/* * fs/eventfd.c * * Copyright (C) 2007 Davide Libenzi <davidel@xmailserver.org> * */ #include <linux/file.h> #include <linux/poll.h> #include <linux/init.h> #include <linux/fs.h> #include <linux/sched/signal.h> #include <linux/kernel.h> #include <linux/slab.h> #include <linux/list.h> #include <linux/spinlock.h> #include <linux/anon_inodes.h> #include <linux/syscalls.h> #include <linux/export.h> #include <linux/kref.h> #include <linux/eventfd.h> #include <linux/proc_fs.h> #include <linux/seq_file.h> struct eventfd_ctx { struct kref kref; wait_queue_head_t wqh; /* * Every time that a write(2) is performed on an eventfd, the * value of the __u64 being written is added to "count" and a * wakeup is performed on "wqh". A read(2) will return the "count" * value to userspace, and will reset "count" to zero. The kernel * side eventfd_signal() also, adds to the "count" counter and * issue a wakeup. */ __u64 count; unsigned int flags; }; /** * eventfd_signal - Adds @n to the eventfd counter. * @ctx: [in] Pointer to the eventfd context. * @n: [in] Value of the counter to be added to the eventfd internal counter. * The value cannot be negative. * * This function is supposed to be called by the kernel in paths that do not * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX * value, and we signal this as overflow condition by returning a POLLERR * to poll(2). * * Returns the amount by which the counter was incremented. This will be less * than @n if the counter has overflowed. */ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) { unsigned long flags; spin_lock_irqsave(&ctx->wqh.lock, flags); if (ULLONG_MAX - ctx->count < n) n = ULLONG_MAX - ctx->count; ctx->count += n; if (waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, POLLIN); spin_unlock_irqrestore(&ctx->wqh.lock, flags); return n; } EXPORT_SYMBOL_GPL(eventfd_signal); static void eventfd_free_ctx(struct eventfd_ctx *ctx) { kfree(ctx); } static void eventfd_free(struct kref *kref) { struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref); eventfd_free_ctx(ctx); } /** * eventfd_ctx_get - Acquires a reference to the internal eventfd context. * @ctx: [in] Pointer to the eventfd context. * * Returns: In case of success, returns a pointer to the eventfd context. */ struct eventfd_ctx *eventfd_ctx_get(struct eventfd_ctx *ctx) { kref_get(&ctx->kref); return ctx; } EXPORT_SYMBOL_GPL(eventfd_ctx_get); /** * eventfd_ctx_put - Releases a reference to the internal eventfd context. * @ctx: [in] Pointer to eventfd context. * * The eventfd context reference must have been previously acquired either * with eventfd_ctx_get() or eventfd_ctx_fdget(). */ void eventfd_ctx_put(struct eventfd_ctx *ctx) { kref_put(&ctx->kref, eventfd_free); } EXPORT_SYMBOL_GPL(eventfd_ctx_put); static int eventfd_release(struct inode *inode, struct file *file) { struct eventfd_ctx *ctx = file->private_data; wake_up_poll(&ctx->wqh, POLLHUP); eventfd_ctx_put(ctx); return 0; } static unsigned int eventfd_poll(struct file *file, poll_table *wait) { struct eventfd_ctx *ctx = file->private_data; unsigned int events = 0; u64 count; poll_wait(file, &ctx->wqh, wait); /* * All writes to ctx->count occur within ctx->wqh.lock. This read * can be done outside ctx->wqh.lock because we know that poll_wait * takes that lock (through add_wait_queue) if our caller will sleep. * * The read _can_ therefore seep into add_wait_queue's critical * section, but cannot move above it! add_wait_queue's spin_lock acts * as an acquire barrier and ensures that the read be ordered properly * against the writes. The following CAN happen and is safe: * * poll write * ----------------- ------------ * lock ctx->wqh.lock (in poll_wait) * count = ctx->count * __add_wait_queue * unlock ctx->wqh.lock * lock ctx->qwh.lock * ctx->count += n * if (waitqueue_active) * wake_up_locked_poll * unlock ctx->qwh.lock * eventfd_poll returns 0 * * but the following, which would miss a wakeup, cannot happen: * * poll write * ----------------- ------------ * count = ctx->count (INVALID!) * lock ctx->qwh.lock * ctx->count += n * **waitqueue_active is false** * **no wake_up_locked_poll!