eventpoll.c

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/*
 *  fs/eventpoll.c (Efficient event retrieval implementation)
 *  Copyright (C) 2001,...,2009  Davide Libenzi
 *
 *  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 <[email protected]>
 *
 */

#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>
#include <linux/mutex.h>
#include <linux/anon_inodes.h>
#include <linux/device.h>
#include <asm/uaccess.h>
#include <asm/io.h>
#include <asm/mman.h>
#include <linux/atomic.h>

/*
 * LOCKING:
 * There are three level of locking required by epoll :
 *
 * 1) epmutex (mutex)
 * 2) ep->mtx (mutex)
 * 3) ep->lock (spinlock)
 *
 * 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
 * mutex (ep->mtx). It is acquired during the event transfer loop,
 * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
 * Then we also need a global mutex to serialize eventpoll_release_file()
 * and ep_free().
 * This mutex is acquired by ep_free() during the epoll file
 * cleanup path and it is also acquired by eventpoll_release_file()
 * if a file has been pushed inside an epoll set and it is then
 * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).
 * It is also acquired when inserting an epoll fd onto another epoll
 * fd. We do this so that we walk the epoll tree and ensure that this
 * insertion does not create a cycle of epoll file descriptors, which
 * could lead to deadlock. We need a global mutex to prevent two
 * simultaneous inserts (A into B and B into A) from racing and
 * constructing a cycle without either insert observing that it is
 * going to.
 * It is necessary to acquire multiple "ep->mtx"es at once in the
 * case when one epoll fd is added to another. In this case, we
 * always acquire the locks in the order of nesting (i.e. after
 * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
 * before e2->mtx). Since we disallow cycles of epoll file
 * descriptors, this ensures that the mutexes are well-ordered. In
 * order to communicate this nesting to lockdep, when walking a tree
 * of epoll file descriptors, we use the current recursion depth as
 * the lockdep subkey.
 * It is possible to drop the "ep->mtx" and to use the global
 * mutex "epmutex" (together with "ep->lock") to have it working,
 * but having "ep->mtx" will make the interface more scalable.
 * Events that require holding "epmutex" are very rare, while for
 * normal operations the epoll private "ep->mtx" will guarantee
 * a better scalability.
 */

/* Epoll private bits inside the event mask */
#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)

/* 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;
};

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

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

/*
 * Each file descriptor added to the eventpoll interface will
 * have an entry of this type linked to the "rbr" RB tree.
 */
struct epitem {
    /* RB tree node used to link this structure to the eventpoll RB tree */
    struct rb_node rbn;

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

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

    /* The file descriptor information this item refers to */
    struct epoll_filefd ffd;

    /* Number of active wait queue attached to poll operations */
    int nwait;

    /* List containing poll wait queues */
    struct list_head pwqlist;

    /* The "container" of this item */
    struct eventpoll *ep;

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

    /* wakeup_source used when EPOLLWAKEUP is set */
    struct wakeup_source *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 {
    /* Protect the access to this structure */
    spinlock_t lock;

    /*
     * 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;

    /* RB tree root used to store monitored fd structs */
    struct rb_root 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 */
    int visited;
    struct list_head visited_list_link;
};

/* Wait structure used by the poll hooks */
struct eppoll_entry {
    /* List header used to link this structure to the "struct epitem" */
    struct list_head llink;

    /* The "base" pointer is set to the container "struct epitem" */
    struct epitem *base;

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

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

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

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

/*
 * Configuration options available inside /proc/sys/fs/epoll/
 */
/* Maximum number of epoll watched descriptors, per user */
static long max_user_watches __read_mostly;

/*
 * This mutex is used to serialize ep_free() and eventpoll_release_file().
 */
static DEFINE_MUTEX(epmutex);

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

/* 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;

/* 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;

/* 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);

#ifdef CONFIG_SYSCTL

#include <linux/sysctl.h>

static long zero;
static long long_max = LONG_MAX;

