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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 returining a POLLERR
 49  * to poll(2).
 50  *
 51  * Returns the amount by which the counter was incrememnted.  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         smp_rmb();
125         count = ctx->count;
126 
127         if (count > 0)
128                 events |= POLLIN;
129         if (count == ULLONG_MAX)
130                 events |= POLLERR;
131         if (ULLONG_MAX - 1 > count)
132                 events |= POLLOUT;
133 
134         return events;
135 }
136 
137 static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
138 {
139         *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
140         ctx->count -= *cnt;
141 }
142 
143 /**
144  * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
145  * @ctx: [in] Pointer to eventfd context.
146  * @wait: [in] Wait queue to be removed.
147  * @cnt: [out] Pointer to the 64-bit counter value.
148  *
149  * Returns %0 if successful, or the following error codes:
150  *
151  * -EAGAIN      : The operation would have blocked.
152  *
153  * This is used to atomically remove a wait queue entry from the eventfd wait
154  * queue head, and read/reset the counter value.
155  */
156 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
157                                   __u64 *cnt)
158 {
159         unsigned long flags;
160 
161         spin_lock_irqsave(&ctx->wqh.lock, flags);
162         eventfd_ctx_do_read(ctx, cnt);
163         __remove_wait_queue(&ctx->wqh, wait);
164         if (*cnt != 0 && waitqueue_active(&ctx->wqh))
165                 wake_up_locked_poll(&ctx->wqh, POLLOUT);
166         spin_unlock_irqrestore(&ctx->wqh.lock, flags);
167 
168         return *cnt != 0 ? 0 : -EAGAIN;
169 }
170 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
171 
172 /**
173  * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
174  * @ctx: [in] Pointer to eventfd context.
175  * @no_wait: [in] Different from zero if the operation should not block.
176  * @cnt: [out] Pointer to the 64-bit counter value.
177  *
178  * Returns %0 if successful, or the following error codes:
179  *
180  * -EAGAIN      : The operation would have blocked but @no_wait was non-zero.
181  * -ERESTARTSYS : A signal interrupted the wait operation.
182  *
183  * If @no_wait is zero, the function might sleep until the eventfd internal
184  * counter becomes greater than zero.
185  */
186 ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
187 {
188         ssize_t res;
189         DECLARE_WAITQUEUE(wait, current);
190 
191         spin_lock_irq(&ctx->wqh.lock);
192         *cnt = 0;
193         res = -EAGAIN;
194         if (ctx->count > 0)
195                 res = 0;
196         else if (!no_wait) {
197                 __add_wait_queue(&ctx->wqh, &wait);
198                 for (;;) {
199                         set_current_state(TASK_INTERRUPTIBLE);
200                         if (ctx->count > 0) {
201                                 res = 0;
202                                 break;
203                         }
204                         if (signal_pending(current)) {
205                                 res = -ERESTARTSYS;
206                                 break;
207                         }
208                         spin_unlock_irq(&ctx->wqh.lock);
209                         schedule();
210                         spin_lock_irq(&ctx->wqh.lock);
211                 }
212                 __remove_wait_queue(&ctx->wqh, &wait);
213                 __set_current_state(TASK_RUNNING);
214         }
215         if (likely(res == 0)) {
216                 eventfd_ctx_do_read(ctx, cnt);
217                 if (waitqueue_active(&ctx->wqh))
218                         wake_up_locked_poll(&ctx->wqh, POLLOUT);
219         }
220         spin_unlock_irq(&ctx->wqh.lock);
221 
222         return res;
223 }
224 EXPORT_SYMBOL_GPL(eventfd_ctx_read);
225 
226 static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
227                             loff_t *ppos)
228 {
229         struct eventfd_ctx *ctx = file->private_data;
230         ssize_t res;
231         __u64 cnt;
232 
233         if (count < sizeof(cnt))
234                 return -EINVAL;
235         res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt);
236         if (res < 0)
237                 return res;
238 
239         return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt);
240 }
241 
242 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
243                              loff_t *ppos)
244 {
245         struct eventfd_ctx *ctx = file->private_data;
246         ssize_t res;
247         __u64 ucnt;
248         DECLARE_WAITQUEUE(wait, current);
249 
250         if (count < sizeof(ucnt))
251                 return -EINVAL;
252         if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
253                 return -EFAULT;
254         if (ucnt == ULLONG_MAX)
255                 return -EINVAL;
256         spin_lock_irq(&ctx->wqh.lock);
257         res = -EAGAIN;
258         if (ULLONG_MAX - ctx->count > ucnt)
259                 res = sizeof(ucnt);
260         else if (!(file->f_flags & O_NONBLOCK)) {
261                 __add_wait_queue(&ctx->wqh, &wait);
262                 for (res = 0;;) {
263                         set_current_state(TASK_INTERRUPTIBLE);
264                         if (ULLONG_MAX - ctx->count > ucnt) {
265                                 res = sizeof(ucnt);
266                                 break;
267                         }
268                         if (signal_pending(current)) {
269                                 res = -ERESTARTSYS;
270                                 break;
271                         }
272                         spin_unlock_irq(&ctx->wqh.lock);
273                         schedule();
274                         spin_lock_irq(&ctx->wqh.lock);
275                 }
276                 __remove_wait_queue(&ctx->wqh, &wait);
277                 __set_current_state(TASK_RUNNING);
278         }
279         if (likely(res > 0)) {
280                 ctx->count += ucnt;
281                 if (waitqueue_active(&ctx->wqh))
282                         wake_up_locked_poll(&ctx->wqh, POLLIN);
283         }
284         spin_unlock_irq(&ctx->wqh.lock);
285 
286         return res;
287 }
288 
289 #ifdef CONFIG_PROC_FS
290 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
291 {
292         struct eventfd_ctx *ctx = f->private_data;
293 
294         spin_lock_irq(&ctx->wqh.lock);
295         seq_printf(m, "eventfd-count: %16llx\n",
296                    (unsigned long long)ctx->count);
297         spin_unlock_irq(&ctx->wqh.lock);
298 }
299 #endif
300 
301 static const struct file_operations eventfd_fops = {
302 #ifdef CONFIG_PROC_FS
303         .show_fdinfo    = eventfd_show_fdinfo,
304 #endif
305         .release        = eventfd_release,
306         .poll           = eventfd_poll,
307         .read           = eventfd_read,
308         .write          = eventfd_write,
309         .llseek         = noop_llseek,
310 };
311 
312 /**
313  * eventfd_fget - Acquire a reference of an eventfd file descriptor.
