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 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 <sust...@250bpm.com> > > 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 <sust...@250bpm.com> > > [dhobs...@igel.co.jp: Rebased, and resubmitted for Linux 4.3] > Signed-off-by: Damian Hobson-Garcia <dhobs...@igel.co.jp> > --- > 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 <davi...@xmailserver.org> > + * Copyright (C) 2013 Martin Sustrik <sust...@250bpm.com> > * > */ > > @@ -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 <sust...@250bpm.com> > + * > + * 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
