zircon/vdso/futex.fidl

// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
library zx;

// TODO(scottmg): This is approximately right, but will need to match the
// current definition of zx_futex_t (atomic_int in some #if branches).
alias Futex = int32;

// TODO(scottmg): The futex is unusual in that by virtue of being an int,
// sometimes it's passed by pointer, and sometimes by value.
@transport("Syscall")
@no_protocol_prefix
closed protocol Futexfuncs {
    /// ## Summary
    ///
    /// Wait on a futex.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_futex_wait(const zx_futex_t* value_ptr,
    ///                           zx_futex_t current_value,
    ///                           zx_handle_t new_futex_owner,
    ///                           zx_instant_mono_t deadline);
    /// ```
    ///
    /// ## Description
    ///
    /// `zx_futex_wait()` atomically verifies that *value_ptr* still contains the value
    /// *current_value* and sleeps until the futex is made available by a call to
    /// `zx_futex_wake`. Optionally, the thread can also be woken up after the
    /// *deadline* (with respect to `ZX_CLOCK_MONOTONIC`) passes. *deadline* may be
    /// automatically adjusted according to the job's [timer slack] policy.
    ///
    /// ## SPURIOUS WAKEUPS
    ///
    /// A component that uses futexes should be prepared to handle spurious
    /// wakeups.  A spurious wakeup is a situation where `zx_futex_wait()`
    /// returns successfully even though the component did not wake the waiter
    /// by calling [`zx_futex_wake()`].
    ///
    /// Zircon's implementation of futexes currently does not generate
    /// spurious wakeups itself.  However, commonly-used algorithms that use
    /// futexes can sometimes generate spurious wakeups.  For example, the
    /// usual implementation of `mutex_unlock` can potentially produce a
    /// [`zx_futex_wake()`] call on a memory location after the location has been
    /// freed and reused for unrelated purposes.
    ///
    /// ## OWNERSHIP
    ///
    /// A successful call to `zx_futex_wait()` results in the owner of the futex being
    /// set to the thread referenced by the *new_futex_owner* handle, or to nothing if
    /// *new_futex_owner* is `ZX_HANDLE_INVALID`.
    ///
    /// See *Ownership and Priority Inheritance* in [futex](/docs/reference/kernel_objects/futex.md) for
    /// details.
    ///
    /// ## Rights
    ///
    /// None.
    ///
    /// ## Return value
    ///
    /// `zx_futex_wait()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  One of the following is true:
    ///
    /// + *value_ptr* is not a valid userspace pointer
    /// + *value_ptr* is not aligned to a `sizeof(zx_futex_t)` boundary.
    /// + *new_futex_owner* is currently a member of the waiters for *value_ptr*.
    /// + *new_futex_owner* has not been started yet.
    ///
    /// `ZX_ERR_BAD_HANDLE`  *new_futex_owner* is not `ZX_HANDLE_INVALID`, and not a valid handle AND
    /// *current_value* matches the value at *value_ptr*
    ///
    /// `ZX_ERR_WRONG_TYPE`  *new_futex_owner* is a valid handle, but is not a handle to a thread.
    ///
    /// `ZX_ERR_BAD_STATE`  *current_value* does not match the value at *value_ptr*.
    ///
    /// `ZX_ERR_TIMED_OUT`  The thread was not woken before *deadline* passed.
    ///
    /// ## See also
    ///
    ///  - [futex objects]
    ///  - [timer slack]
    ///  - [`zx_futex_requeue()`]
    ///  - [`zx_futex_wake()`]
    ///
    /// [futex objects]: /docs/reference/kernel_objects/futex.md
    /// [timer slack]: /docs/concepts/kernel/timer_slack.md
    /// [`zx_futex_requeue()`]: futex_requeue.md
    /// [`zx_futex_wake()`]: futex_wake.md
    @blocking
    strict FutexWait(resource struct {
        @embedded_alias("zx/Futex")
        value_ptr experimental_pointer<Futex>;
        current_value int32;
        new_futex_owner Handle;
        deadline InstantMono;
    }) -> () error Status;

