Date:   Thu, 25 Jun 2020 05:18:31 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Damian Hobson-Garcia <dhobsong@...l.co.jp>
Cc:     Paul Elder <paul.elder@...asonboard.com>, viro@...iv.linux.org.uk,
        sustrik@...bpm.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        netdev@...r.kernel.org, David.Laight@...lab.com
Subject: Re: [PATCH v3 1/1] eventfd: implementation of EFD_MASK flag

Hi Damian,

On Tue, Jun 23, 2020 at 06:21:28PM +0900, Damian Hobson-Garcia wrote:
> 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 would have thought that defining EFD_SEMAPHORE in a public API header
would be best, but it seems that glibc has its own definition in
bits/eventfd.h. I don't know what is usually preferred in these cases.

> 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.

Thanks for your reply. It seems a good way forward would be to resubmit
the patch then.

> > 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.
> > 
> > [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@...bpm.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 <sustrik@...bpm.com>
> >>
> >> [dhobsong@...l.co.jp: Rebased, and resubmitted for Linux 4.3]
> >> Signed-off-by: Damian Hobson-Garcia <dhobsong@...l.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 <davidel@...ilserver.org>
> >> + *  Copyright (C) 2013  Martin Sustrik <sustrik@...bpm.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 <sustrik@...bpm.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 */

-- 
Regards,

Laurent Pinchart

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