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Date:   Thu, 23 Sep 2021 14:11:11 -0300
From:   André Almeida <>
To:     Thomas Gleixner <>,
        Ingo Molnar <>,
        Peter Zijlstra <>,
        Darren Hart <>,, Steven Rostedt <>,
        Sebastian Andrzej Siewior <>
Cc:,,,,, Davidlohr Bueso <>,
        Arnd Bergmann <>,
        André Almeida <>
Subject: [PATCH v2 22/22] futex2: Documentation: Document sys_futex_waitv() uAPI

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

Signed-off-by: André Almeida <>
 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
+:Author: André Almeida <>
+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
+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``.
+``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.
+   futex2
 .. only::  subproject and html

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