/*
   *  fs/eventfd.c
   *
   *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
   *
   */
  
  #include <linux/file.h>
  #include <linux/poll.h>
  #include <linux/init.h>
  #include <linux/fs.h>

  #include <linux/sched/signal.h>

  #include <linux/kernel.h>

  #include <linux/slab.h>

  #include <linux/list.h>
  #include <linux/spinlock.h>
  #include <linux/anon_inodes.h>

  #include <linux/syscalls.h>

  #include <linux/export.h>

  #include <linux/kref.h>
  #include <linux/eventfd.h>

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

  
  struct eventfd_ctx {

  	struct kref kref;

  	wait_queue_head_t wqh;
  	/*
  	 * Every time that a write(2) is performed on an eventfd, the
  	 * value of the __u64 being written is added to "count" and a
  	 * wakeup is performed on "wqh". A read(2) will return the "count"
  	 * value to userspace, and will reset "count" to zero. The kernel

  	 * side eventfd_signal() also, adds to the "count" counter and

  	 * issue a wakeup.
  	 */
  	__u64 count;

  	unsigned int flags;

  };

  /**
   * eventfd_signal - Adds @n to the eventfd counter.
   * @ctx: [in] Pointer to the eventfd context.
   * @n: [in] Value of the counter to be added to the eventfd internal counter.
   *          The value cannot be negative.
   *
   * This function is supposed to be called by the kernel in paths that do not
   * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX

   * value, and we signal this as overflow condition by returning a EPOLLERR

   * to poll(2).
   *

   * Returns the amount by which the counter was incremented.  This will be less

   * than @n if the counter has overflowed.

   */

  __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)

  {

  	unsigned long flags;

  	spin_lock_irqsave(&ctx->wqh.lock, flags);

  	if (ULLONG_MAX - ctx->count < n)

  		n = ULLONG_MAX - ctx->count;

  	ctx->count += n;
  	if (waitqueue_active(&ctx->wqh))

  		wake_up_locked_poll(&ctx->wqh, EPOLLIN);

  	spin_unlock_irqrestore(&ctx->wqh.lock, flags);

  
  	return n;
  }

  EXPORT_SYMBOL_GPL(eventfd_signal);

  static void eventfd_free_ctx(struct eventfd_ctx *ctx)
  {
  	kfree(ctx);
  }

  static void eventfd_free(struct kref *kref)
  {
  	struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);

  	eventfd_free_ctx(ctx);

  }
  
  /**

   * eventfd_ctx_put - Releases a reference to the internal eventfd context.
   * @ctx: [in] Pointer to eventfd context.
   *
   * The eventfd context reference must have been previously acquired either

   * with eventfd_ctx_fdget() or eventfd_ctx_fileget().

   */
  void eventfd_ctx_put(struct eventfd_ctx *ctx)
  {
  	kref_put(&ctx->kref, eventfd_free);
  }
  EXPORT_SYMBOL_GPL(eventfd_ctx_put);

  static int eventfd_release(struct inode *inode, struct file *file)
  {

  	struct eventfd_ctx *ctx = file->private_data;

  	wake_up_poll(&ctx->wqh, EPOLLHUP);

  	eventfd_ctx_put(ctx);

  	return 0;
  }

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

  {
  	struct eventfd_ctx *ctx = file->private_data;

  	__poll_t events = 0;

  	u64 count;

  	poll_wait(file, &ctx->wqh, wait);

  	/*
  	 * All writes to ctx->count occur within ctx->wqh.lock.  This read
  	 * can be done outside ctx->wqh.lock because we know that poll_wait
  	 * takes that lock (through add_wait_queue) if our caller will sleep.
  	 *
  	 * The read _can_ therefore seep into add_wait_queue's critical
  	 * section, but cannot move above it!  add_wait_queue's spin_lock acts
  	 * as an acquire barrier and ensures that the read be ordered properly
  	 * against the writes.  The following CAN happen and is safe:
  	 *
  	 *     poll                               write
  	 *     -----------------                  ------------
  	 *     lock ctx->wqh.lock (in poll_wait)
  	 *     count = ctx->count
  	 *     __add_wait_queue
  	 *     unlock ctx->wqh.lock
  	 *                                        lock ctx->qwh.lock
  	 *                                        ctx->count += n
  	 *                                        if (waitqueue_active)
  	 *                                          wake_up_locked_poll
  	 *                                        unlock ctx->qwh.lock
  	 *     eventfd_poll returns 0
  	 *
  	 * but the following, which would miss a wakeup, cannot happen:
  	 *
  	 *     poll                               write
  	 *     -----------------                  ------------
  	 *     count = ctx->count (INVALID!)
  	 *                                        lock ctx->qwh.lock
  	 *                                        ctx->count += n
  	 *                                        **waitqueue_active is false**
  	 *                                        **no wake_up_locked_poll!**
  	 *                                        unlock ctx->qwh.lock
  	 *     lock ctx->wqh.lock (in poll_wait)
  	 *     __add_wait_queue
  	 *     unlock ctx->wqh.lock
  	 *     eventfd_poll returns 0
  	 */
  	count = READ_ONCE(ctx->count);