    /// ## Summary
    ///
    /// Wake some number of threads waiting on a futex, and set the ownership of the futex to nothing.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_futex_wake(const zx_futex_t* value_ptr, uint32_t wake_count);
    /// ```
    ///
    /// ## Description
    ///
    /// Waking a futex causes *wake_count* threads waiting on the *value_ptr*
    /// futex to be woken up.
    ///
    /// Waking up zero threads is not an error condition.  Passing in an unallocated
    /// address for *value_ptr* is not an error condition.
    ///
    /// ## OWNERSHIP
    ///
    /// A successful call to `zx_futex_wake()` results in the owner of the futex being
    /// set to nothing, regardless of the wake count.  In order to transfer ownership of
    /// a futex, use the [`zx_futex_wake_single_owner()`] variant instead.
    /// [`zx_futex_wake_single_owner()`] will attempt to wake exactly one thread from the
    /// futex wait queue.  If there is at least one thread to wake, the owner of the
    /// futex will be set to the thread that was woken.  Otherwise, the futex will have
    /// no owner.
    ///
    /// See *Ownership and Priority Inheritance* in [futex](/docs/reference/kernel_objects/futex.md) for
    /// details.
    ///
    /// ## Rights
    ///
    /// None.
    ///
    /// ## Return value
    ///
    /// `zx_futex_wake()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  *value_ptr* is not aligned.
    ///
    /// ## See also
    ///
    ///  - [futex objects]
    ///  - [`zx_futex_requeue()`]
    ///  - [`zx_futex_wait()`]
    ///  - [`zx_futex_wake_single_owner()`]
    ///
    /// [futex objects]: /docs/reference/kernel_objects/futex.md
    /// [`zx_futex_requeue()`]: futex_requeue.md
    /// [`zx_futex_wait()`]: futex_wait.md
    /// [`zx_futex_wake_single_owner()`]: futex_wake_single_owner.md
    strict FutexWake(struct {
        @embedded_alias("zx/Futex")
        value_ptr experimental_pointer<Futex>;
        wake_count uint32;
    }) -> () error Status;

