futex2: Documentation: Document futex_waitv() uAPI

Create userspace documentation for futex_waitv() syscall, detailing how
the arguments are used.

Signed-off-by: André Almeida <andrealmeid@collabora.com>

Documentation/userspace-api/futex2.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+======
+futex2
+======
+
+:Author: André Almeida <andrealmeid@collabora.com>
+
+futex, or fast user mutex, is a set of syscalls to allow userspace to create
+performant synchronization mechanisms, such as mutexes, semaphores and
+conditional variables in userspace. C standard libraries, like glibc, uses it
+as a means to implement more high level interfaces like pthreads.
+
+futex2 is a followup version of the initial futex syscall, designed to overcome
+limitations of the original interface.
+
+User API
+========
+
+``futex_waitv()``
+-----------------
+
+Wait on an array of futexes, wake on any::
+
+  futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes, unsigned int flags, struct timespec *timo)
+
+  struct futex_waitv {
+        __u64 val;
+        __u64 uaddr;
+        __u32 flags;
+        __u32 __reserved;
+  };
+
+Userspace set an array of struct futex_waitv (up to a max of 128 entries),
+using ``uaddr`` for the address to wait for, ``val`` for the expected value
+and ``flags`` to specify the type (shared, private) and size of futex.
+``__reserved`` is here for explicit padding in 32 and 64-bit platforms. It can
+be used for future extensions, but currently it should be always 0.
+
+``uaddr`` uses a 64-bit unsigned integer to store the userspace address. Given
+signal extension and promotion rules, platforms that uses 32-bit pointers should
+explicit cast the address value. Casting using ``(uintptr_t)`` solves the
+problem and make the code work on both 64 and 32-bit platforms without further
+tweaks::
+
+  waitv[i].uaddr = (uintptr_t) uaddr;
+
+The pointer for the first item of the array is passed as ``waiters``. An invalid
+address for ``waiters`` or for any ``uaddr`` returns ``-EFAULT``.
+
+``nr_futexes`` specifies the size of the array. Numbers out of [1, 128]
+interval will make the syscall return ``-EINVAL``.
+
+``flags`` can be used to set the timeout clock.
+
+For each entry in ``waiters`` array, the current value at ``uaddr`` is compared
+to ``val``. If it's different, the syscall undo all the work done so far and
+return ``-EAGAIN``. If all tests and verifications succeeds, syscall waits until
+one of the following happens:
+
+- The timeout expires, returning ``-ETIMEOUT``.
+- A signal was sent to the sleeping task, returning ``-ERESTARTSYS``.
+- Some futex at the list was awaken, returning the index of some waked futex.
+
+An example of how to use the interface can be found at ``tools/testing/selftests/futex/futenctional/futex_waitv.c``.
+
+Timeout
+-------
+
+For every operation that has a ``struct timespec timo`` argument, it is an
+optional argument that points to an absolute timeout. By default, it's measured
+against ``CLOCK_MONOTONIC``, but the flag ``FUTEX_CLOCK_REALTIME`` can be used
+to measure it against ``CLOCK_REALTIME``. This syscall accepts only 64bit
+timespec structs.
+
+Types of futex
+--------------
+
+A futex can be either private or shared. Private is used for processes that
+shares the same memory space and the virtual address of the futex will be the
+same for all processes. This allows for optimizations in the kernel and is the
+default type of a futex. To use private futexes, set the flag
+``FUTEX_PRIVATE_FLAG``. For processes that doesn't share the same memory space
+and therefore can have different virtual addresses for the same futex (using,
+for instance, a file-backed shared memory) requires different internal
+mechanisms to be get properly enqueued in the kernel. This is the default mode.
+
+Futexes can be of different sizes: 8, 16, 32 or 64 bits. Currently, the only
+supported one is 32 bit sized futex, and it need to be specified using
+``FUTEX_32`` flag.

Documentation/userspace-api/index.rst

@@ -28,6 +28,7 @@ place where this information is gathered.
    media/index
    sysfs-platform_profile
    vduse
+   futex2
 
 .. only::  subproject and html
 

MAINTAINERS

@@ -7723,6 +7723,7 @@ L:	linux-kernel@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/core
 F:	Documentation/locking/*futex*
+F:	Documentation/userspace-api/futex2.rst
 F:	include/asm-generic/futex.h
 F:	include/linux/futex.h
 F:	include/uapi/linux/futex.h