passed to the wait queue wakeup
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 * mechanism. It is called by the stored file descriptors when they
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 * have events to report.
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 */
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static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
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{
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	int pwake = 0;
	unsigned long flags;
	struct epitem *epi = ep_item_from_wait(wait);
	struct eventpoll *ep = epi->ep;
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	if ((unsigned long)key & POLLFREE) {
		ep_pwq_from_wait(wait)->whead = NULL;
		/*
		 * whead = NULL above can race with ep_remove_wait_queue()
		 * which can do another remove_wait_queue() after us, so we
		 * can't use __remove_wait_queue(). whead->lock is held by
		 * the caller.
		 */
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		list_del_init(&wait->task_list);
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	}
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1017
	spin_lock_irqsave(&ep->lock, flags);
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	/*
	 * 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))
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		goto out_unlock;

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	/*
	 * 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 (key && !((unsigned long) key & epi->event.events))
		goto out_unlock;

1037
	/*
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	 * If we are transferring events to userspace, we can hold no locks
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	 * (because we're accessing user memory, and because of linux f_op->poll()
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	 * semantics). All the events that happen during that period of time are
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	 * chained in ep->ovflist and requeued later on.
	 */
	if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
		if (epi->next == EP_UNACTIVE_PTR) {
			epi->next = ep->ovflist;
			ep->ovflist = epi;
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			if (epi->ws) {
				/*
				 * Activate ep->ws since epi->ws may get
				 * deactivated at any time.
				 */
				__pm_stay_awake(ep->ws);
			}

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		}
		goto out_unlock;
	}
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1059
	/* If this file is already in the ready list we exit soon */
1060
	if (!ep_is_linked(&epi->rdllink)) {
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		list_add_tail(&epi->rdllink, &ep->rdllist);
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		ep_pm_stay_awake_rcu(epi);
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	}
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	/*
	 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
	 * wait list.
	 */
	if (waitqueue_active(&ep->wq))
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		wake_up_locked(&ep->wq);
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	if (waitqueue_active(&ep->poll_wait))
		pwake++;

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out_unlock:
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	spin_unlock_irqrestore(&ep->lock, flags);
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	/* We have to call this outside the lock */
	if (pwake)
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		ep_poll_safewake(&ep->poll_wait);
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	return 1;
}
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/*
 * 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)
{
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	struct epitem *epi = ep_item_from_epqueue(pt);
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	struct eppoll_entry *pwq;

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	if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
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		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
		pwq->whead = whead;
		pwq->base = epi;
		add_wait_queue(whead, &pwq->wait);
		list_add_tail(&pwq->llink, &epi->pwqlist);
		epi->nwait++;
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	} else {
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		/* We have to signal that an error occurred */
		epi->nwait = -1;
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	}
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}

static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
{
	int kcmp;
	struct rb_node **p = &ep->rbr.rb_node, *parent = NULL;
	struct epitem *epic;

	while (*p) {
		parent = *p;
		epic = rb_entry(parent, struct epitem, rbn);
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		kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);
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		if (kcmp > 0)
			p = &parent->rb_right;
		else
			p = &parent->rb_left;
	}
	rb_link_node(&epi->rbn, parent, p);
	rb_insert_color(&epi->rbn, &ep->rbr);
}

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

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	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;

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	/* 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) {
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		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,
							EP_MAX_NESTS,
							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!\n");
		}
	}
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	rcu_read_unlock();
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	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, EP_MAX_NESTS,
					reverse_path_check_proc, current_file,
					current_file, current);
		if (error)
			break;
	}
	return error;
}

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static int ep_create_wakeup_source(struct epitem *epi)
{
	const char *name;
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	struct wakeup_source *ws;
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	if (!epi->ep->ws) {
		epi->ep->ws = wakeup_source_register("eventpoll");
		if (!epi->ep->ws)
			return -ENOMEM;
	}

	name = epi->ffd.file->f_path.dentry->d_name.name;
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	ws = wakeup_source_register(name);

	if (!ws)
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		return -ENOMEM;
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	rcu_assign_pointer(epi->ws, ws);
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	return 0;
}

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/* rare code path, only used when EPOLL_CTL_MOD removes a wakeup source */
static noinline void ep_destroy_wakeup_source(struct epitem *epi)
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{
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	struct wakeup_source *ws = ep_wakeup_source(epi);

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	RCU_INIT_POINTER(epi->ws, NULL);
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	/*
	 * 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);
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}

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/*
 * Must be called with "mtx" held.
 */
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static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
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		     struct file *tfile, int fd, int full_check)
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{
	int error, revents, pwake = 0;
	unsigned long flags;
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	long user_watches;
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	struct epitem *epi;
	struct ep_pqueue epq;

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	user_watches = atomic_long_read(&ep->user->epoll_watches);
	if (unlikely(user_watches >= max_user_watches))
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		return -ENOSPC;
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	if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
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		return -ENOMEM;
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	/* Item initialization follow here ... */
	INIT_LIST_HEAD(&epi->rdllink);
	INIT_LIST_HEAD(&epi->fllink);
	INIT_LIST_HEAD(&epi->pwqlist);
	epi->ep = ep;
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	ep_set_ffd(&epi->ffd, tfile, fd);
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	epi->event = *event;
	epi->nwait = 0;
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	epi->next = EP_UNACTIVE_PTR;
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