    /// ## Summary
    ///
    /// Wake some number of threads waiting on a futex, and move more waiters to another wait queue.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_futex_requeue(const zx_futex_t* value_ptr,
    ///                              uint32_t wake_count,
    ///                              zx_futex_t current_value,
    ///                              const zx_futex_t* requeue_ptr,
    ///                              uint32_t requeue_count,
    ///                              zx_handle_t new_requeue_owner);
    /// ```
    ///
    /// ## Description
    ///
    /// Requeuing is a generalization of waking. First, the kernel verifies
    /// that the value in *current_value* matches the value of the futex at
    /// *value_ptr*, and if not reports `ZX_ERR_BAD_STATE`. After waking *wake_count*
    /// threads, *requeue_count* threads are moved from the original futex's
    /// wait queue to the wait queue corresponding to *requeue_ptr*, another
    /// futex.
    ///
    /// This requeueing behavior may be used to avoid thundering herds on wake.
    ///
    /// ## OWNERSHIP
    ///
    /// A requeue operation targets two futexes, the _wake futex_ and the _requeue
    /// futex_.  The ownership implications for each are discussed separately.
    /// Generally, if the call fails for any reason, no changes to ownership for either
    /// futex are made.
    ///
    /// See *Ownership and Priority Inheritance* in [futex](/docs/reference/kernel_objects/futex.md) for
    /// details.
    ///
    /// ### Effects on the _wake futex_ target
    ///
    /// A successful call to `zx_futex_requeue()` results in the owner of the futex being
    /// set to nothing, regardless of the wake count.  In order to transfer ownership of
    /// a futex, use the [`zx_futex_requeue_single_owner()`] variant instead.
    /// [`zx_futex_requeue_single_owner()`] will attempt to wake exactly one thread from the
    /// futex wait queue.  If there is at least one thread to wake, the owner of the futex will be
    /// set to the thread that was woken.  Otherwise, the futex
    /// will have no owner.
    ///
    /// ### Effects on the _requeue futex_ target
    ///
    /// A successful call to `zx_futex_requeue()` or [`zx_futex_requeue_single_owner()`]
    /// results in the owner of the futex being set to the thread referenced by the
    /// *new_requeue_owner* handle, or to nothing if *new_requeue_owner* is
    /// `ZX_HANDLE_INVALID`.
    ///
    /// ## Rights
    ///
    /// None.
    ///
    /// ## Return value
    ///
    /// `zx_futex_requeue()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  One of the following is true:
    ///
    /// + Either *value_ptr* or *requeue_ptr* is not a valid userspace pointer
    /// + Either *value_ptr* or *requeue_ptr* is not aligned to a `sizeof(zx_futex_t)` boundary.
    /// + *value_ptr* is the same futex as *requeue_ptr*
    /// + *new_requeue_owner* is currently a member of the waiters for either *value_ptr* or *requeue_ptr*
    /// + *new_requeue_owner* has not been started yet.
    ///
    /// `ZX_ERR_BAD_HANDLE`  *new_requeue_owner* is not `ZX_HANDLE_INVALID`, and not a valid handle AND
    /// *current_value* matches the value at *value_ptr*
    ///
    /// `ZX_ERR_WRONG_TYPE`  *new_requeue_owner* is a valid handle, but is not a handle to a thread.
    ///
    /// `ZX_ERR_BAD_STATE`  *current_value* does not match the value at *value_ptr*.
    ///
    /// ## See also
    ///
    ///  - [futex objects]
    ///  - [`zx_futex_requeue_single_owner()`]
    ///  - [`zx_futex_wait()`]
    ///  - [`zx_futex_wake()`]
    ///
    /// [futex objects]: /docs/reference/kernel_objects/futex.md
    /// [`zx_futex_requeue_single_owner()`]: futex_requeue_single_owner.md
    /// [`zx_futex_wait()`]: futex_wait.md
    /// [`zx_futex_wake()`]: futex_wake.md
    strict FutexRequeue(resource struct {
        @embedded_alias("zx/Futex")
        value_ptr experimental_pointer<Futex>;
        wake_count uint32;
        current_value int32;
        @embedded_alias("zx/Futex")
        requeue_ptr experimental_pointer<Futex>;
        requeue_count uint32;
        new_requeue_owner Handle;
    }) -> () error Status;

    /// ## Summary
    ///
    /// Wake one thread waiting on a futex, and set the ownership of the futex to that thread.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_futex_wake_single_owner(const zx_futex_t* value_ptr);
    /// ```
    ///
    /// ## Description
    ///
    /// Wake one thread waiting on a futex.
    /// If a thread is woken, ownership of the futex is transferred to that thread. If no
    /// thread is woken (because none are waiting), ownership of the futex is set to none.
    ///
    /// See [`zx_futex_wake()`] for a full description.
    ///
    /// ## Rights
    ///
    /// None.
    ///
    /// ## Return value
    ///
    /// `zx_futex_wake_single_owner()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  *value_ptr* is not aligned.
    ///
    /// ## See also
    ///
    ///  - [futex objects]
    ///  - [`zx_futex_requeue()`]
    ///  - [`zx_futex_wait()`]
    ///  - [`zx_futex_wake()`]
    ///
    /// [futex objects]: /docs/reference/kernel_objects/futex.md
    /// [`zx_futex_requeue()`]: futex_requeue.md
    /// [`zx_futex_wait()`]: futex_wait.md
    /// [`zx_futex_wake()`]: futex_wake.md
    strict FutexWakeSingleOwner(struct {
        @embedded_alias("zx/Futex")
        value_ptr experimental_pointer<Futex>;
    }) -> () error Status;