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     = &zero,
        .extra2     = &long_max,
    },
    { }
};
#endif /* CONFIG_SYSCTL */

static const struct file_operations eventpoll_fops;

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

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

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

/* Tells us if the item is currently linked */
static inline int ep_is_linked(struct list_head *p)
{
    return !list_empty(p);
}

static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_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_t *p)
{
    return container_of(p, struct eppoll_entry, wait)->base;
}

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

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

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

static inline int ep_events_available(struct eventpoll *ep)
{
    return !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
}

static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
              int (*nproc)(void *, void *, int), void *priv,
              void *cookie, void *ctx)
{
    int error, call_nests = 0;
    unsigned long flags;
    struct list_head *lsthead = &ncalls->tasks_call_list;
    struct nested_call_node *tncur;
    struct nested_call_node tnode;

    spin_lock_irqsave(&ncalls->lock, flags);

    /*
     * Try to see if the current task is already inside this wakeup call.
     * We use a list here, since the population inside this set is always
     * very much limited.
     */
    list_for_each_entry(tncur, lsthead, llink) {
        if (tncur->ctx == ctx &&
            (tncur->cookie == cookie || ++call_nests > max_nests)) {
            /*
             * Ops ... loop detected or maximum nest level reached.
             * We abort this wake by breaking the cycle itself.
             */
            error = -1;
            goto out_unlock;
        }
    }

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

    spin_unlock_irqrestore(&ncalls->lock, flags);

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

    /* Remove the current task from the list */
    spin_lock_irqsave(&ncalls->lock, flags);
    list_del(&tnode.llink);
out_unlock:
    spin_unlock_irqrestore(&ncalls->lock, flags);

    return error;
}

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

static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)
{
    ep_wake_up_nested((wait_queue_head_t *) cookie, POLLIN,
              1 + call_nests);
    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)
{
    int this_cpu = get_cpu();

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

    put_cpu();
}

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();
}

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

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

        list_del(&pwq->llink);
        ep_remove_wait_queue(pwq);
        kmem_cache_free(pwq_cache, pwq);
    }
}

static int ep_scan_ready_list(struct eventpoll *ep,
                  int (*sproc)(struct eventpoll *,
                       struct list_head *, void *),
                  void *priv,
                  int depth)
{
    int error, pwake = 0;
    unsigned long flags;
    struct epitem *epi, *nepi;
    LIST_HEAD(txlist);

    /*
     * We need to lock this because we could be hit by
     * eventpoll_release_file() and epoll_ctl().
     */
    mutex_lock_nested(&ep->mtx, depth);

    /*
     * Steal the ready list, and re-init the original one to the
     * empty list. Also, set ep->ovflist to NULL so that events
     * happening while looping w/out locks, are not lost. We cannot
     * have the poll callback to queue directly on ep->rdllist,
     * because we want the "sproc" callback to be able to do it
     * in a lockless way.
     */
    spin_lock_irqsave(&ep->lock, flags);
    list_splice_init(&ep->rdllist, &txlist);
    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.
         */
        if (!ep_is_linked(&epi->rdllink)) {
            list_add_tail(&epi->rdllink, &ep->rdllist);
            __pm_stay_awake(epi->ws);
        }
    }
    /*
     * 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);
    __pm_relax(ep->ws);

    if (!list_empty(&ep->rdllist)) {
        /*
         * Wake up (if active) both the eventpoll wait list and
         * the ->poll() wait list (delayed after we release the lock).
         */
        if (waitqueue_active(&ep->wq))
            wake_up_locked(&ep->wq);
        if (waitqueue_active(&ep->poll_wait))
            pwake++;
    }
    spin_unlock_irqrestore(&ep->lock, flags);

    mutex_unlock(&ep->mtx);

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

    return error;
}

/*
 * Removes a "struct epitem" from the eventpoll RB tree and deallocates
 * all the associated resources. Must be called with "mtx" held.
 */
static int ep_remove(struct eventpoll *ep, struct epitem *epi)
{
    unsigned long flags;
    struct file *file = epi->ffd.file;

