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    Subject[PATCH v2 22/22] futex2: Documentation: Document sys_futex_waitv() uAPI
    Date
    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 | 86 ++++++++++++++++++++++++++
    Documentation/userspace-api/index.rst | 1 +
    2 files changed, 87 insertions(+)
    create mode 100644 Documentation/userspace-api/futex2.rst

    diff --git a/Documentation/userspace-api/futex2.rst b/Documentation/userspace-api/futex2.rst
    new file mode 100644
    index 000000000000..7d37409df355
    --- /dev/null
    +++ b/Documentation/userspace-api/futex2.rst
    @@ -0,0 +1,86 @@
    +.. 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 *timeout, clockid_t clockid)
    +
    + struct futex_waitv {
    + __u64 val;
    + __u64 uaddr;
    + __u32 flags;
    + __u32 __reserved;
    + };
    +
    +Userspace sets 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 (e.g. private) and size of futex.
    +``__reserved`` needs to be 0, but it can be used for future extension. The
    +pointer for the first item of the array is passed as ``waiters``. An invalid
    +address for ``waiters`` or for any ``uaddr`` returns ``-EFAULT``.
    +
    +If userspace has 32-bit pointers, it should do a explicit cast to make sure
    +the upper bits are zeroed. ``uintptr_t`` does the tricky and it works for
    +both 32/64-bit pointers.
    +
    +``nr_futexes`` specifies the size of the array. Numbers out of [1, 128]
    +interval will make the syscall return ``-EINVAL``.
    +
    +The ``flags`` argument of the syscall needs to be 0, but it can be used for
    +future extension.
    +
    +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/functional/futex_waitv.c``.
    +
    +Timeout
    +-------
    +
    +``struct timespec *timeout`` argument is an optional argument that points to an
    +absolute timeout. You need to specify the type of clock being used at
    +``clockid`` argument. ``CLOCK_MONOTONIC`` and ``CLOCK_REALTIME`` are supported.
    +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. To use
    +private futexes, it's necessary to specify ``FUTEX_PRIVATE_FLAG`` in the futex
    +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. This is the default behavior, and it works with both private
    +and shared futexes.
    +
    +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.
    diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
    index c432be070f67..a61eac0c73f8 100644
    --- a/Documentation/userspace-api/index.rst
    +++ b/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

    --
    2.33.0
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