From 9cc63f8bcd560c760d0b12e15bba8f81c86237cf Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 14 Aug 2026 15:43:51 -0700 Subject: Merge branches 'expcb.2026.07.24a', 'misc.2026.07.30a', 'rcu-tasks.2026.07.30a', 'srcu.2026.08.11a' and 'torture.2026.08.14a' into HEAD Changes: Make expedited grace periods expedite normal RCU callbacks Miscellaneous fixes: * Improve diagnostic output with character task states. * Mark accesses to inform KCSAN of concurrency design. * Move from kmalloc() to kmalloc_obj(). * Documentation updates. * Improve handling of RCU deferred quiescent states. * Clean up unused function arguments and structure fields. * Reduce show_rcu_gp_kthreads() stack space. Tasks RCU updates: * Clean up after SRCU re-implementation of Tasks Trace RCU. * Mark accesses to inform KCSAN of concurrency design. * Add ->lazy_timer status to diagnostic output. * Remove an unnecessary memory barrier. * Fix a data race, courtesy of KCSAN. * Documentation updates. * Convert cond_resched_tasks_rcu_qs() from macro to static inline function. SRCU updates: * Add Rust helpers for SRCU. * Avoid losing queued work at cleanup_srcu_struct() time. Torture-test updates: * Preparation work for immediate RCU priority deboosting. * Test RCU readers from real interrupt handlers (as opposed to softirq). * Simplify code through use of cpumask_next_wrap(). * Improve diagnostic output with character task states. * Add rcutorture.nwriters parameter to allow lightweight stall testing, and rcutorture.stall_only to make doing so easier. * Test an RCU Tasks Trace grace period implying an RCU grace period. * Make RCU Tasks Trace torturing track reader batches. * Fix a data race, courtesy of KCSAN. * Plug a shuffle_tmp_mask memory leak on kthread spawn failure. --- Documentation/userspace-api/futex2.rst | 86 ++++++++++++++++++++++++++++++++++ 1 file changed, 86 insertions(+) create mode 100644 Documentation/userspace-api/futex2.rst (limited to 'Documentation/userspace-api/futex2.rst') diff --git a/Documentation/userspace-api/futex2.rst b/Documentation/userspace-api/futex2.rst new file mode 100644 index 000000000..9693f47a7 --- /dev/null +++ b/Documentation/userspace-api/futex2.rst @@ -0,0 +1,86 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====== +futex2 +====== + +: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 +======== + +``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 woken, 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. -- cgit v1.3.1