    /*
     * 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".
     */
    ep_unregister_pollwait(ep, epi);

    /* Remove the current item from the list of epoll hooks */
    spin_lock(&file->f_lock);
    if (ep_is_linked(&epi->fllink))
        list_del_init(&epi->fllink);
    spin_unlock(&file->f_lock);

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

    spin_lock_irqsave(&ep->lock, flags);
    if (ep_is_linked(&epi->rdllink))
        list_del_init(&epi->rdllink);
    spin_unlock_irqrestore(&ep->lock, flags);

    wakeup_source_unregister(epi->ws);

    /* At this point it is safe to free the eventpoll item */
    kmem_cache_free(epi_cache, epi);

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

    return 0;
}

static void ep_free(struct eventpoll *ep)
{
    struct rb_node *rbp;
    struct epitem *epi;

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

    /*
     * We need to lock this because we could be hit by
     * eventpoll_release_file() while we're freeing the "struct eventpoll".
     * We do not need to hold "ep->mtx" here because the epoll file
     * is on the way to be removed and no one has references to it
     * anymore. The only hit might come from eventpoll_release_file() but
     * holding "epmutex" is sufficient here.
     */
    mutex_lock(&epmutex);

    /*
     * Walks through the whole tree by unregistering poll callbacks.
     */
    for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
        epi = rb_entry(rbp, struct epitem, rbn);

        ep_unregister_pollwait(ep, epi);
    }

    /*
     * Walks through the whole tree by freeing each "struct epitem". At this
     * point we are sure no poll callbacks will be lingering around, and also by
     * holding "epmutex" we can be sure that no file cleanup code will hit
     * us during this operation. So we can avoid the lock on "ep->lock".
     */
    while ((rbp = rb_first(&ep->rbr)) != NULL) {
        epi = rb_entry(rbp, struct epitem, rbn);
        ep_remove(ep, epi);
    }

    mutex_unlock(&epmutex);
    mutex_destroy(&ep->mtx);
    free_uid(ep->user);
    wakeup_source_unregister(ep->ws);
    kfree(ep);
}

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

    if (ep)
        ep_free(ep);

    return 0;
}

static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
                   void *priv)
{
    struct epitem *epi, *tmp;
    poll_table pt;

    init_poll_funcptr(&pt, NULL);
    list_for_each_entry_safe(epi, tmp, head, rdllink) {
        pt._key = epi->event.events;
        if (epi->ffd.file->f_op->poll(epi->ffd.file, &pt) &
            epi->event.events)
            return POLLIN | POLLRDNORM;
        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(epi->ws);
            list_del_init(&epi->rdllink);
        }
    }

    return 0;
}

static int ep_poll_readyevents_proc(void *priv, void *cookie, int call_nests)
{
    return ep_scan_ready_list(priv, ep_read_events_proc, NULL, call_nests + 1);
}

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

    /* 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. 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,
                   ep_poll_readyevents_proc, ep, ep, current);

    return pollflags != -1 ? pollflags : 0;
}

/* File callbacks that implement the eventpoll file behaviour */
static const struct file_operations eventpoll_fops = {
    .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 list_head *lsthead = &file->f_ep_links;
    struct eventpoll *ep;
    struct epitem *epi;

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

    while (!list_empty(lsthead)) {
        epi = list_first_entry(lsthead, struct epitem, fllink);

        ep = epi->ep;
        list_del_init(&epi->fllink);
        mutex_lock_nested(&ep->mtx, 0);
        ep_remove(ep, epi);
        mutex_unlock(&ep->mtx);
    }

    mutex_unlock(&epmutex);
}

static int ep_alloc(struct eventpoll **pep)
{
    int error;
    struct user_struct *user;
    struct eventpoll *ep;