314  * @fd: [in] Eventfd file descriptor.
315  *
316  * Returns a pointer to the eventfd file structure in case of success, or the
317  * following error pointer:
318  *
319  * -EBADF    : Invalid @fd file descriptor.
320  * -EINVAL   : The @fd file descriptor is not an eventfd file.
321  */
322 struct file *eventfd_fget(int fd)
323 {
324         struct file *file;
325 
326         file = fget(fd);
327         if (!file)
328                 return ERR_PTR(-EBADF);
329         if (file->f_op != &eventfd_fops) {
330                 fput(file);
331                 return ERR_PTR(-EINVAL);
332         }
333 
334         return file;
335 }
336 EXPORT_SYMBOL_GPL(eventfd_fget);
337 
338 /**
339  * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
340  * @fd: [in] Eventfd file descriptor.
341  *
342  * Returns a pointer to the internal eventfd context, otherwise the error
343  * pointers returned by the following functions:
344  *
345  * eventfd_fget
346  */
347 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
348 {
349         struct eventfd_ctx *ctx;
350         struct fd f = fdget(fd);
351         if (!f.file)
352                 return ERR_PTR(-EBADF);
353         ctx = eventfd_ctx_fileget(f.file);
354         fdput(f);
355         return ctx;
356 }
357 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
358 
359 /**
360  * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
361  * @file: [in] Eventfd file pointer.
362  *
363  * Returns a pointer to the internal eventfd context, otherwise the error
364  * pointer:
365  *
366  * -EINVAL   : The @fd file descriptor is not an eventfd file.
367  */
368 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
369 {
370         if (file->f_op != &eventfd_fops)
371                 return ERR_PTR(-EINVAL);
372 
373         return eventfd_ctx_get(file->private_data);
374 }
375 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
376 
377 /**
378  * eventfd_file_create - Creates an eventfd file pointer.
379  * @count: Initial eventfd counter value.
380  * @flags: Flags for the eventfd file.
381  *
382  * This function creates an eventfd file pointer, w/out installing it into
383  * the fd table. This is useful when the eventfd file is used during the
384  * initialization of data structures that require extra setup after the eventfd
385  * creation. So the eventfd creation is split into the file pointer creation
386  * phase, and the file descriptor installation phase.
387  * In this way races with userspace closing the newly installed file descriptor
388  * can be avoided.
389  * Returns an eventfd file pointer, or a proper error pointer.
390  */
391 struct file *eventfd_file_create(unsigned int count, int flags)
392 {
393         struct file *file;
394         struct eventfd_ctx *ctx;
395 
396         /* Check the EFD_* constants for consistency.  */
397         BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
398         BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
399 
400         if (flags & ~EFD_FLAGS_SET)
401                 return ERR_PTR(-EINVAL);
402 
403         ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
404         if (!ctx)
405                 return ERR_PTR(-ENOMEM);
406 
407         kref_init(&ctx->kref);
408         init_waitqueue_head(&ctx->wqh);
409         ctx->count = count;
410         ctx->flags = flags;
411 
412         file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,
413                                   O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
414         if (IS_ERR(file))
415                 eventfd_free_ctx(ctx);
416 
417         return file;
418 }
419 
420 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
421 {
422         int fd, error;
423         struct file *file;
424 
425         error = get_unused_fd_flags(flags & EFD_SHARED_FCNTL_FLAGS);
426         if (error < 0)
427                 return error;
428         fd = error;
429 
430         file = eventfd_file_create(count, flags);
431         if (IS_ERR(file)) {
432                 error = PTR_ERR(file);
433                 goto err_put_unused_fd;
434         }
435         fd_install(fd, file);
436 
437         return fd;
438 
439 err_put_unused_fd:
440         put_unused_fd(fd);
441 
442         return error;
443 }
444 
445 SYSCALL_DEFINE1(eventfd, unsigned int, count)
446 {
447         return sys_eventfd2(count, 0);
448 }
449 
450 

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