1 // SPDX-License-Identifier: GPL-2.0-only
5 * Copyright (C) 2007 Davide Libenzi <davidel@xmailserver.org>
9 #include <linux/file.h>
10 #include <linux/poll.h>
11 #include <linux/init.h>
13 #include <linux/sched/signal.h>
14 #include <linux/kernel.h>
15 #include <linux/slab.h>
16 #include <linux/list.h>
17 #include <linux/spinlock.h>
18 #include <linux/anon_inodes.h>
19 #include <linux/syscalls.h>
20 #include <linux/export.h>
21 #include <linux/kref.h>
22 #include <linux/eventfd.h>
23 #include <linux/proc_fs.h>
24 #include <linux/seq_file.h>
25 #include <linux/idr.h>
26 #include <linux/uio.h>
28 static DEFINE_IDA(eventfd_ida
);
32 wait_queue_head_t wqh
;
34 * Every time that a write(2) is performed on an eventfd, the
35 * value of the __u64 being written is added to "count" and a
36 * wakeup is performed on "wqh". A read(2) will return the "count"
37 * value to userspace, and will reset "count" to zero. The kernel
38 * side eventfd_signal() also, adds to the "count" counter and
47 * eventfd_signal - Adds @n to the eventfd counter.
48 * @ctx: [in] Pointer to the eventfd context.
49 * @n: [in] Value of the counter to be added to the eventfd internal counter.
50 * The value cannot be negative.
52 * This function is supposed to be called by the kernel in paths that do not
53 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
54 * value, and we signal this as overflow condition by returning a EPOLLERR
57 * Returns the amount by which the counter was incremented. This will be less
58 * than @n if the counter has overflowed.
60 __u64
eventfd_signal(struct eventfd_ctx
*ctx
, __u64 n
)
65 * Deadlock or stack overflow issues can happen if we recurse here
66 * through waitqueue wakeup handlers. If the caller users potentially
67 * nested waitqueues with custom wakeup handlers, then it should
68 * check eventfd_signal_allowed() before calling this function. If
69 * it returns false, the eventfd_signal() call should be deferred to a
72 if (WARN_ON_ONCE(current
->in_eventfd_signal
))
75 spin_lock_irqsave(&ctx
->wqh
.lock
, flags
);
76 current
->in_eventfd_signal
= 1;
77 if (ULLONG_MAX
- ctx
->count
< n
)
78 n
= ULLONG_MAX
- ctx
->count
;
80 if (waitqueue_active(&ctx
->wqh
))
81 wake_up_locked_poll(&ctx
->wqh
, EPOLLIN
);
82 current
->in_eventfd_signal
= 0;
83 spin_unlock_irqrestore(&ctx
->wqh
.lock
, flags
);
87 EXPORT_SYMBOL_GPL(eventfd_signal
);
89 static void eventfd_free_ctx(struct eventfd_ctx
*ctx
)
92 ida_simple_remove(&eventfd_ida
, ctx
->id
);
96 static void eventfd_free(struct kref
*kref
)
98 struct eventfd_ctx
*ctx
= container_of(kref
, struct eventfd_ctx
, kref
);
100 eventfd_free_ctx(ctx
);
104 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
105 * @ctx: [in] Pointer to eventfd context.
107 * The eventfd context reference must have been previously acquired either
108 * with eventfd_ctx_fdget() or eventfd_ctx_fileget().
110 void eventfd_ctx_put(struct eventfd_ctx
*ctx
)
112 kref_put(&ctx
->kref
, eventfd_free
);
114 EXPORT_SYMBOL_GPL(eventfd_ctx_put
);
116 static int eventfd_release(struct inode
*inode
, struct file
*file
)
118 struct eventfd_ctx
*ctx
= file
->private_data
;
120 wake_up_poll(&ctx
->wqh
, EPOLLHUP
);
121 eventfd_ctx_put(ctx
);
125 static __poll_t
eventfd_poll(struct file
*file
, poll_table
*wait
)
127 struct eventfd_ctx
*ctx
= file
->private_data
;
131 poll_wait(file
, &ctx
->wqh
, wait
);
134 * All writes to ctx->count occur within ctx->wqh.lock. This read
135 * can be done outside ctx->wqh.lock because we know that poll_wait
136 * takes that lock (through add_wait_queue) if our caller will sleep.