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

    spin_lock_init(&ep->lock);
    mutex_init(&ep->mtx);
    init_waitqueue_head(&ep->wq);
    init_waitqueue_head(&ep->poll_wait);
    INIT_LIST_HEAD(&ep->rdllist);
    ep->rbr = RB_ROOT;
    ep->ovflist = EP_UNACTIVE_PTR;
    ep->user = user;

    *pep = ep;

    return 0;

free_uid:
    free_uid(user);
    return error;
}

/*
 * Search the file inside the eventpoll tree. The RB tree operations
 * are protected by the "mtx" mutex, and ep_find() must be called with
 * "mtx" held.
 */
static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
{
    int kcmp;
    struct rb_node *rbp;
    struct epitem *epi, *epir = NULL;
    struct epoll_filefd ffd;

    ep_set_ffd(&ffd, file, fd);
    for (rbp = ep->rbr.rb_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;
}

/*
 * 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.
 */
static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
    int pwake = 0;
    unsigned long flags;
    struct epitem *epi = ep_item_from_wait(wait);
    struct eventpoll *ep = epi->ep;

    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.
         */
        list_del_init(&wait->task_list);
    }

    spin_lock_irqsave(&ep->lock, flags);

    /*
     * 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 (key && !((unsigned long) key & 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 (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
        if (epi->next == EP_UNACTIVE_PTR) {
            epi->next = ep->ovflist;
            ep->ovflist = epi;
            if (epi->ws) {
                /*
                 * Activate ep->ws since epi->ws may get
                 * deactivated at any time.
                 */
                __pm_stay_awake(ep->ws);
            }

        }
        goto out_unlock;
    }

    /* If this file is already in the ready list we exit soon */
    if (!ep_is_linked(&epi->rdllink)) {
        list_add_tail(&epi->rdllink, &ep->rdllist);
        __pm_stay_awake(epi->ws);
    }

    /*
     * Wake up ( if active ) both the eventpoll wait list and the ->poll()
     * wait list.
     */
    if (waitqueue_active(&ep->wq))
        wake_up_locked(&ep->wq);
    if (waitqueue_active(&ep->poll_wait))
        pwake++;

out_unlock:
    spin_unlock_irqrestore(&ep->lock, flags);

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

    return 1;
}

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

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

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

    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;
        else
            p = &parent->rb_left;
    }
    rb_link_node(&epi->rbn, parent, p);
    rb_insert_color(&epi->rbn, &ep->rbr);
}



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

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

    if (++path_count[nests] > path_limits[nests])
        return -1;
    return 0;
}

static void path_count_init(void)
{
    int i;

    for (i = 0; i < PATH_ARR_SIZE; i++)
        path_count[i] = 0;
}

static int reverse_path_check_proc(void *priv, void *cookie, int call_nests)
{
    int error = 0;
    struct file *file = priv;
    struct file *child_file;
    struct epitem *epi;

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

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;
}

static int ep_create_wakeup_source(struct epitem *epi)
{
    const char *name;

    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;
    epi->ws = wakeup_source_register(name);
    if (!epi->ws)
        return -ENOMEM;

    return 0;
}

static void ep_destroy_wakeup_source(struct epitem *epi)
{
    wakeup_source_unregister(epi->ws);
    epi->ws = NULL;
}

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

    user_watches = atomic_long_read(&ep->user->epoll_watches);
    if (unlikely(user_watches >= max_user_watches))
        return -ENOSPC;
    if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
        return -ENOMEM;

    /* Item initialization follow here ... */
    INIT_LIST_HEAD(&epi->rdllink);
    INIT_LIST_HEAD(&epi->fllink);
    INIT_LIST_HEAD(&epi->pwqlist);
    epi->ep = ep;
    ep_set_ffd(&epi->ffd, tfile, fd);
    epi->event = *event;
    epi->nwait = 0;
    epi->next = EP_UNACTIVE_PTR;
    if (epi->event.events & EPOLLWAKEUP) {
        error = ep_create_wakeup_source(epi);
        if (error)
            goto error_create_wakeup_source;
    } else {
        epi->ws = NULL;
    }