** * unlock ctx->qwh.lock * lock ctx->wqh.lock (in poll_wait) * __add_wait_queue * unlock ctx->wqh.lock * eventfd_poll returns 0 */ count = READ_ONCE(ctx->count); if (count > 0) events |= POLLIN; if (count == ULLONG_MAX) events |= POLLERR; if (ULLONG_MAX - 1 > count) events |= POLLOUT; return events; } static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt) { *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count; ctx->count -= *cnt; } /** * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue. * @ctx: [in] Pointer to eventfd context. * @wait: [in] Wait queue to be removed. * @cnt: [out] Pointer to the 64-bit counter value. * * Returns %0 if successful, or the following error codes: * * -EAGAIN : The operation would have blocked. * * This is used to atomically remove a wait queue entry from the eventfd wait * queue head, and read/reset the counter value. */ int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait, __u64 *cnt) { unsigned long flags; spin_lock_irqsave(&ctx->wqh.lock, flags); eventfd_ctx_do_read(ctx, cnt); __remove_wait_queue(&ctx->wqh, wait); if (*cnt != 0 && waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, POLLOUT); spin_unlock_irqrestore(&ctx->wqh.lock, flags); return *cnt != 0 ? 0 : -EAGAIN; } EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue); /** * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero. * @ctx: [in] Pointer to eventfd context. * @no_wait: [in] Different from zero if the operation should not block. * @cnt: [out] Pointer to the 64-bit counter value. * * Returns %0 if successful, or the following error codes: * * - -EAGAIN : The operation would have blocked but @no_wait was non-zero. * - -ERESTARTSYS : A signal interrupted the wait operation. * * If @no_wait is zero, the function might sleep until the eventfd internal * counter becomes greater than zero. */ ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt) { ssize_t res; DECLARE_WAITQUEUE(wait, current); spin_lock_irq(&ctx->wqh.lock); *cnt = 0; res = -EAGAIN; if (ctx->count > 0) res = 0; else if (!no_wait) { __add_wait_queue(&ctx->wqh, &wait); for (;;) { set_current_state(TASK_INTERRUPTIBLE); if (ctx->count > 0) { res = 0; break; } if (signal_pending(current)) { res = -ERESTARTSYS; break; } spin_unlock_irq(&ctx->wqh.lock); schedule(); spin_lock_irq(&ctx->wqh.lock); } __remove_wait_queue(&ctx->wqh, &wait); __set_current_state(TASK_RUNNING); } if (likely(res == 0)) { eventfd_ctx_do_read(ctx, cnt); if (waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, POLLOUT); } spin_unlock_irq(&ctx->wqh.lock); return res; } EXPORT_SYMBOL_GPL(eventfd_ctx_read); static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count, loff_t *ppos) { struct eventfd_ctx *ctx = file->private_data; ssize_t res; __u64 cnt; if (count < sizeof(cnt)) return -EINVAL; res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt); if (res < 0) return res; return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt); } static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos) { struct eventfd_ctx *ctx = file->private_data; ssize_t res; __u64 ucnt; DECLARE_WAITQUEUE(wait, current); if (count < sizeof(ucnt)) return -EINVAL; if (copy_from_user(&ucnt, buf, sizeof(ucnt))) return -EFAULT; if (ucnt == ULLONG_MAX) return -EINVAL; spin_lock_irq(&ctx->wqh.lock); res = -EAGAIN; if (ULLONG_MAX - ctx->count > ucnt) res = sizeof(ucnt); else if (!