87 lines
3.4 KiB
ReStructuredText
87 lines
3.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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======
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futex2
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======
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:Author: André Almeida <andrealmeid@collabora.com>
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futex, or fast user mutex, is a set of syscalls to allow userspace to create
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performant synchronization mechanisms, such as mutexes, semaphores and
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conditional variables in userspace. C standard libraries, like glibc, uses it
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as a means to implement more high level interfaces like pthreads.
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futex2 is a followup version of the initial futex syscall, designed to overcome
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limitations of the original interface.
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User API
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========
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``futex_waitv()``
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-----------------
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Wait on an array of futexes, wake on any::
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futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes,
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unsigned int flags, struct timespec *timeout, clockid_t clockid)
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struct futex_waitv {
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__u64 val;
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__u64 uaddr;
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__u32 flags;
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__u32 __reserved;
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};
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Userspace sets an array of struct futex_waitv (up to a max of 128 entries),
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using ``uaddr`` for the address to wait for, ``val`` for the expected value
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and ``flags`` to specify the type (e.g. private) and size of futex.
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``__reserved`` needs to be 0, but it can be used for future extension. The
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pointer for the first item of the array is passed as ``waiters``. An invalid
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address for ``waiters`` or for any ``uaddr`` returns ``-EFAULT``.
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If userspace has 32-bit pointers, it should do a explicit cast to make sure
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the upper bits are zeroed. ``uintptr_t`` does the tricky and it works for
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both 32/64-bit pointers.
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``nr_futexes`` specifies the size of the array. Numbers out of [1, 128]
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interval will make the syscall return ``-EINVAL``.
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The ``flags`` argument of the syscall needs to be 0, but it can be used for
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future extension.
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For each entry in ``waiters`` array, the current value at ``uaddr`` is compared
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to ``val``. If it's different, the syscall undo all the work done so far and
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return ``-EAGAIN``. If all tests and verifications succeeds, syscall waits until
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one of the following happens:
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- The timeout expires, returning ``-ETIMEOUT``.
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- A signal was sent to the sleeping task, returning ``-ERESTARTSYS``.
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- Some futex at the list was woken, returning the index of some waked futex.
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An example of how to use the interface can be found at ``tools/testing/selftests/futex/functional/futex_waitv.c``.
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Timeout
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-------
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``struct timespec *timeout`` argument is an optional argument that points to an
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absolute timeout. You need to specify the type of clock being used at
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``clockid`` argument. ``CLOCK_MONOTONIC`` and ``CLOCK_REALTIME`` are supported.
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This syscall accepts only 64bit timespec structs.
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Types of futex
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--------------
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A futex can be either private or shared. Private is used for processes that
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shares the same memory space and the virtual address of the futex will be the
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same for all processes. This allows for optimizations in the kernel. To use
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private futexes, it's necessary to specify ``FUTEX_PRIVATE_FLAG`` in the futex
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flag. For processes that doesn't share the same memory space and therefore can
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have different virtual addresses for the same futex (using, for instance, a
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file-backed shared memory) requires different internal mechanisms to be get
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properly enqueued. This is the default behavior, and it works with both private
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and shared futexes.
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Futexes can be of different sizes: 8, 16, 32 or 64 bits. Currently, the only
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supported one is 32 bit sized futex, and it need to be specified using
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``FUTEX_32`` flag.
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