    /* Initialize the poll table using the queue callback */
    epq.epi = epi;
    init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
    epq.pt._key = event->events;

    /*
     * 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 = tfile->f_op->poll(tfile, &epq.pt);

    /*
     * We have to check if something went wrong during the poll wait queue
     * install process. Namely an allocation for a wait queue failed due
     * high memory pressure.
     */
    error = -ENOMEM;
    if (epi->nwait < 0)
        goto error_unregister;

    /* Add the current item to the list of active epoll hook for this file */
    spin_lock(&tfile->f_lock);
    list_add_tail(&epi->fllink, &tfile->f_ep_links);
    spin_unlock(&tfile->f_lock);

    /*
     * Add the current item to the RB tree. All RB tree operations are
     * protected by "mtx", and ep_insert() is called with "mtx" held.
     */
    ep_rbtree_insert(ep, epi);

    /* now check if we've created too many backpaths */
    error = -EINVAL;
    if (reverse_path_check())
        goto error_remove_epi;

    /* We have to drop the new item inside our item list to keep track of it */
    spin_lock_irqsave(&ep->lock, flags);

    /* If the file is already "ready" we drop it inside the ready list */
    if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
        list_add_tail(&epi->rdllink, &ep->rdllist);
        __pm_stay_awake(epi->ws);

        /* Notify waiting tasks that events are available */
        if (waitqueue_active(&ep->wq))
            wake_up_locked(&ep->wq);
        if (waitqueue_active(&ep->poll_wait))
            pwake++;
    }

    spin_unlock_irqrestore(&ep->lock, flags);

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

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

    return 0;

error_remove_epi:
    spin_lock(&tfile->f_lock);
    if (ep_is_linked(&epi->fllink))
        list_del_init(&epi->fllink);
    spin_unlock(&tfile->f_lock);

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

error_unregister:
    ep_unregister_pollwait(ep, epi);

    /*
     * We need to do this because an event could have been arrived on some
     * allocated wait queue. Note that we don't care about the ep->ovflist
     * list, since that is used/cleaned only inside a section bound by "mtx".
     * And ep_insert() is called with "mtx" held.
     */
    spin_lock_irqsave(&ep->lock, flags);
    if (ep_is_linked(&epi->rdllink))
        list_del_init(&epi->rdllink);
    spin_unlock_irqrestore(&ep->lock, flags);

    wakeup_source_unregister(epi->ws);

error_create_wakeup_source:
    kmem_cache_free(epi_cache, epi);

    return error;
}

/*
 * 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, struct epoll_event *event)
{
    int pwake = 0;
    unsigned int revents;
    poll_table pt;

    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;
    pt._key = event->events;
    epi->event.data = event->data; /* protected by mtx */
    if (epi->event.events & EPOLLWAKEUP) {
        if (!epi->ws)
            ep_create_wakeup_source(epi);
    } else if (epi->ws) {
        ep_destroy_wakeup_source(epi);
    }

    /*
     * Get current event bits. We can safely use the file* here because
     * its usage count has been increased by the caller of this function.
     */
    revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt);

    /*
     * If the item is "hot" and it is not registered inside the ready
     * list, push it inside.
     */
    if (revents & event->events) {
        spin_lock_irq(&ep->lock);
        if (!ep_is_linked(&epi->rdllink)) {
            list_add_tail(&epi->rdllink, &ep->rdllist);
            __pm_stay_awake(epi->ws);

            /* Notify waiting tasks that events are available */
            if (waitqueue_active(&ep->wq))
                wake_up_locked(&ep->wq);
            if (waitqueue_active(&ep->poll_wait))
                pwake++;
        }
        spin_unlock_irq(&ep->lock);
    }