  	if (count > 0)

  		events |= EPOLLIN;

  	if (count == ULLONG_MAX)

  		events |= EPOLLERR;

  	if (ULLONG_MAX - 1 > count)

  		events |= EPOLLOUT;

  
  	return events;
  }

  static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
  {
  	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
  	ctx->count -= *cnt;
  }
  
  /**
   * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
   * @ctx: [in] Pointer to eventfd context.
   * @wait: [in] Wait queue to be removed.

   * @cnt: [out] Pointer to the 64-bit counter value.

   *

   * Returns %0 if successful, or the following error codes:

   *
   * -EAGAIN      : The operation would have blocked.
   *
   * This is used to atomically remove a wait queue entry from the eventfd wait
   * queue head, and read/reset the counter value.
   */

  int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,

  				  __u64 *cnt)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&ctx->wqh.lock, flags);
  	eventfd_ctx_do_read(ctx, cnt);
  	__remove_wait_queue(&ctx->wqh, wait);
  	if (*cnt != 0 && waitqueue_active(&ctx->wqh))

  		wake_up_locked_poll(&ctx->wqh, EPOLLOUT);

  	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
  
  	return *cnt != 0 ? 0 : -EAGAIN;
  }
  EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);

  static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
  			    loff_t *ppos)

  {

  	struct eventfd_ctx *ctx = file->private_data;

  	ssize_t res;

  	__u64 ucnt = 0;

  	DECLARE_WAITQUEUE(wait, current);

  	if (count < sizeof(ucnt))
  		return -EINVAL;

  	spin_lock_irq(&ctx->wqh.lock);

  	res = -EAGAIN;

  	if (ctx->count > 0)

  		res = sizeof(ucnt);
  	else if (!(file->f_flags & O_NONBLOCK)) {

  		__add_wait_queue(&ctx->wqh, &wait);

  		for (;;) {

  			set_current_state(TASK_INTERRUPTIBLE);
  			if (ctx->count > 0) {

  				res = sizeof(ucnt);

  				break;
  			}
  			if (signal_pending(current)) {
  				res = -ERESTARTSYS;
  				break;
  			}

  			spin_unlock_irq(&ctx->wqh.lock);

  			schedule();

  			spin_lock_irq(&ctx->wqh.lock);

  		}
  		__remove_wait_queue(&ctx->wqh, &wait);
  		__set_current_state(TASK_RUNNING);
  	}

  	if (likely(res > 0)) {
  		eventfd_ctx_do_read(ctx, &ucnt);

  		if (waitqueue_active(&ctx->wqh))

  			wake_up_locked_poll(&ctx->wqh, EPOLLOUT);

  	}

  	spin_unlock_irq(&ctx->wqh.lock);

  	if (res > 0 && put_user(ucnt, (__u64 __user *)buf))
  		return -EFAULT;

  	return res;

  }

  
  static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
  			     loff_t *ppos)
  {
  	struct eventfd_ctx *ctx = file->private_data;
  	ssize_t res;
  	__u64 ucnt;
  	DECLARE_WAITQUEUE(wait, current);
  
  	if (count < sizeof(ucnt))
  		return -EINVAL;
  	if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
  		return -EFAULT;
  	if (ucnt == ULLONG_MAX)
  		return -EINVAL;

  	spin_lock_irq(&ctx->wqh.lock);

  	res = -EAGAIN;
  	if (ULLONG_MAX - ctx->count > ucnt)
  		res = sizeof(ucnt);
  	else if (!(file->f_flags & O_NONBLOCK)) {
  		__add_wait_queue(&ctx->wqh, &wait);
  		for (res = 0;;) {
  			set_current_state(TASK_INTERRUPTIBLE);
  			if (ULLONG_MAX - ctx->count > ucnt) {
  				res = sizeof(ucnt);
  				break;
  			}
  			if (signal_pending(current)) {
  				res = -ERESTARTSYS;
  				break;
  			}

  			spin_unlock_irq(&ctx->wqh.lock);

  			schedule();

  			spin_lock_irq(&ctx->wqh.lock);