138 * The read _can_ therefore seep into add_wait_queue's critical
139 * section, but cannot move above it! add_wait_queue's spin_lock acts
140 * as an acquire barrier and ensures that the read be ordered properly
141 * against the writes. The following CAN happen and is safe:
144 * ----------------- ------------
145 * lock ctx->wqh.lock (in poll_wait)
148 * unlock ctx->wqh.lock
151 * if (waitqueue_active)
152 * wake_up_locked_poll
153 * unlock ctx->qwh.lock
154 * eventfd_poll returns 0
156 * but the following, which would miss a wakeup, cannot happen:
159 * ----------------- ------------
160 * count = ctx->count (INVALID!)
163 * **waitqueue_active is false**
164 * **no wake_up_locked_poll!**
165 * unlock ctx->qwh.lock
166 * lock ctx->wqh.lock (in poll_wait)
168 * unlock ctx->wqh.lock
169 * eventfd_poll returns 0
171 count
= READ_ONCE(ctx
->count
);
175 if (count
== ULLONG_MAX
)
177 if (ULLONG_MAX
- 1 > count
)
183 void eventfd_ctx_do_read(struct eventfd_ctx
*ctx
, __u64
*cnt
)
185 lockdep_assert_held(&ctx
->wqh
.lock
);
187 *cnt
= (ctx
->flags
& EFD_SEMAPHORE
) ? 1 : ctx
->count
;
190 EXPORT_SYMBOL_GPL(eventfd_ctx_do_read
);
193 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
194 * @ctx: [in] Pointer to eventfd context.
195 * @wait: [in] Wait queue to be removed.
196 * @cnt: [out] Pointer to the 64-bit counter value.
198 * Returns %0 if successful, or the following error codes:
200 * -EAGAIN : The operation would have blocked.
202 * This is used to atomically remove a wait queue entry from the eventfd wait
203 * queue head, and read/reset the counter value.
205 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx
*ctx
, wait_queue_entry_t
*wait
,
210 spin_lock_irqsave(&ctx
->wqh
.lock
, flags
);
211 eventfd_ctx_do_read(ctx
, cnt
);
212 __remove_wait_queue(&ctx
->wqh
, wait
);
213 if (*cnt
!= 0 && waitqueue_active(&ctx
->wqh
))
214 wake_up_locked_poll(&ctx
->wqh
, EPOLLOUT
);
215 spin_unlock_irqrestore(&ctx
->wqh
.lock
, flags
);
217 return *cnt
!= 0 ? 0 : -EAGAIN
;
219 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue
);
221 static ssize_t
eventfd_read(struct kiocb
*iocb
, struct iov_iter
*to
)
223 struct file
*file
= iocb
->ki_filp
;
224 struct eventfd_ctx
*ctx
= file
->private_data
;
226 DECLARE_WAITQUEUE(wait
, current
);
228 if (iov_iter_count(to
) < sizeof(ucnt
))
230 spin_lock_irq(&ctx
->wqh
.lock
);
232 if ((file
->f_flags
& O_NONBLOCK
) ||
233 (iocb
->ki_flags
& IOCB_NOWAIT
)) {
234 spin_unlock_irq(&ctx
->wqh
.lock
);
237 __add_wait_queue(&ctx
->wqh
, &wait
);
239 set_current_state(TASK_INTERRUPTIBLE
);
242 if (signal_pending(current
)) {
243 __remove_wait_queue(&ctx
->wqh
, &wait
);
244 __set_current_state(TASK_RUNNING
);
245 spin_unlock_irq(&ctx
->wqh
.lock
);
248 spin_unlock_irq(&ctx
->wqh
.lock
);
250 spin_lock_irq(&ctx
->wqh
.lock
);
252 __remove_wait_queue(&ctx
->wqh
, &wait
);
253 __set_current_state(TASK_RUNNING
);
255 eventfd_ctx_do_read(ctx
, &ucnt
);
256 if (waitqueue_active(&ctx
->wqh
))
257 wake_up_locked_poll(&ctx
->wqh
, EPOLLOUT
);
258 spin_unlock_irq(&ctx
->wqh
.lock
);
259 if (unlikely(copy_to_iter(&ucnt
, sizeof(ucnt
), to
) != sizeof(ucnt
)))
265 static ssize_t
eventfd_write(struct file
*file
, const char __user
*buf
, size_t count
,
268 struct eventfd_ctx
*ctx
= file
->private_data
;
271 DECLARE_WAITQUEUE(wait
, current
);
273 if (count
< sizeof(ucnt
))
275 if (copy_from_user(&ucnt
, buf
, sizeof(ucnt
)))
277 if (ucnt
== ULLONG_MAX
)
279 spin_lock_irq(&ctx
->wqh
.lock
);
281 if (ULLONG_MAX
- ctx
->count
> ucnt
)
283 else if (!(file