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

    return 0;
}

static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
                   void *priv)
{
    struct ep_send_events_data *esed = priv;
    int eventcnt;
    unsigned int revents;
    struct epitem *epi;
    struct epoll_event __user *uevent;
    poll_table pt;

    init_poll_funcptr(&pt, NULL);

    /*
     * We can loop without lock because we are passed a task private list.
     * Items cannot vanish during the loop because ep_scan_ready_list() is
     * holding "mtx" during this call.
     */
    for (eventcnt = 0, uevent = esed->events;
         !list_empty(head) && eventcnt < esed->maxevents;) {
        epi = list_first_entry(head, struct epitem, rdllink);

        /*
         * 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().
         */
        if (epi->ws && epi->ws->active)
            __pm_stay_awake(ep->ws);
        __pm_relax(epi->ws);
        list_del_init(&epi->rdllink);

        pt._key = epi->event.events;
        revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt) &
            epi->event.events;

        /*
         * If the event mask intersect the caller-requested one,
         * deliver the event to userspace. Again, ep_scan_ready_list()
         * is holding "mtx", so no operations coming from userspace
         * can change the item.
         */
        if (revents) {
            if (__put_user(revents, &uevent->events) ||
                __put_user(epi->event.data, &uevent->data)) {
                list_add(&epi->rdllink, head);
                __pm_stay_awake(epi->ws);
                return eventcnt ? eventcnt : -EFAULT;
            }
            eventcnt++;
            uevent++;
            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);
                __pm_stay_awake(epi->ws);
            }
        }
    }

    return eventcnt;
}

static int ep_send_events(struct eventpoll *ep,
              struct epoll_event __user *events, int maxevents)
{
    struct ep_send_events_data esed;

    esed.maxevents = maxevents;
    esed.events = events;

    return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0);
}

static inline struct timespec ep_set_mstimeout(long ms)
{
    struct timespec now, ts = {
        .tv_sec = ms / MSEC_PER_SEC,
        .tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC),
    };

    ktime_get_ts(&now);
    return timespec_add_safe(now, ts);
}

static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
           int maxevents, long timeout)
{
    int res = 0, eavail, timed_out = 0;
    unsigned long flags;
    long slack = 0;
    wait_queue_t wait;
    ktime_t expires, *to = NULL;

    if (timeout > 0) {
        struct timespec end_time = ep_set_mstimeout(timeout);

        slack = select_estimate_accuracy(&end_time);
        to = &expires;
        *to = timespec_to_ktime(end_time);
    } else if (timeout == 0) {
        /*
         * Avoid the unnecessary trip to the wait queue loop, if the
         * caller specified a non blocking operation.
         */
        timed_out = 1;
        spin_lock_irqsave(&ep->lock, flags);
        goto check_events;
    }

fetch_events:
    spin_lock_irqsave(&ep->lock, flags);

    if (!ep_events_available(ep)) {
        /*
         * We don't have any available event to return to the caller.
         * We need to sleep here, and we will be wake up by
         * ep_poll_callback() when events will become available.
         */
        init_waitqueue_entry(&wait, current);
        __add_wait_queue_exclusive(&ep->wq, &wait);

        for (;;) {
            /*
             * We don't want to sleep if the ep_poll_callback() sends us
             * a wakeup in between. That's why we set the task state
             * to TASK_INTERRUPTIBLE before doing the checks.
             */
            set_current_state(TASK_INTERRUPTIBLE);
            if (ep_events_available(ep) || timed_out)
                break;
            if (signal_pending(current)) {
                res = -EINTR;
                break;
            }

            spin_unlock_irqrestore(&ep->lock, flags);
            if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
                timed_out = 1;

            spin_lock_irqsave(&ep->lock, flags);
        }
        __remove_wait_queue(&ep->wq, &wait);

        set_current_state(TASK_RUNNING);
    }
check_events:
    /* Is it worth to try to dig for events ? */
    eavail = ep_events_available(ep);

    spin_unlock_irqrestore(&ep->lock, flags);

