Re: [PATCH v4 00/30] NT synchronization primitive driver

Elizabeth Figura
2024-04-17T06:05:58Z

On Wednesday, 17 April 2024 00:22:18 CDT Peter Zijlstra wrote:
> On Tue, Apr 16, 2024 at 04:18:19PM -0500, Elizabeth Figura wrote:
> > Let me know if that's good enough or if I should try to render it into
> > plain text somehow.
>
> Plain text is much preferred. I'm more of a text editor kinda guy --
> being a programmer and all that.

I can certainly sympathize with that ;-)

Here's a (slightly ad-hoc) simplification of the patch into text form inlined
into this message; hopefully it's readable enough.

===================================
NT synchronization primitive driver
===================================

This page documents the user-space API for the ntsync driver.

ntsync is a support driver for emulation of NT synchronization
primitives by user-space NT emulators. It exists because implementation
in user-space, using existing tools, cannot match Windows performance
while offering accurate semantics. It is implemented entirely in
software, and does not drive any hardware device.

This interface is meant as a compatibility tool only, and should not
be used for general synchronization. Instead use generic, versatile
interfaces such as futex(2) and poll(2).

Synchronization primitives
==========================

The ntsync driver exposes three types of synchronization primitives:
semaphores, mutexes, and events.

A semaphore holds a single volatile 32-bit counter, and a static 32-bit
integer denoting the maximum value. It is considered signaled when the
counter is nonzero. The counter is decremented by one when a wait is
satisfied. Both the initial and maximum count are established when the
semaphore is created.

A mutex holds a volatile 32-bit recursion count, and a volatile 32-bit
identifier denoting its owner. A mutex is considered signaled when its
owner is zero (indicating that it is not owned). The recursion count is
incremented when a wait is satisfied, and ownership is set to the given
identifier.

A mutex also holds an internal flag denoting whether its previous owner
has died; such a mutex is said to be abandoned. Owner death is not
tracked automatically based on thread death, but rather must be
communicated using NTSYNC_IOC_MUTEX_KILL. An abandoned mutex is
inherently considered unowned.

Except for the "unowned" semantics of zero, the actual value of the
owner identifier is not interpreted by the ntsync driver at all. The
intended use is to store a thread identifier; however, the ntsync
driver does not actually validate that a calling thread provides
consistent or unique identifiers.

An event holds a volatile boolean state denoting whether it is signaled
or not. There are two types of events, auto-reset and manual-reset. An
auto-reset event is designaled when a wait is satisfied; a manual-reset
event is not. The event type is specified when the event is created.

Unless specified otherwise, all operations on an object are atomic and
totally ordered with respect to other operations on the same object.

Objects are represented by files. When all file descriptors to an
object are closed, that object is deleted.

Char device
===========

The ntsync driver creates a single char device /dev/ntsync. Each file
description opened on the device represents a unique instance intended
to back an individual NT virtual machine. Objects created by one ntsync
instance may only be used with other objects created by the same
instance.

ioctl reference
===============

All operations on the device are done through ioctls. There are four
structures used in ioctl calls::

struct ntsync_sem_args {
__u32 sem;
__u32 count;
__u32 max;
};

struct ntsync_mutex_args {
__u32 mutex;
__u32 owner;
__u32 count;
};

struct ntsync_event_args {
__u32 event;
__u32 signaled;
__u32 manual;
};

struct ntsync_wait_args {
__u64 timeout;
__u64 objs;
__u32 count;
__u32 owner;
__u32 index;
__u32 alert;
__u32 flags;
__u32 pad;
};

Depending on the ioctl, members of the structure may be used as input,
output, or not at all. All ioctls return 0 on success.

The ioctls on the device file are as follows:

.. NTSYNC_IOC_CREATE_SEM

Create a semaphore object. Takes a pointer to struct ntsync_sem_args,
which is used as follows:

* sem: On output, contains a file descriptor to the created semaphore.
* count: Initial count of the semaphore.
* max: Maximum count of the semaphore.

Fails with EINVAL if `count` is greater than `max`.

.. NTSYNC_IOC_CREATE_MUTEX

Create a mutex object. Takes a pointer to struct ntsync_mutex_args,
which is used as follows:

* mutex: On output, contains a file descriptor to the created mutex.
* count: Initial recursion count of the mutex.
* owner: Initial owner of the mutex.

If ``owner`` is nonzero and ``count`` is zero, or if ``owner`` is zero
and ``count`` is nonzero, the function fails with EINVAL.

