linux/fs/eventfd.c
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   1/*
   2 *  fs/eventfd.c
   3 *
   4 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
   5 *
   6 */
   7
   8#include <linux/file.h>
   9#include <linux/poll.h>
  10#include <linux/init.h>
  11#include <linux/fs.h>
  12#include <linux/sched.h>
  13#include <linux/kernel.h>
  14#include <linux/slab.h>
  15#include <linux/list.h>
  16#include <linux/spinlock.h>
  17#include <linux/anon_inodes.h>
  18#include <linux/syscalls.h>
  19#include <linux/module.h>
  20#include <linux/kref.h>
  21#include <linux/eventfd.h>
  22
  23struct eventfd_ctx {
  24        struct kref kref;
  25        wait_queue_head_t wqh;
  26        /*
  27         * Every time that a write(2) is performed on an eventfd, the
  28         * value of the __u64 being written is added to "count" and a
  29         * wakeup is performed on "wqh". A read(2) will return the "count"
  30         * value to userspace, and will reset "count" to zero. The kernel
  31         * side eventfd_signal() also, adds to the "count" counter and
  32         * issue a wakeup.
  33         */
  34        __u64 count;
  35        unsigned int flags;
  36};
  37
  38/**
  39 * eventfd_signal - Adds @n to the eventfd counter.
  40 * @ctx: [in] Pointer to the eventfd context.
  41 * @n: [in] Value of the counter to be added to the eventfd internal counter.
  42 *          The value cannot be negative.
  43 *
  44 * This function is supposed to be called by the kernel in paths that do not
  45 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
  46 * value, and we signal this as overflow condition by returining a POLLERR
  47 * to poll(2).
  48 *
  49 * Returns @n in case of success, a non-negative number lower than @n in case
  50 * of overflow, or the following error codes:
  51 *
  52 * -EINVAL    : The value of @n is negative.
  53 */
  54int eventfd_signal(struct eventfd_ctx *ctx, int n)
  55{
  56        unsigned long flags;
  57
  58        if (n < 0)
  59                return -EINVAL;
  60        spin_lock_irqsave(&ctx->wqh.lock, flags);
  61        if (ULLONG_MAX - ctx->count < n)
  62                n = (int) (ULLONG_MAX - ctx->count);
  63        ctx->count += n;
  64        if (waitqueue_active(&ctx->wqh))
  65                wake_up_locked_poll(&ctx->wqh, POLLIN);
  66        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
  67
  68        return n;
  69}
  70EXPORT_SYMBOL_GPL(eventfd_signal);
  71
  72static void eventfd_free_ctx(struct eventfd_ctx *ctx)
  73{
  74        kfree(ctx);
  75}
  76
  77static void eventfd_free(struct kref *kref)
  78{
  79        struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
  80
  81        eventfd_free_ctx(ctx);
  82}
  83
  84/**
  85 * eventfd_ctx_get - Acquires a reference to the internal eventfd context.
  86 * @ctx: [in] Pointer to the eventfd context.
  87 *
  88 * Returns: In case of success, returns a pointer to the eventfd context.
  89 */
  90struct eventfd_ctx *eventfd_ctx_get(struct eventfd_ctx *ctx)
  91{
  92        kref_get(&ctx->kref);
  93        return ctx;
  94}
  95EXPORT_SYMBOL_GPL(eventfd_ctx_get);
  96
  97/**
  98 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
  99 * @ctx: [in] Pointer to eventfd context.
 100 *
 101 * The eventfd context reference must have been previously acquired either
 102 * with eventfd_ctx_get() or eventfd_ctx_fdget().
 103 */
 104void eventfd_ctx_put(struct eventfd_ctx *ctx)
 105{
 106        kref_put(&ctx->kref, eventfd_free);
 107}
 108EXPORT_SYMBOL_GPL(eventfd_ctx_put);
 109
 110static int eventfd_release(struct inode *inode, struct file *file)
 111{
 112        struct eventfd_ctx *ctx = file->private_data;
 113
 114        wake_up_poll(&ctx->wqh, POLLHUP);
 115        eventfd_ctx_put(ctx);
 116        return 0;
 117}
 118
 119static unsigned int eventfd_poll(struct file *file, poll_table *wait)
 120{
 121        struct eventfd_ctx *ctx = file->private_data;
 122        unsigned int events = 0;
 123        unsigned long flags;
 124
 125        poll_wait(file, &ctx->wqh, wait);
 126
 127        spin_lock_irqsave(&ctx->wqh.lock, flags);
 128        if (ctx->count > 0)
 129                events |= POLLIN;
 130        if (ctx->count == ULLONG_MAX)
 131                events |= POLLERR;
 132        if (ULLONG_MAX - 1 > ctx->count)
 133                events |= POLLOUT;
 134        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 135
 136        return events;
 137}
 138
 139static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
 140{
 141        *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
 142        ctx->count -= *cnt;
 143}
 144
 145/**
 146 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
 147 * @ctx: [in] Pointer to eventfd context.