    /*
     * 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.
     */
    if (!res && eavail &&
        !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
        goto fetch_events;

    return res;
}

static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
{
    int error = 0;
    struct file *file = priv;
    struct eventpoll *ep = file->private_data;
    struct eventpoll *ep_tovisit;
    struct rb_node *rbp;
    struct epitem *epi;

    mutex_lock_nested(&ep->mtx, call_nests + 1);
    ep->visited = 1;
    list_add(&ep->visited_list_link, &visited_list);
    for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
        epi = rb_entry(rbp, struct epitem, rbn);
        if (unlikely(is_file_epoll(epi->ffd.file))) {
            ep_tovisit = epi->ffd.file->private_data;
            if (ep_tovisit->visited)
                continue;
            error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
                    ep_loop_check_proc, epi->ffd.file,
                    ep_tovisit, current);
            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().
             */
            if (list_empty(&epi->ffd.file->f_tfile_llink))
                list_add(&epi->ffd.file->f_tfile_llink,
                     &tfile_check_list);
        }
    }
    mutex_unlock(&ep->mtx);

    return error;
}

static int ep_loop_check(struct eventpoll *ep, struct file *file)
{
    int ret;
    struct eventpoll *ep_cur, *ep_next;

    ret = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
                  ep_loop_check_proc, file, ep, current);
    /* clear visited list */
    list_for_each_entry_safe(ep_cur, ep_next, &visited_list,
                            visited_list_link) {
        ep_cur->visited = 0;
        list_del(&ep_cur->visited_list_link);
    }
    return ret;
}

static void clear_tfile_check_list(void)
{
    struct file *file;

    /* first clear the tfile_check_list */
    while (!list_empty(&tfile_check_list)) {
        file = list_first_entry(&tfile_check_list, struct file,
                    f_tfile_llink);
        list_del_init(&file->f_tfile_llink);
    }
    INIT_LIST_HEAD(&tfile_check_list);
}

/*
 * Open an eventpoll file descriptor.
 */
SYSCALL_DEFINE1(epoll_create1, 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_free(ep);
    return error;
}

SYSCALL_DEFINE1(epoll_create, int, size)
{
    if (size <= 0)
        return -EINVAL;

    return sys_epoll_create1(0);
}

/*
 * 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)
{
    int error;
    int did_lock_epmutex = 0;
    struct file *file, *tfile;
    struct eventpoll *ep;
    struct epitem *epi;
    struct epoll_event epds;

    error = -EFAULT;
    if (ep_op_has_event(op) &&
        copy_from_user(&epds, event, sizeof(struct epoll_event)))
        goto error_return;

    /* Get the "struct file *" for the eventpoll file */
    error = -EBADF;
    file = fget(epfd);
    if (!file)
        goto error_return;

    /* Get the "struct file *" for the target file */
    tfile = fget(fd);
    if (!tfile)
        goto error_fput;

    /* The target file descriptor must support poll */
    error = -EPERM;
    if (!tfile->f_op || !tfile->f_op->poll)
        goto error_tgt_fput;

    /* Check if EPOLLWAKEUP is allowed */
    if ((epds.events & EPOLLWAKEUP) && !capable(CAP_BLOCK_SUSPEND))
        epds.events &= ~EPOLLWAKEUP;

    /*
     * 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 (file == tfile || !is_file_epoll(file))
        goto error_tgt_fput;

    /*
     * At this point it is safe to assume that the "private_data" contains
     * our own data structure.
     */
    ep = file->private_data;

