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Linux/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/export.h>
 20 #include <linux/kref.h>
 21 #include <linux/eventfd.h>
 22 #include <linux/proc_fs.h>
 23 #include <linux/seq_file.h>
 24 
 25 struct eventfd_ctx {
 26         struct kref kref;
 27         wait_queue_head_t wqh;
 28         /*
 29          * Every time that a write(2) is performed on an eventfd, the
 30          * value of the __u64 being written is added to "count" and a
 31          * wakeup is performed on "wqh". A read(2) will return the "count"
 32          * value to userspace, and will reset "count" to zero. The kernel
 33          * side eventfd_signal() also, adds to the "count" counter and
 34          * issue a wakeup.
 35          */
 36         __u64 count;
 37         unsigned int flags;
 38 };
 39 
 40 /**
 41  * eventfd_signal - Adds @n to the eventfd counter.
 42  * @ctx: [in] Pointer to the eventfd context.
 43  * @n: [in] Value of the counter to be added to the eventfd internal counter.
 44  *          The value cannot be negative.
 45  *
 46  * This function is supposed to be called by the kernel in paths that do not
 47  * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
 48  * value, and we signal this as overflow condition by returning a POLLERR
 49  * to poll(2).
 50  *
 51  * Returns the amount by which the counter was incremented.  This will be less
 52  * than @n if the counter has overflowed.
 53  */
 54 __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
 55 {
 56         unsigned long flags;
 57 
 58         spin_lock_irqsave(&ctx->wqh.lock, flags);
 59         if (ULLONG_MAX - ctx->count < n)
 60                 n = ULLONG_MAX - ctx->count;
 61         ctx->count += n;
 62         if (waitqueue_active(&ctx->wqh))
 63                 wake_up_locked_poll(&ctx->wqh, POLLIN);
 64         spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 65 
 66         return n;
 67 }
 68 EXPORT_SYMBOL_GPL(eventfd_signal);
 69 
 70 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
 71 {
 72         kfree(ctx);
 73 }
 74 
 75 static void eventfd_free(struct kref *kref)
 76 {
 77         struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
 78 
 79         eventfd_free_ctx(ctx);
 80 }
 81 
 82 /**
 83  * eventfd_ctx_get - Acquires a reference to the internal eventfd context.
 84  * @ctx: [in] Pointer to the eventfd context.
 85  *
 86  * Returns: In case of success, returns a pointer to the eventfd context.
 87  */
 88 struct eventfd_ctx *eventfd_ctx_get(struct eventfd_ctx *ctx)
 89 {
 90         kref_get(&ctx->kref);
 91         return ctx;
 92 }
 93 EXPORT_SYMBOL_GPL(eventfd_ctx_get);
 94 
 95 /**
 96  * eventfd_ctx_put - Releases a reference to the internal eventfd context.
 97  * @ctx: [in] Pointer to eventfd context.
 98  *
 99  * The eventfd context reference must have been previously acquired either
100  * with eventfd_ctx_get() or eventfd_ctx_fdget().
101  */
102 void eventfd_ctx_put(struct eventfd_ctx *ctx)
103 {
104         kref_put(&ctx->kref, eventfd_free);
105 }
106 EXPORT_SYMBOL_GPL(eventfd_ctx_put);
107 
108 static int eventfd_release(struct inode *inode, struct file *file)
109 {
110         struct eventfd_ctx *ctx = file->private_data;
111 
112         wake_up_poll(&ctx->wqh, POLLHUP);
113         eventfd_ctx_put(ctx);
114         return 0;
115 }
116 
117 static unsigned int eventfd_poll(struct file *file, poll_table *wait)
118 {
119         struct eventfd_ctx *ctx = file->private_data;
120         unsigned int events = 0;
121         u64 count;
122 
123         poll_wait(file, &ctx->wqh, wait);
124 
125         /*
126          * All writes to ctx->count occur within ctx->wqh.lock.  This read
127          * can be done outside ctx->wqh.lock because we know that poll_wait
128          * takes that lock (through add_wait_queue) if our caller will sleep.
