Hi Paul, On 2020-06-19 7:16 p.m., Paul Elder wrote: > Hello Damian, Martin, and all, > > I came across this (quite old by now) patch to extend eventfd's polling > functionality. I was wondering what happened to it (why it hasn't been > merged yet) and if we could, or what is needed to, move it forward. I think there was an open question about whether it was best to move the definitions of EFD_SEMAPHORE, etc out of /include/linux/eventfd.h and into a newly created /include/uapi/linux/eventfd.h as this patch does. I don't know if the maintainers have any concerns on this matter, or the patch in general, that would prevent this from moving forward. Thank you, Damian > > I was thinking to use it for V4L2 events support via polling in the V4L2 > compatibility layer for libcamera [1]. We can signal V4L2 buffer > availability POLLOUT via write(), but there is no way to signal V4L2 > events, as they are signaled via POLLPRI. > > > Thank you, > > Paul > > [1] https://libcamera.org/docs.html#id1 > > On Thu, Oct 15, 2015 at 10:42:08AM +0900, Damian Hobson-Garcia wrote: >> From: Martin Sustrik <sustrik@xxxxxxxxxx> >> >> When implementing network protocols in user space, one has to implement >> fake file descriptors to represent the sockets for the protocol. >> >> Polling on such fake file descriptors is a problem (poll/select/epoll >> accept only true file descriptors) and forces protocol implementers to use >> various workarounds resulting in complex, non-standard and convoluted APIs. >> >> More generally, ability to create full-blown file descriptors for >> userspace-to-userspace signalling is missing. While eventfd(2) goes half >> the way towards this goal it has follwoing shorcomings: >> >> I. There's no way to signal POLLPRI, POLLHUP etc. >> II. There's no way to signal arbitrary combination of POLL* flags. Most >> notably, simultaneous !POLLIN and !POLLOUT, which is a perfectly valid >> combination for a network protocol (rx buffer is empty and tx buffer is >> full), cannot be signaled using eventfd. >> >> This patch implements new EFD_MASK flag which solves the above problems. >> >> The semantics of EFD_MASK are as follows: >> >> eventfd(2): >> >> If eventfd is created with EFD_MASK flag set, it is initialised in such a >> way as to signal no events on the file descriptor when it is polled on. >> The 'initval' argument is ignored. >> >> write(2): >> >> User is allowed to write only buffers containing a 32-bit value >> representing any combination of event flags as defined by the poll(2) >> function (POLLIN, POLLOUT, POLLERR, POLLHUP etc.). Specified events >> will be signaled when polling (select, poll, epoll) on the eventfd is >> done later on. >> >> read(2): >> >> read is not supported and will fail with EINVAL. >> >> select(2), poll(2) and similar: >> >> When polling on the eventfd marked by EFD_MASK flag, all the events >> specified in last written event flags shall be signaled. >> >> Signed-off-by: Martin Sustrik <sustrik@xxxxxxxxxx> >> >> [dhobsong@xxxxxxxxxx: Rebased, and resubmitted for Linux 4.3] >> Signed-off-by: Damian Hobson-Garcia <dhobsong@xxxxxxxxxx> >> --- >> fs/eventfd.c | 102 ++++++++++++++++++++++++++++++++++++++----- >> include/linux/eventfd.h | 16 +------ >> include/uapi/linux/eventfd.h | 33 ++++++++++++++ >> 3 files changed, 126 insertions(+), 25 deletions(-) >> create mode 100644 include/uapi/linux/eventfd.h >> >> diff --git a/fs/eventfd.c b/fs/eventfd.c >> index 8d0c0df..1310779 100644 >> --- a/fs/eventfd.c >> +++ b/fs/eventfd.c >> @@ -2,6 +2,7 @@ >> * fs/eventfd.c >> * >> * Copyright (C) 2007 Davide Libenzi <davidel@xxxxxxxxxxxxxxx> >> + * Copyright (C) 2013 Martin Sustrik <sustrik@xxxxxxxxxx> >> * >> */ >> >> @@ -22,18 +23,31 @@ >> #include <linux/proc_fs.h> >> #include <linux/seq_file.h> >> >> +#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK) >> +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE | EFD_MASK) >> +#define EFD_MASK_VALID_EVENTS (POLLIN | POLLPRI | POLLOUT | POLLERR | POLLHUP) >> + >> 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; >> + union { >> + /* >> + * 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; >> + >> + /* >> + * When using eventfd in EFD_MASK mode this stracture stores the >> + * current events to be signaled on the eventfd (events member) >> + * along with opaque user-defined data (data member). >> + */ >> + __u32 events; >> + }; >> unsigned int flags; >> }; >> >> @@ -134,6 +148,14 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait) >> return events; >> } >> >> +static unsigned int eventfd_mask_poll(struct file *file, poll_table *wait) >> +{ >> + struct eventfd_ctx *ctx = file->private_data; >> + >> + poll_wait(file, &ctx->wqh, wait); >> + return ctx->events; >> +} >> + >> static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt) >> { >> *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count; >> @@ -239,6 +261,14 @@ static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count, >> return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt); >> } >> >> +static ssize_t eventfd_mask_read(struct file *file, char __user *buf, >> + size_t count, >> + loff_t *ppos) >> +{ >> + return -EINVAL; >> +} >> + >> + >> static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count, >> loff_t *ppos) >> { >> @@ -286,6 +316,28 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c >> return res; >> } >> >> +static ssize_t eventfd_mask_write(struct file *file, const char __user *buf, >> + size_t count, >> + loff_t *ppos) >> +{ >> + struct eventfd_ctx *ctx = file->private_data; >> + __u32 events; >> + >> + if (count < sizeof(events)) >> + return -EINVAL; >> + if (copy_from_user(&events, buf, sizeof(events))) >> + return -EFAULT; >> + if (events & ~EFD_MASK_VALID_EVENTS) >> + return -EINVAL; >> + spin_lock_irq(&ctx->wqh.lock); >> + memcpy(&ctx->events, &events, sizeof(ctx->events)); >> + if (waitqueue_active(&ctx->wqh)) >> + wake_up_locked_poll(&ctx->wqh, >> + (unsigned long)ctx->events); >> + spin_unlock_irq(&ctx->wqh.lock); >> + return sizeof(ctx->events); >> +} >> + >> #ifdef CONFIG_PROC_FS >> static void eventfd_show_fdinfo(struct seq_file *m, struct file *f) >> { >> @@ -296,6 +348,16 @@ static void eventfd_show_fdinfo(struct seq_file *m, struct file *f) >> (unsigned long long)ctx->count); >> spin_unlock_irq(&ctx->wqh.lock); >> } >> + >> +static void eventfd_mask_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-mask: %x\n", >> + ctx->events); >> + spin_unlock_irq(&ctx->wqh.lock); >> +} >> #endif >> >> static const struct file_operations eventfd_fops = { >> @@ -309,6 +371,17 @@ static const struct file_operations eventfd_fops = { >> .llseek = noop_llseek, >> }; >> >> +static const struct file_operations eventfd_mask_fops = { >> +#ifdef CONFIG_PROC_FS >> + .show_fdinfo = eventfd_mask_show_fdinfo, >> +#endif >> + .release = eventfd_release, >> + .poll = eventfd_mask_poll, >> + .read = eventfd_mask_read, >> + .write = eventfd_mask_write, >> + .llseek = noop_llseek, >> +}; >> + >> /** >> * eventfd_fget - Acquire a reference of an eventfd file descriptor. >> * @fd: [in] Eventfd file descriptor. >> @@ -392,6 +465,7 @@ struct file *eventfd_file_create(unsigned int count, int flags) >> { >> struct file *file; >> struct eventfd_ctx *ctx; >> + const struct file_operations *fops; >> >> /* Check the EFD_* constants for consistency. */ >> BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC); >> @@ -406,10 +480,16 @@ struct file *eventfd_file_create(unsigned int count, int flags) >> >> kref_init(&ctx->kref); >> init_waitqueue_head(&ctx->wqh); >> - ctx->count = count; >> + if (flags & EFD_MASK) { >> + ctx->events = 0; >> + fops = &eventfd_mask_fops; >> + } else { >> + ctx->count = count; >> + fops = &eventfd_fops; >> + } >> ctx->flags = flags; >> >> - file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, >> + file = anon_inode_getfile("[eventfd]", fops, ctx, >> O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS)); >> if (IS_ERR(file)) >> eventfd_free_ctx(ctx); >> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h >> index ff0b981..87de343 100644 >> --- a/include/linux/eventfd.h >> +++ b/include/linux/eventfd.h >> @@ -8,23 +8,11 @@ >> #ifndef _LINUX_EVENTFD_H >> #define _LINUX_EVENTFD_H >> >> +#include <uapi/linux/eventfd.h> >> + >> #include <linux/fcntl.h> >> #include <linux/wait.h> >> >> -/* >> - * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining >> - * new flags, since they might collide with O_* ones. We want >> - * to re-use O_* flags that couldn't possibly have a meaning >> - * from eventfd, in order to leave a free define-space for >> - * shared O_* flags. >> - */ >> -#define EFD_SEMAPHORE (1 << 0) >> -#define EFD_CLOEXEC O_CLOEXEC >> -#define EFD_NONBLOCK O_NONBLOCK >> - >> -#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK) >> -#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE) >> - >> struct file; >> >> #ifdef CONFIG_EVENTFD >> diff --git a/include/uapi/linux/eventfd.h b/include/uapi/linux/eventfd.h >> new file mode 100644 >> index 0000000..097dcad >> --- /dev/null >> +++ b/include/uapi/linux/eventfd.h >> @@ -0,0 +1,33 @@ >> +/* >> + * Copyright (C) 2013 Martin Sustrik <sustrik@xxxxxxxxxx> >> + * >> + * This program is free software; you can redistribute it and/or modify >> + * it under the terms of the GNU General Public License as published by >> + * the Free Software Foundation; either version 2 of the License, or >> + * (at your option) any later version. >> + */ >> + >> +#ifndef _UAPI_LINUX_EVENTFD_H >> +#define _UAPI_LINUX_EVENTFD_H >> + >> +/* For O_CLOEXEC */ >> +#include <linux/fcntl.h> >> +#include <linux/types.h> >> + >> +/* >> + * CAREFUL: Check include/asm-generic/fcntl.h when defining >> + * new flags, since they might collide with O_* ones. We want >> + * to re-use O_* flags that couldn't possibly have a meaning >> + * from eventfd, in order to leave a free define-space for >> + * shared O_* flags. >> + */ >> + >> +/* Provide semaphore-like semantics for reads from the eventfd. */ >> +#define EFD_SEMAPHORE (1 << 0) >> +/* Provide event mask semantics for the eventfd. */ >> +#define EFD_MASK (1 << 1) >> +/* Set the close-on-exec (FD_CLOEXEC) flag on the eventfd. */ >> +#define EFD_CLOEXEC O_CLOEXEC >> +/* Create the eventfd in non-blocking mode. */ >> +#define EFD_NONBLOCK O_NONBLOCK >> +#endif /* _UAPI_LINUX_EVENTFD_H */ >> -- >> 1.9.1