->f_flags
& O_NONBLOCK
)) {
284 __add_wait_queue(&ctx
->wqh
, &wait
);
286 set_current_state(TASK_INTERRUPTIBLE
);
287 if (ULLONG_MAX
- ctx
->count
> ucnt
) {
291 if (signal_pending(current
)) {
295 spin_unlock_irq(&ctx
->wqh
.lock
);
297 spin_lock_irq(&ctx
->wqh
.lock
);
299 __remove_wait_queue(&ctx
->wqh
, &wait
);
300 __set_current_state(TASK_RUNNING
);
302 if (likely(res
> 0)) {
304 if (waitqueue_active(&ctx
->wqh
))
305 wake_up_locked_poll(&ctx
->wqh
, EPOLLIN
);
307 spin_unlock_irq(&ctx
->wqh
.lock
);
312 #ifdef CONFIG_PROC_FS
313 static void eventfd_show_fdinfo(struct seq_file
*m
, struct file
*f
)
315 struct eventfd_ctx
*ctx
= f
->private_data
;
317 spin_lock_irq(&ctx
->wqh
.lock
);
318 seq_printf(m
, "eventfd-count: %16llx\n",
319 (unsigned long long)ctx
->count
);
320 spin_unlock_irq(&ctx
->wqh
.lock
);
321 seq_printf(m
, "eventfd-id: %d\n", ctx
->id
);
325 static const struct file_operations eventfd_fops
= {
326 #ifdef CONFIG_PROC_FS
327 .show_fdinfo
= eventfd_show_fdinfo
,
329 .release
= eventfd_release
,
330 .poll
= eventfd_poll
,
331 .read_iter
= eventfd_read
,
332 .write
= eventfd_write
,
333 .llseek
= noop_llseek
,
337 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
338 * @fd: [in] Eventfd file descriptor.
340 * Returns a pointer to the eventfd file structure in case of success, or the
341 * following error pointer:
343 * -EBADF : Invalid @fd file descriptor.
344 * -EINVAL : The @fd file descriptor is not an eventfd file.
346 struct file
*eventfd_fget(int fd
)
352 return ERR_PTR(-EBADF
);
353 if (file
->f_op
!= &eventfd_fops
) {
355 return ERR_PTR(-EINVAL
);
360 EXPORT_SYMBOL_GPL(eventfd_fget
);
363 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
364 * @fd: [in] Eventfd file descriptor.
366 * Returns a pointer to the internal eventfd context, otherwise the error
367 * pointers returned by the following functions:
371 struct eventfd_ctx
*eventfd_ctx_fdget(int fd
)
373 struct eventfd_ctx
*ctx
;
374 struct fd f
= fdget(fd
);
376 return ERR_PTR(-EBADF
);
377 ctx
= eventfd_ctx_fileget(f
.file
);
381 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget
);
384 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
385 * @file: [in] Eventfd file pointer.
387 * Returns a pointer to the internal eventfd context, otherwise the error
390 * -EINVAL : The @fd file descriptor is not an eventfd file.
392 struct eventfd_ctx
*eventfd_ctx_fileget(struct file
*file
)
394 struct eventfd_ctx
*ctx
;
396 if (file
->f_op
!= &eventfd_fops
)
397 return ERR_PTR(-EINVAL
);
399 ctx
= file
->private_data
;
400 kref_get(&ctx
->kref
);
403 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget
);
405 static int do_eventfd(unsigned int count
, int flags
)
407 struct eventfd_ctx
*ctx
;
411 /* Check the EFD_* constants for consistency. */
412 BUILD_BUG_ON(EFD_CLOEXEC
!= O_CLOEXEC
);
413 BUILD_BUG_ON(EFD_NONBLOCK
!= O_NONBLOCK
);
415 if (flags
& ~EFD_FLAGS_SET
)
418 ctx
= kmalloc(sizeof(*ctx
), GFP_KERNEL
);
422 kref_init(&ctx
->kref
);
423 init_waitqueue_head(&ctx
->wqh
);
426 ctx
->id
= ida_simple_get(&eventfd_ida
, 0, 0, GFP_KERNEL
);
428 flags
&= EFD_SHARED_FCNTL_FLAGS
;
430 fd
= get_unused_fd_flags(flags
);
434 file
= anon_inode_getfile("[eventfd]", &eventfd_fops
, ctx
, flags
);
441 file
->f_mode
|= FMODE_NOWAIT
;
442 fd_install(fd
, file
);
445 eventfd_free_ctx(ctx
);
449 SYSCALL_DEFINE2(eventfd2
, unsigned int, count
, int, flags
)
451 return do_eventfd(count
, flags
);
454 SYSCALL_DEFINE1(eventfd
, unsigned int, count
)
456 return do_eventfd(count
, 0);