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Subject: Re: [PATCH v3 1/1] eventfd: implementation of EFD_MASK flag
To:     Paul Elder <paul.elder@ideasonboard.com>
Cc:     viro@zeniv.linux.org.uk, sustrik@250bpm.com,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-api@vger.kernel.org, netdev@vger.kernel.org,
        David.Laight@aculab.com, laurent.pinchart@ideasonboard.com
References: <1444873328-8466-1-git-send-email-dhobsong@igel.co.jp>
 <1444873328-8466-2-git-send-email-dhobsong@igel.co.jp>
 <20200619101619.GD2073@jade.amanokami.net>
From:   Damian Hobson-Garcia <dhobsong@igel.co.jp>
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Date:   Tue, 23 Jun 2020 18:21:28 +0900
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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@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 <sustrik@250bpm.com>
>>
>> [dhobsong@igel.co.jp: Rebased, and resubmitted for Linux 4.3]
>> Signed-off-by: Damian Hobson-Garcia <dhobsong@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 <davidel@xmailserver.org>
>> + *  Copyright (C) 2013  Martin Sustrik <sustrik@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 <sustrik@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