129          *
130          * The read _can_ therefore seep into add_wait_queue's critical
131          * section, but cannot move above it!  add_wait_queue's spin_lock acts
132          * as an acquire barrier and ensures that the read be ordered properly
133          * against the writes.  The following CAN happen and is safe:
134          *
135          *     poll                               write
136          *     -----------------                  ------------
137          *     lock ctx->wqh.lock (in poll_wait)
138          *     count = ctx->count
139          *     __add_wait_queue
140          *     unlock ctx->wqh.lock
141          *                                        lock ctx->qwh.lock
142          *                                        ctx->count += n
143          *                                        if (waitqueue_active)
144          *                                          wake_up_locked_poll
145          *                                        unlock ctx->qwh.lock
146          *     eventfd_poll returns 0
147          *
148          * but the following, which would miss a wakeup, cannot happen:
149          *
150          *     poll                               write
151          *     -----------------                  ------------
152          *     count = ctx->count (INVALID!)
153          *                                        lock ctx->qwh.lock
154          *                                        ctx->count += n
155          *                                        **waitqueue_active is false**
156          *                                        **no wake_up_locked_poll!**
157          *                                        unlock ctx->qwh.lock
158          *     lock ctx->wqh.lock (in poll_wait)
159          *     __add_wait_queue
160          *     unlock ctx->wqh.lock
161          *     eventfd_poll returns 0
162          */
163         count = READ_ONCE(ctx->count);
164 
165         if (count > 0)
166                 events |= POLLIN;
167         if (count == ULLONG_MAX)
168                 events |= POLLERR;
169         if (ULLONG_MAX - 1 > count)
170                 events |= POLLOUT;
171 
172         return events;
173 }
174 
175 static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
176 {
177         *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
178         ctx->count -= *cnt;
179 }
180 
181 /**
182  * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
183  * @ctx: [in] Pointer to eventfd context.
184  * @wait: [in] Wait queue to be removed.
185  * @cnt: [out] Pointer to the 64-bit counter value.
186  *
187  * Returns %0 if successful, or the following error codes:
188  *
189  * -EAGAIN      : The operation would have blocked.
190  *
191  * This is used to atomically remove a wait queue entry from the eventfd wait
192  * queue head, and read/reset the counter value.
193  */
194 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
195                                   __u64 *cnt)
196 {
197         unsigned long flags;
198 
199         spin_lock_irqsave(&ctx->wqh.lock, flags);
200         eventfd_ctx_do_read(ctx, cnt);
201         __remove_wait_queue(&ctx->wqh, wait);
202         if (*cnt != 0 && waitqueue_active(&ctx->wqh))
203                 wake_up_locked_poll(&ctx->wqh, POLLOUT);
204         spin_unlock_irqrestore(&ctx->wqh.lock, flags);
205 
206         return *cnt != 0 ? 0 : -EAGAIN;
207 }
208 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
209 
210 /**
211  * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
212  * @ctx: [in] Pointer to eventfd context.
213  * @no_wait: [in] Different from zero if the operation should not block.
214  * @cnt: [out] Pointer to the 64-bit counter value.
215  *
216  * Returns %0 if successful, or the following error codes:
217  *
218  * -EAGAIN      : The operation would have blocked but @no_wait was non-zero.
219  * -ERESTARTSYS : A signal interrupted the wait operation.
220  *
221  * If @no_wait is zero, the function might sleep until the eventfd internal
222  * counter becomes greater than zero.