    /*
     * When we insert an epoll file descriptor, inside another epoll file
     * descriptor, there is the change 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 need to hold the epmutex across both ep_insert and ep_remove
     * b/c we want to make sure we are looking at a coherent view of
     * epoll network.
     */
    if (op == EPOLL_CTL_ADD || op == EPOLL_CTL_DEL) {
        mutex_lock(&epmutex);
        did_lock_epmutex = 1;
    }
    if (op == EPOLL_CTL_ADD) {
        if (is_file_epoll(tfile)) {
            error = -ELOOP;
            if (ep_loop_check(ep, tfile) != 0) {
                clear_tfile_check_list();
                goto error_tgt_fput;
            }
        } else
            list_add(&tfile->f_tfile_llink, &tfile_check_list);
    }

    mutex_lock_nested(&ep->mtx, 0);

    /*
     * 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, tfile, fd);

    error = -EINVAL;
    switch (op) {
    case EPOLL_CTL_ADD:
        if (!epi) {
            epds.events |= POLLERR | POLLHUP;
            error = ep_insert(ep, &epds, tfile, fd);
        } else
            error = -EEXIST;
        clear_tfile_check_list();
        break;
    case EPOLL_CTL_DEL:
        if (epi)
            error = ep_remove(ep, epi);
        else
            error = -ENOENT;
        break;
    case EPOLL_CTL_MOD:
        if (epi) {
            epds.events |= POLLERR | POLLHUP;
            error = ep_modify(ep, epi, &epds);
        } else
            error = -ENOENT;
        break;
    }
    mutex_unlock(&ep->mtx);

error_tgt_fput:
    if (did_lock_epmutex)
        mutex_unlock(&epmutex);

    fput(tfile);
error_fput:
    fput(file);
error_return:

    return error;
}

/*
 * Implement the event wait interface for the eventpoll file. It is the kernel
 * part of the user space epoll_wait(2).
 */
SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
        int, maxevents, int, timeout)
{
    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(VERIFY_WRITE, 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, timeout);

error_fput:
    fdput(f);
    return error;
}

/*
 * Implement the event wait interface for the eventpoll file. It is the kernel
 * part of the user space epoll_pwait(2).
 */
SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
        int, maxevents, int, timeout, const sigset_t __user *, sigmask,
        size_t, sigsetsize)
{
    int error;
    sigset_t ksigmask, sigsaved;

    /*
     * If the caller wants a certain signal mask to be set during the wait,
     * we apply it here.
     */
    if (sigmask) {
        if (sigsetsize != sizeof(sigset_t))
            return -EINVAL;
        if (copy_from_user(&ksigmask, sigmask, sizeof(ksigmask)))
            return -EFAULT;
        sigdelsetmask(&ksigmask, sigmask(SIGKILL) | sigmask(SIGSTOP));
        sigprocmask(SIG_SETMASK, &ksigmask, &sigsaved);
    }

    error = sys_epoll_wait(epfd, events, maxevents, timeout);

    /*
     * If we changed the signal mask, we need to restore the original one.
     * In case we've got a signal while waiting, we do not restore the
     * signal mask yet, and we allow do_signal() to deliver the signal on
     * the way back to userspace, before the signal mask is restored.
     */
    if (sigmask) {
        if (error == -EINTR) {
            memcpy(&current->saved_sigmask, &sigsaved,
                   sizeof(sigsaved));
            set_restore_sigmask();
        } else
            sigprocmask(SIG_SETMASK, &sigsaved, NULL);
    }

    return error;
}

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);

    /*
     * Initialize the structure used to perform epoll file descriptor
     * inclusion loops checks.
     */
    ep_nested_calls_init(&poll_loop_ncalls);

    /* Initialize the structure used to perform safe poll wait head wake ups */
    ep_nested_calls_init(&poll_safewake_ncalls);

    /* Initialize the structure used to perform file's f_op->poll() calls */
    ep_nested_calls_init(&poll_readywalk_ncalls);

    /* 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, NULL);

    /* Allocates slab cache used to allocate "struct eppoll_entry" */
    pwq_cache = kmem_cache_create("eventpoll_pwq",
            sizeof(struct eppoll_entry), 0, SLAB_PANIC, NULL);

    return 0;
}
fs_initcall(eventpoll_init);