Root/fs/eventfd.c

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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