223  */
224 ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
225 {
226         ssize_t res;
227         DECLARE_WAITQUEUE(wait, current);
228 
229         spin_lock_irq(&ctx->wqh.lock);
230         *cnt = 0;
231         res = -EAGAIN;
232         if (ctx->count > 0)
233                 res = 0;
234         else if (!no_wait) {
235                 __add_wait_queue(&ctx->wqh, &wait);
236                 for (;;) {
237                         set_current_state(TASK_INTERRUPTIBLE);
238                         if (ctx->count > 0) {
239                                 res = 0;
240                                 break;
241                         }
242                         if (signal_pending(current)) {
243                                 res = -ERESTARTSYS;
244                                 break;
245                         }
246                         spin_unlock_irq(&ctx->wqh.lock);
247                         schedule();
248                         spin_lock_irq(&ctx->wqh.lock);
249                 }
250                 __remove_wait_queue(&ctx->wqh, &wait);
251                 __set_current_state(TASK_RUNNING);
252         }
253         if (likely(res == 0)) {
254                 eventfd_ctx_do_read(ctx, cnt);
255                 if (waitqueue_active(&ctx->wqh))
256                         wake_up_locked_poll(&ctx->wqh, POLLOUT);
257         }
258         spin_unlock_irq(&ctx->wqh.lock);
259 
260         return res;
261 }
262 EXPORT_SYMBOL_GPL(eventfd_ctx_read);
263 
264 static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
265                             loff_t *ppos)
266 {
267         struct eventfd_ctx *ctx = file->private_data;
268         ssize_t res;
269         __u64 cnt;
270 
271         if (count < sizeof(cnt))
272                 return -EINVAL;
273         res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt);
274         if (res < 0)
275                 return res;
276 
277         return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt);
278 }
279 
280 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
281                              loff_t *ppos)
282 {
283         struct eventfd_ctx *ctx = file->private_data;
284         ssize_t res;
285         __u64 ucnt;
286         DECLARE_WAITQUEUE(wait, current);
287 
288         if (count < sizeof(ucnt))
289                 return -EINVAL;
290         if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
291                 return -EFAULT;
292         if (ucnt == ULLONG_MAX)
293                 return -EINVAL;
294         spin_lock_irq(&ctx->wqh.lock);
295         res = -EAGAIN;
296         if (ULLONG_MAX - ctx->count > ucnt)
297                 res = sizeof(ucnt);
298         else if (!(file->f_flags & O_NONBLOCK)) {
299                 __add_wait_queue(&ctx->wqh, &wait);
300                 for (res = 0;;) {
301                         set_current_state(TASK_INTERRUPTIBLE);
302                         if (ULLONG_MAX - ctx->count > ucnt) {
303                                 res = sizeof(ucnt);
304                                 break;
305                         }
306                         if (signal_pending(current)) {
307                                 res = -ERESTARTSYS;
308                                 break;
309                         }
310                         spin_unlock_irq(&ctx->wqh.lock);
311                         schedule();
312                         spin_lock_irq(&ctx->wqh.lock);
313                 }
314                 __remove_wait_queue(&ctx->wqh, &wait);
315                 __set_current_state(TASK_RUNNING);
316         }
317         if (likely(res > 0)) {
318                 ctx->count += ucnt;
319                 if (waitqueue_active(&ctx->wqh))
320                         wake_up_locked_poll(&ctx->wqh, POLLIN);
321         }
322         spin_unlock_irq(&ctx->wqh.lock);
323 
324         return res;
325 }
326 
327 #ifdef CONFIG_PROC_FS
328 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
329 {
330         struct eventfd_ctx *ctx = f->private_data;
331 
332         spin_lock_irq(&ctx->wqh.lock);
333         seq_printf(m, "eventfd-count: %16llx\n",
334                    (unsigned long long)ctx->count);
335         spin_unlock_irq(&ctx->wqh.lock);
336 }
337 #endif
338 
339 static const struct file_operations eventfd_fops = {
340 #ifdef CONFIG_PROC_FS
341         .show_fdinfo    = eventfd_show_fdinfo,
342 #endif
343         .release        = eventfd_release,
344         .poll           = eventfd_poll,
345         .read           = eventfd_read,
346         .write          = eventfd_write,
347         .llseek         = noop_llseek,
348 };
349 
350 /**
351  * eventfd_fget - Acquire a reference of an eventfd file descriptor.