(file->f_flags & O_NONBLOCK)) { __add_wait_queue(&ctx->wqh, &wait); for (res = 0;;) { set_current_state(TASK_INTERRUPTIBLE); if (ULLONG_MAX - ctx->count > ucnt) { res = sizeof(ucnt); break; } if (signal_pending(current)) { res = -ERESTARTSYS; break; } spin_unlock_irq(&ctx->wqh.lock); schedule(); spin_lock_irq(&ctx->wqh.lock); } __remove_wait_queue(&ctx->wqh, &wait); __set_current_state(TASK_RUNNING); } if (likely(res > 0)) { ctx->count += ucnt; if (waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, POLLIN); } spin_unlock_irq(&ctx->wqh.lock); return res; } #ifdef CONFIG_PROC_FS static void eventfd_show_fdinfo(struct seq_file *m, struct file *f) { struct eventfd_ctx *ctx = f->private_data; spin_lock_irq(&ctx->wqh.lock); seq_printf(m, "eventfd-count: %16llx\n", (unsigned long long)ctx->count); spin_unlock_irq(&ctx->wqh.lock); } #endif static const struct file_operations eventfd_fops = { #ifdef CONFIG_PROC_FS .show_fdinfo = eventfd_show_fdinfo, #endif .release = eventfd_release, .poll = eventfd_poll, .read = eventfd_read, .write = eventfd_write, .llseek = noop_llseek, }; /** * eventfd_fget - Acquire a reference of an eventfd file descriptor. * @fd: [in] Eventfd file descriptor. * * Returns a pointer to the eventfd file structure in case of success, or the * following error pointer: * * -EBADF : Invalid @fd file descriptor. * -EINVAL : The @fd file descriptor is not an eventfd file. */ struct file *eventfd_fget(int fd) { struct file *file; file = fget(fd); if (!file) return ERR_PTR(-EBADF); if (file->f_op != &eventfd_fops) { fput(file); return ERR_PTR(-EINVAL); } return file; } EXPORT_SYMBOL_GPL(eventfd_fget); /** * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context. * @fd: [in] Eventfd file descriptor. * * Returns a pointer to the internal eventfd context, otherwise the error * pointers returned by the following functions: * * eventfd_fget */ struct eventfd_ctx *eventfd_ctx_fdget(int fd) { struct eventfd_ctx *ctx; struct fd f = fdget(fd); if (!f.file) return ERR_PTR(-EBADF); ctx = eventfd_ctx_fileget(f.file); fdput(f); return ctx; } EXPORT_SYMBOL_GPL(eventfd_ctx_fdget); /** * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context. * @file: [in] Eventfd file pointer. * * Returns a pointer to the internal eventfd context, otherwise the error * pointer: * * -EINVAL : The @fd file descriptor is not an eventfd file. */ struct eventfd_ctx *eventfd_ctx_fileget(struct file *file) { if (file->f_op != &eventfd_fops) return ERR_PTR(-EINVAL); return eventfd_ctx_get(file->private_data); } EXPORT_SYMBOL_GPL(eventfd_ctx_fileget); /** * eventfd_file_create - Creates an eventfd file pointer. * @count: Initial eventfd counter value. * @flags: Flags for the eventfd file. * * This function creates an eventfd file pointer, w/out installing it into * the fd table. This is useful when the eventfd file is used during the * initialization of data structures that require extra setup after the eventfd * creation. So the eventfd creation is split into the file pointer creation * phase, and the file descriptor installation phase. * In this way races with userspace closing the newly installed file descriptor * can be avoided. * Returns an eventfd file pointer, or a proper error pointer. */ struct file *eventfd_file_create(unsigned int count, int flags) { struct file *file; struct eventfd_ctx *ctx; /* Check the EFD_* constants for consistency. */ BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC); BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK); if (flags & ~EFD_FLAGS_SET) return ERR_PTR(-EINVAL); ctx = kmalloc(sizeof(*ctx), GFP_KERNEL); if (!ctx) return ERR_PTR(-ENOMEM); kref_init(&ctx->kref); init_waitqueue_head(&ctx->wqh); ctx->count = count; ctx->flags = flags; file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS)); if (IS_ERR(file)) eventfd_free_ctx(ctx); return file; } SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags) { int fd, error; struct file *file; error = get_unused_fd_flags(flags & EFD_SHARED_FCNTL_FLAGS); if (error < 0) return error; fd = error; file = eventfd_file_create(count, flags); if (IS_ERR(file)) { error = PTR_ERR(file); goto err_put_unused_fd; } fd_install(fd, file); return fd; err_put_unused_fd: put_unused_fd(fd); return error; } SYSCALL_DEFINE1(eventfd, unsigned int, count) { return sys_eventfd2(count, 0); }