  		}
  		__remove_wait_queue(&ctx->wqh, &wait);
  		__set_current_state(TASK_RUNNING);
  	}

  	if (likely(res > 0)) {

  		ctx->count += ucnt;
  		if (waitqueue_active(&ctx->wqh))

  			wake_up_locked_poll(&ctx->wqh, EPOLLIN);

  	}

  	spin_unlock_irq(&ctx->wqh.lock);

  
  	return res;
  }

  #ifdef CONFIG_PROC_FS

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

  {
  	struct eventfd_ctx *ctx = f->private_data;

  
  	spin_lock_irq(&ctx->wqh.lock);

  	seq_printf(m, "eventfd-count: %16llx
  ",
  		   (unsigned long long)ctx->count);

  	spin_unlock_irq(&ctx->wqh.lock);

  }
  #endif

  static const struct file_operations eventfd_fops = {

  #ifdef CONFIG_PROC_FS
  	.show_fdinfo	= eventfd_show_fdinfo,
  #endif

  	.release	= eventfd_release,

  	.poll		= eventfd_poll,

  	.read		= eventfd_read,
  	.write		= eventfd_write,

  	.llseek		= noop_llseek,

  };

  /**
   * eventfd_fget - Acquire a reference of an eventfd file descriptor.
   * @fd: [in] Eventfd file descriptor.
   *
   * Returns a pointer to the eventfd file structure in case of success, or the
   * following error pointer:
   *
   * -EBADF    : Invalid @fd file descriptor.
   * -EINVAL   : The @fd file descriptor is not an eventfd file.
   */

  struct file *eventfd_fget(int fd)
  {
  	struct file *file;
  
  	file = fget(fd);
  	if (!file)
  		return ERR_PTR(-EBADF);
  	if (file->f_op != &eventfd_fops) {
  		fput(file);
  		return ERR_PTR(-EINVAL);
  	}
  
  	return file;
  }

  EXPORT_SYMBOL_GPL(eventfd_fget);

  /**
   * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
   * @fd: [in] Eventfd file descriptor.
   *
   * Returns a pointer to the internal eventfd context, otherwise the error
   * pointers returned by the following functions:
   *
   * eventfd_fget
   */
  struct eventfd_ctx *eventfd_ctx_fdget(int fd)
  {

  	struct eventfd_ctx *ctx;

  	struct fd f = fdget(fd);
  	if (!f.file)
  		return ERR_PTR(-EBADF);
  	ctx = eventfd_ctx_fileget(f.file);
  	fdput(f);

  	return ctx;
  }
  EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
  
  /**
   * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
   * @file: [in] Eventfd file pointer.
   *
   * Returns a pointer to the internal eventfd context, otherwise the error
   * pointer:
   *
   * -EINVAL   : The @fd file descriptor is not an eventfd file.
   */
  struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
  {

  	struct eventfd_ctx *ctx;

  	if (file->f_op != &eventfd_fops)
  		return ERR_PTR(-EINVAL);

  	ctx = file->private_data;
  	kref_get(&ctx->kref);
  	return ctx;

  }
  EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);

  static int do_eventfd(unsigned int count, int flags)

  {

  	struct eventfd_ctx *ctx;

  	int fd;

  	/* Check the EFD_* constants for consistency.  */
  	BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
  	BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);

  	if (flags & ~EFD_FLAGS_SET)

  		return -EINVAL;

  	ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
  	if (!ctx)

  		return -ENOMEM;

  	kref_init(&ctx->kref);

  	init_waitqueue_head(&ctx->wqh);

  	ctx->count = count;

  	ctx->flags = flags;

  	fd = anon_inode_getfd("[eventfd]", &eventfd_fops, ctx,
  			      O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
  	if (fd < 0)

  		eventfd_free_ctx(ctx);

  	return fd;

  }

  SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
  {
  	return do_eventfd(count, flags);
  }

  SYSCALL_DEFINE1(eventfd, unsigned int, count)

  {

  	return do_eventfd(count, 0);

  }