 148 * @wait: [in] Wait queue to be removed.
 149 * @cnt: [out] Pointer to the 64-bit counter value.
 150 *
 151 * Returns %0 if successful, or the following error codes:
 152 *
 153 * -EAGAIN      : The operation would have blocked.
 154 *
 155 * This is used to atomically remove a wait queue entry from the eventfd wait
 156 * queue head, and read/reset the counter value.
 157 */
 158int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
 159                                  __u64 *cnt)
 160{
 161        unsigned long flags;
 162
 163        spin_lock_irqsave(&ctx->wqh.lock, flags);
 164        eventfd_ctx_do_read(ctx, cnt);
 165        __remove_wait_queue(&ctx->wqh, wait);
 166        if (*cnt != 0 && waitqueue_active(&ctx->wqh))
 167                wake_up_locked_poll(&ctx->wqh, POLLOUT);
 168        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 169
 170        return *cnt != 0 ? 0 : -EAGAIN;
 171}
 172EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
 173
 174/**
 175 * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
 176 * @ctx: [in] Pointer to eventfd context.
 177 * @no_wait: [in] Different from zero if the operation should not block.
 178 * @cnt: [out] Pointer to the 64-bit counter value.
 179 *
 180 * Returns %0 if successful, or the following error codes:
 181 *
 182 * -EAGAIN      : The operation would have blocked but @no_wait was non-zero.
 183 * -ERESTARTSYS : A signal interrupted the wait operation.
 184 *
 185 * If @no_wait is zero, the function might sleep until the eventfd internal
 186 * counter becomes greater than zero.
 187 */
 188ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
 189{
 190        ssize_t res;
 191        DECLARE_WAITQUEUE(wait, current);
 192
 193        spin_lock_irq(&ctx->wqh.lock);
 194        *cnt = 0;
 195        res = -EAGAIN;
 196        if (ctx->count > 0)
 197                res = 0;
 198        else if (!no_wait) {
 199                __add_wait_queue(&ctx->wqh, &wait);
 200                for (;;) {
 201                        set_current_state(TASK_INTERRUPTIBLE);
 202                        if (ctx->count > 0) {
 203                                res = 0;
 204                                break;
 205                        }
 206                        if (signal_pending(current)) {
 207                                res = -ERESTARTSYS;
 208                                break;
 209                        }
 210                        spin_unlock_irq(&ctx->wqh.lock);
 211                        schedule();
 212                        spin_lock_irq(&ctx->wqh.lock);
 213                }
 214                __remove_wait_queue(&ctx->wqh, &wait);
 215                __set_current_state(TASK_RUNNING);
 216        }
 217        if (likely(res == 0)) {
 218                eventfd_ctx_do_read(ctx, cnt);
 219                if (waitqueue_active(&ctx->wqh))
 220                        wake_up_locked_poll(&ctx->wqh, POLLOUT);
 221        }
 222        spin_unlock_irq(&ctx->wqh.lock);
 223
 224        return res;
 225}
 226EXPORT_SYMBOL_GPL(eventfd_ctx_read);
 227
 228static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
 229                            loff_t *ppos)
 230{
 231        struct eventfd_ctx *ctx = file->private_data;
 232        ssize_t res;
 233        __u64 cnt;
 234
 235        if (count < sizeof(cnt))
 236                return -EINVAL;
 237        res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt);
 238        if (res < 0)
 239                return res;
 240
 241        return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt);
 242}
 243
 244static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
 245                             loff_t *ppos)
 246{
 247        struct eventfd_ctx *ctx = file->private_data;
 248        ssize_t res;
 249        __u64 ucnt;
 250        DECLARE_WAITQUEUE(wait, current);
 251
 252        if (count < sizeof(ucnt))
 253                return -EINVAL;
 254        if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
 255                return -EFAULT;
 256        if (ucnt == ULLONG_MAX)
 257                return -EINVAL;
 258        spin_lock_irq(&ctx->wqh.lock);
 259        res = -EAGAIN;
 260        if (ULLONG_MAX - ctx->count > ucnt)
 261                res = sizeof(ucnt);
 262        else if (!(file->f_flags & O_NONBLOCK)) {
 263                __add_wait_queue(&ctx->wqh, &wait);
 264                for (res = 0;;) {
 265                        set_current_state(TASK_INTERRUPTIBLE);
 266                        if (ULLONG_MAX - ctx->count > ucnt) {
 267                                res = sizeof(ucnt);
 268                                break;
 269                        }
 270                        if (signal_pending(current)) {
 271                                res = -ERESTARTSYS;
 272                                break;
 273                        }
 274                        spin_unlock_irq(&ctx->wqh.lock);
 275                        schedule();
 276                        spin_lock_irq(&ctx->wqh.lock);
 277                }
 278                __remove_wait_queue(&ctx->wqh, &wait);
 279                __set_current_state(TASK_RUNNING);
 280        }
 281        if (likely(res > 0)) {
 282                ctx->count += ucnt;
 283                if (waitqueue_active(&ctx->wqh))
 284                        wake_up_locked_poll(&ctx->wqh, POLLIN);
 285        }
 286        spin_unlock_irq(&ctx->wqh.lock);
 287
 288        return res;
 289}
 290
 291static const struct file_operations eventfd_fops = {
 292        .release        = eventfd_release,
 293        .poll           = eventfd_poll,
 294        .read           = eventfd_read,
 295        .write          = eventfd_write,
 296        .llseek         = noop_llseek,
 297};
 298
 299/**
 300 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
 301 * @fd: [in] Eventfd file descriptor.