.. NTSYNC_IOC_CREATE_EVENT

Create an event object. Takes a pointer to struct ntsync_event_args,
which is used as follows:

* event: On output, contains a file descriptor to the created event.
* signaled: If nonzero, the event is initially signaled, otherwise
nonsignaled.
* manual: If nonzero, the event is a manual-reset event, otherwise
auto-reset.

The ioctls on the individual objects are as follows:

.. NTSYNC_IOC_SEM_POST

Post to a semaphore object. Takes a pointer to a 32-bit integer,
which on input holds the count to be added to the semaphore, and on
output contains its previous count.

If adding to the semaphore's current count would raise the latter
past the semaphore's maximum count, the ioctl fails with
EOVERFLOW and the semaphore is not affected. If raising the
semaphore's count causes it to become signaled, eligible threads
waiting on this semaphore will be woken and the semaphore's count
decremented appropriately.

.. NTSYNC_IOC_MUTEX_UNLOCK

Release a mutex object. Takes a pointer to struct ntsync_mutex_args,
which is used as follows:

* mutex: Ignored.
* owner: Specifies the owner trying to release this mutex.
* count: On output, contains the previous recursion count.

If ``owner`` is zero, the ioctl fails with EINVAL. If ``owner``
is not the current owner of the mutex, the ioctl fails with
EPERM.

The mutex's count will be decremented by one. If decrementing the
mutex's count causes it to become zero, the mutex is marked as
unowned and signaled, and eligible threads waiting on it will be
woken as appropriate.

.. NTSYNC_IOC_SET_EVENT

Signal an event object. Takes a pointer to a 32-bit integer, which on
output contains the previous state of the event.

Eligible threads will be woken, and auto-reset events will be
designaled appropriately.

.. NTSYNC_IOC_RESET_EVENT

Designal an event object. Takes a pointer to a 32-bit integer, which
on output contains the previous state of the event.

.. NTSYNC_IOC_PULSE_EVENT

Wake threads waiting on an event object while leaving it in an
unsignaled state. Takes a pointer to a 32-bit integer, which on
output contains the previous state of the event.

A pulse operation can be thought of as a set followed by a reset,
performed as a single atomic operation. If two threads are waiting on
an auto-reset event which is pulsed, only one will be woken. If two
threads are waiting a manual-reset event which is pulsed, both will
be woken. However, in both cases, the event will be unsignaled
afterwards, and a simultaneous read operation will always report the
event as unsignaled.

.. NTSYNC_IOC_READ_SEM

Read the current state of a semaphore object. Takes a pointer to
struct ntsync_sem_args, which is used as follows:

* sem: Ignored.
* count: On output, contains the current count of the semaphore.
* max: On output, contains the maximum count of the semaphore.

.. NTSYNC_IOC_READ_MUTEX

Read the current state of a mutex object. Takes a pointer to struct
ntsync_mutex_args, which is used as follows:

* mutex: Ignored.
* owner: On output, contains the current owner of the mutex, or zero
if the mutex is not currently owned.
* count: On output, contains the current recursion count of the mutex.

If the mutex is marked as abandoned, the function fails with
EOWNERDEAD. In this case, ``count`` and ``owner`` are set to zero.

.. NTSYNC_IOC_READ_EVENT

Read the current state of an event object. Takes a pointer to struct
ntsync_event_args, which is used as follows:

* event: Ignored.
* signaled: On output, contains the current state of the event.
* manual: On output, contains 1 if the event is a manual-reset event,
and 0 otherwise.

.. NTSYNC_IOC_KILL_OWNER

Mark a mutex as unowned and abandoned if it is owned by the given
owner. Takes an input-only pointer to a 32-bit integer denoting the
owner. If the owner is zero, the ioctl fails with EINVAL. If the
owner does not own the mutex, the function fails with EPERM.

Eligible threads waiting on the mutex will be woken as appropriate
(and such waits will fail with EOWNERDEAD, as described below).

.. NTSYNC_IOC_WAIT_ANY

Poll on any of a list of objects, atomically acquiring at most one.
Takes a pointer to struct ntsync_wait_args, which is used as follows:

* timeout: Absolute timeout in nanoseconds. If NTSYNC_WAIT_REALTIME
is set, the timeout is measured against the REALTIME
clock; otherwise it is measured against the MONOTONIC
clock. If the timeout is equal to or earlier than the
current time, the function returns immediately without
sleeping. If ``timeout`` is U64_MAX, the function will
sleep until an object is signaled, and will not fail
with ETIMEDOUT.