352  * @fd: [in] Eventfd file descriptor.
353  *
354  * Returns a pointer to the eventfd file structure in case of success, or the
355  * following error pointer:
356  *
357  * -EBADF    : Invalid @fd file descriptor.
358  * -EINVAL   : The @fd file descriptor is not an eventfd file.
359  */
360 struct file *eventfd_fget(int fd)
361 {
362         struct file *file;
363 
364         file = fget(fd);
365         if (!file)
366                 return ERR_PTR(-EBADF);
367         if (file->f_op != &eventfd_fops) {
368                 fput(file);
369                 return ERR_PTR(-EINVAL);
370         }
371 
372         return file;
373 }
374 EXPORT_SYMBOL_GPL(eventfd_fget);
375 
376 /**
377  * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
378  * @fd: [in] Eventfd file descriptor.
379  *
380  * Returns a pointer to the internal eventfd context, otherwise the error
381  * pointers returned by the following functions:
382  *
383  * eventfd_fget
384  */
385 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
386 {
387         struct eventfd_ctx *ctx;
388         struct fd f = fdget(fd);
389         if (!f.file)
390                 return ERR_PTR(-EBADF);
391         ctx = eventfd_ctx_fileget(f.file);
392         fdput(f);
393         return ctx;
394 }
395 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
396 
397 /**
398  * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
399  * @file: [in] Eventfd file pointer.
400  *
401  * Returns a pointer to the internal eventfd context, otherwise the error
402  * pointer:
403  *
404  * -EINVAL   : The @fd file descriptor is not an eventfd file.
405  */
406 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
407 {
408         if (file->f_op != &eventfd_fops)
409                 return ERR_PTR(-EINVAL);
410 
411         return eventfd_ctx_get(file->private_data);
412 }
413 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
414 
415 /**
416  * eventfd_file_create - Creates an eventfd file pointer.
417  * @count: Initial eventfd counter value.
418  * @flags: Flags for the eventfd file.
419  *
420  * This function creates an eventfd file pointer, w/out installing it into
421  * the fd table. This is useful when the eventfd file is used during the
422  * initialization of data structures that require extra setup after the eventfd
423  * creation. So the eventfd creation is split into the file pointer creation
424  * phase, and the file descriptor installation phase.
425  * In this way races with userspace closing the newly installed file descriptor
426  * can be avoided.
427  * Returns an eventfd file pointer, or a proper error pointer.
428  */
429 struct file *eventfd_file_create(unsigned int count, int flags)
430 {
431         struct file *file;
432         struct eventfd_ctx *ctx;
433 
434         /* Check the EFD_* constants for consistency.  */
435         BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
436         BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
437 
438         if (flags & ~EFD_FLAGS_SET)
439                 return ERR_PTR(-EINVAL);
440 
441         ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
442         if (!ctx)
443                 return ERR_PTR(-ENOMEM);
444 
445         kref_init(&ctx->kref);
446         init_waitqueue_head(&ctx->wqh);
447         ctx->count = count;
448         ctx->flags = flags;
449 
450         file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,
451                                   O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
452         if (IS_ERR(file))
453                 eventfd_free_ctx(ctx);
454 
455         return file;
456 }
457 
458 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
459 {
460         int fd, error;
461         struct file *file;
462 
463         error = get_unused_fd_flags(flags & EFD_SHARED_FCNTL_FLAGS);
464         if (error < 0)
465                 return error;
466         fd = error;
467 
468         file = eventfd_file_create(count, flags);
469         if (IS_ERR(file)) {
470                 error = PTR_ERR(file);
471                 goto err_put_unused_fd;
472         }
473         fd_install(fd, file);
474 
475         return fd;
476 
477 err_put_unused_fd:
478         put_unused_fd(fd);
479 
480         return error;
481 }
482 
483 SYSCALL_DEFINE1(eventfd, unsigned int, count)
484 {
485         return sys_eventfd2(count, 0);
486 }
487 
488 

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