 302 *
 303 * Returns a pointer to the eventfd file structure in case of success, or the
 304 * following error pointer:
 305 *
 306 * -EBADF    : Invalid @fd file descriptor.
 307 * -EINVAL   : The @fd file descriptor is not an eventfd file.
 308 */
 309struct file *eventfd_fget(int fd)
 310{
 311        struct file *file;
 312
 313        file = fget(fd);
 314        if (!file)
 315                return ERR_PTR(-EBADF);
 316        if (file->f_op != &eventfd_fops) {
 317                fput(file);
 318                return ERR_PTR(-EINVAL);
 319        }
 320
 321        return file;
 322}
 323EXPORT_SYMBOL_GPL(eventfd_fget);
 324
 325/**
 326 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
 327 * @fd: [in] Eventfd file descriptor.
 328 *
 329 * Returns a pointer to the internal eventfd context, otherwise the error
 330 * pointers returned by the following functions:
 331 *
 332 * eventfd_fget
 333 */
 334struct eventfd_ctx *eventfd_ctx_fdget(int fd)
 335{
 336        struct file *file;
 337        struct eventfd_ctx *ctx;
 338
 339        file = eventfd_fget(fd);
 340        if (IS_ERR(file))
 341                return (struct eventfd_ctx *) file;
 342        ctx = eventfd_ctx_get(file->private_data);
 343        fput(file);
 344
 345        return ctx;
 346}
 347EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
 348
 349/**
 350 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
 351 * @file: [in] Eventfd file pointer.
 352 *
 353 * Returns a pointer to the internal eventfd context, otherwise the error
 354 * pointer:
 355 *
 356 * -EINVAL   : The @fd file descriptor is not an eventfd file.
 357 */
 358struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
 359{
 360        if (file->f_op != &eventfd_fops)
 361                return ERR_PTR(-EINVAL);
 362
 363        return eventfd_ctx_get(file->private_data);
 364}
 365EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
 366
 367/**
 368 * eventfd_file_create - Creates an eventfd file pointer.
 369 * @count: Initial eventfd counter value.
 370 * @flags: Flags for the eventfd file.
 371 *
 372 * This function creates an eventfd file pointer, w/out installing it into
 373 * the fd table. This is useful when the eventfd file is used during the
 374 * initialization of data structures that require extra setup after the eventfd
 375 * creation. So the eventfd creation is split into the file pointer creation
 376 * phase, and the file descriptor installation phase.
 377 * In this way races with userspace closing the newly installed file descriptor
 378 * can be avoided.
 379 * Returns an eventfd file pointer, or a proper error pointer.
 380 */
 381struct file *eventfd_file_create(unsigned int count, int flags)
 382{
 383        struct file *file;
 384        struct eventfd_ctx *ctx;
 385
 386        /* Check the EFD_* constants for consistency.  */
 387        BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
 388        BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
 389
 390        if (flags & ~EFD_FLAGS_SET)
 391                return ERR_PTR(-EINVAL);
 392
 393        ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
 394        if (!ctx)
 395                return ERR_PTR(-ENOMEM);
 396
 397        kref_init(&ctx->kref);
 398        init_waitqueue_head(&ctx->wqh);
 399        ctx->count = count;
 400        ctx->flags = flags;
 401
 402        file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,
 403                                  O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
 404        if (IS_ERR(file))
 405                eventfd_free_ctx(ctx);
 406
 407        return file;
 408}
 409
 410SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
 411{
 412        int fd, error;
 413        struct file *file;
 414
 415        error = get_unused_fd_flags(flags & EFD_SHARED_FCNTL_FLAGS);
 416        if (error < 0)
 417                return error;
 418        fd = error;
 419
 420        file = eventfd_file_create(count, flags);
 421        if (IS_ERR(file)) {
 422                error = PTR_ERR(file);
 423                goto err_put_unused_fd;
 424        }
 425        fd_install(fd, file);
 426
 427        return fd;
 428
 429err_put_unused_fd:
 430        put_unused_fd(fd);
 431
 432        return error;
 433}
 434
 435SYSCALL_DEFINE1(eventfd, unsigned int, count)
 436{
 437        return sys_eventfd2(count, 0);
 438}
 439
 440
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