* objs: Pointer to an array of ``count`` file descriptors
(specified as an integer so that the structure has the
same size regardless of architecture). If any object is
invalid, the function fails with EINVAL.

* count: Number of objects specified in the ``objs`` array. If
greater than NTSYNC_MAX_WAIT_COUNT, the function fails
with EINVAL.

* owner: Mutex owner identifier. If any object in ``objs`` is a
mutex, the ioctl will attempt to acquire that mutex on
behalf of ``owner``. If ``owner`` is zero, the ioctl
fails with EINVAL.

* index: On success, contains the index (into ``objs``) of the
object which was signaled. If ``alert`` was signaled
instead, this contains ``count``.

* alert: Optional event object file descriptor. If nonzero, this
specifies an "alert" event object which, if signaled,
will terminate the wait. If nonzero, the identifier must
point to a valid event.

* flags: Zero or more flags. Currently the only flag is
NTSYNC_WAIT_REALTIME, which causes the timeout to be
measured against the REALTIME clock instead of
MONOTONIC.

* pad: Unused, must be set to zero.

This function attempts to acquire one of the given objects. If unable
to do so, it sleeps until an object becomes signaled, subsequently
acquiring it, or the timeout expires. In the latter case the ioctl
fails with ETIMEDOUT. The function only acquires one object, even if
multiple objects are signaled.

A semaphore is considered to be signaled if its count is nonzero, and
is acquired by decrementing its count by one. A mutex is considered
to be signaled if it is unowned or if its owner matches the ``owner``
argument, and is acquired by incrementing its recursion count by one
and setting its owner to the ``owner`` argument. An auto-reset event
is acquired by designaling it; a manual-reset event is not affected
by acquisition.

Acquisition is atomic and totally ordered with respect to other
operations on the same object. If two wait operations (with different
``owner`` identifiers) are queued on the same mutex, only one is
signaled. If two wait operations are queued on the same semaphore,
and a value of one is posted to it, only one is signaled. The order
in which threads are signaled is not specified.

If an abandoned mutex is acquired, the ioctl fails with
EOWNERDEAD. Although this is a failure return, the function may
otherwise be considered successful. The mutex is marked as owned by
the given owner (with a recursion count of 1) and as no longer
abandoned, and ``index`` is still set to the index of the mutex.

The ``alert`` argument is an "extra" event which can terminate the
wait, independently of all other objects. If members of ``objs`` and
``alert`` are both simultaneously signaled, a member of ``objs`` will
always be given priority and acquired first.

It is valid to pass the same object more than once, including by
passing the same event in the ``objs`` array and in ``alert``. If a
wakeup occurs due to that object being signaled, ``index`` is set to
the lowest index corresponding to that object.

The function may fail with EINTR if a signal is received.

.. NTSYNC_IOC_WAIT_ALL

Poll on a list of objects, atomically acquiring all of them. Takes a
pointer to struct ntsync_wait_args, which is used identically to
NTSYNC_IOC_WAIT_ANY, except that ``index`` is always filled with zero
on success if not woken via alert.

This function attempts to simultaneously acquire all of the given
objects. If unable to do so, it sleeps until all objects become
simultaneously signaled, subsequently acquiring them, or the timeout
expires. In the latter case the ioctl fails with ETIMEDOUT and no
objects are modified.

Objects may become signaled and subsequently designaled (through
acquisition by other threads) while this thread is sleeping. Only
once all objects are simultaneously signaled does the ioctl acquire
them and return. The entire acquisition is atomic and totally ordered
with respect to other operations on any of the given objects.

If an abandoned mutex is acquired, the ioctl fails with
EOWNERDEAD. Similarly to NTSYNC_IOC_WAIT_ANY, all objects are
nevertheless marked as acquired. Note that if multiple mutex objects
are specified, there is no way to know which were marked as
abandoned.

As with "any" waits, the ``alert`` argument is an "extra" event which
can terminate the wait. Critically, however, an "all" wait will
succeed if all members in ``objs`` are signaled, *or* if ``alert`` is
signaled. In the latter case ``index`` will be set to ``count``. As
with "any" waits, if both conditions are filled, the former takes
priority, and objects in ``objs`` will be acquired.

Unlike NTSYNC_IOC_WAIT_ANY, it is not valid to pass the same
object more than once, nor is it valid to pass the same object in
``objs`` and in ``alert``. If this is attempted, the function fails
with EINVAL.