[PATCH v3 0/1] Generalize poll events from eventfd

Using eventfd user space can generate POLLIN/POLLOUT events but some
applications may want to generate POLLPRI/POLLERR events as well.
This patch submission aims to generalize the events generated by an
eventfd. This is a resubmission of a patch from Feb 2013[1]. The original
discussion trailed off without any conclusion, but the original author
has recently confirmed[2] that this functionality is still useful, so I
volunteered to rebase and resubmit the patch for discussion.

[1] https://lkml.org/lkml/2013/2/18/147
[2] https://lkml.org/lkml/2015/7/9/153

Changes in v3
-------------
* replace efd_mask structure with scalar 'events' variable.

Changes in v2
-------------

* rebased on Linux v4.3-rc1
* Move file operation implementations for EFD_MASK to a seperate structure
* Remove 'data' element from efd_mask structure
* read() is no longer supported when EFD_MASK is set (fails with EINVAL)
* eventfd_ctx_fileget() now returns EINVAL when EFD_MASK is set, eliminating
  the possibility of triggering the orginal BUG_ON() macros which have now
  been removed.

Thank you,
Damian

Martin Sustrik (1):
  eventfd: implementation of EFD_MASK flag

 fs/eventfd.c                 | 91 ++++++++++++++++++++++++++++++++++++++------
 include/linux/eventfd.h      | 16 +-------
 include/uapi/linux/eventfd.h | 40 +++++++++++++++++++
 3 files changed, 121 insertions(+), 26 deletions(-)
 create mode 100644 include/uapi/linux/eventfd.h

-- 
1.9.1

From: Martin Sustrik <redacted>

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 <redacted>

[dhobsong@igel.co.jp: Rebased, and resubmitted for Linux 4.3]
Signed-off-by: Damian Hobson-Garcia <redacted>
---
 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

Hello,

<snip>

<snip>

+
+/*
+ * 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 */

Since the latest version of this patch adds only the EFD_MASK definition
to the eventfd header, I was wondering if it was really
necessary/recommended to move the definitions from linux/eventfd.h to
linux/uapi/eventfd.h.  From my understanding, the EFD_SEMAPHORE (and now
EFD_MASK) define(s) are provided to user space from the libc headers
only. Any advice would be greatly appreciated.

Thank you,
Damian

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:

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:

quoted

From: Martin Sustrik <redacted>

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 <redacted>

[dhobsong@igel.co.jp: Rebased, and resubmitted for Linux 4.3]
Signed-off-by: Damian Hobson-Garcia <redacted>
---
 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

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:

quoted

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.

quoted

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:

quoted

From: Martin Sustrik <redacted>

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 <redacted>

[dhobsong@igel.co.jp: Rebased, and resubmitted for Linux 4.3]
Signed-off-by: Damian Hobson-Garcia <redacted>
---
 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 */