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