    /// ## Summary
    ///
    /// Wake one thread waiting on a futex, and requeue more waiters to another futex wait queue.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_futex_requeue_single_owner(const zx_futex_t* value_ptr,
    ///                                           zx_futex_t current_value,
    ///                                           const zx_futex_t* requeue_ptr,
    ///                                           uint32_t requeue_count,
    ///                                           zx_handle_t new_requeue_owner);
    /// ```
    ///
    /// ## Description
    ///
    /// Wake one thread waiting on *value_ptr* and assign ownership of *value_ptr* to
    /// the thread that was woken. If there are no threads waiting on *value_ptr* then
    /// the ownership of *value_ptr* is set to none.
    ///
    /// Then move up to *requeue_count* threads that are still waiting on *value_ptr* from
    /// the *value_ptr* futex to the *requeue_ptr* futex.
    ///
    /// `zx_futex_requeue_single_owner` is similar to `zx_futex_requeue` with a
    /// *wake_count* of 1, except that `zx_futex_requeue_single_owner` changes the
    /// ownership of *value_ptr* to the woken thread.
    /// See [`zx_futex_requeue()`] for a full description.
    ///
    /// ## Rights
    ///
    /// None.
    ///
    /// ## Return value
    ///
    /// `zx_futex_requeue_single_owner()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  One of the following is true:
    ///
    /// + Either *value_ptr* or *requeue_ptr* is not a valid userspace pointer
    /// + Either *value_ptr* or *requeue_ptr* is not aligned to a `sizeof(zx_futex_t)` boundary.
    /// + *value_ptr* is the same futex as *requeue_ptr*
    /// + *new_requeue_owner* is currently a member of the waiters for either *value_ptr* or *requeue_ptr*
    ///
    /// `ZX_ERR_BAD_HANDLE`  *new_requeue_owner* is not `ZX_HANDLE_INVALID`, and not a valid handle.
    ///
    /// `ZX_ERR_WRONG_TYPE`  *new_requeue_owner* is a valid handle, but is not a handle to a thread.
    ///
    /// `ZX_ERR_BAD_STATE`  *current_value* does not match the value at *value_ptr*.
    ///
    /// ## See also
    ///
    ///  - [futex objects]
    ///  - [`zx_futex_requeue()`]
    ///  - [`zx_futex_wait()`]
    ///  - [`zx_futex_wake()`]
    ///
    /// [futex objects]: /docs/reference/kernel_objects/futex.md
    /// [`zx_futex_requeue()`]: futex_requeue.md
    /// [`zx_futex_wait()`]: futex_wait.md
    /// [`zx_futex_wake()`]: futex_wake.md
    strict FutexRequeueSingleOwner(resource struct {
        @embedded_alias("zx/Futex")
        value_ptr experimental_pointer<Futex>;
        current_value int32;
        @embedded_alias("zx/Futex")
        requeue_ptr experimental_pointer<Futex>;
        requeue_count uint32;
        new_requeue_owner Handle;
    }) -> () error Status;

    /// ## Summary
    ///
    /// Fetch the koid current owner of a futex, if any.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_futex_get_owner(const zx_futex_t* value_ptr, zx_koid_t* koid);
    /// ```
    ///
    /// ## Description
    ///
    /// Fetch the koid of the current owner of the futex identified by *value_ptr*, or
    /// `ZX_KOID_INVALID` if there is no current owner.  Knowledge of the ownership of
    /// a futex typically serves no purpose when building synchronization primitives
    /// from futexes.  This syscall is used primarily for testing.
    ///
    /// See *Ownership and Priority Inheritance* in [futex](/docs/reference/kernel_objects/futex.md) for
    /// details.
    ///
    /// ## Rights
    ///
    /// None.
    ///
    /// ## Return value
    ///
    /// `zx_futex_get_owner()` returns `ZX_OK` on success, and koids hold the owner of
    /// the futex at the time of the syscall, or `ZX_KOID_INVALID` if there was no
    /// owner.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  One of the following is true:
    ///
    /// + *value_ptr* is not a valid userspace pointer.
    /// + *value_ptr* is not aligned to a `sizeof(zx_futex_t)` boundary.
    /// + *koid* is not a valid userspace pointer.
    ///
    /// ## See also
    ///
    /// [futex objects](/docs/reference/kernel_objects/futex.md)
    strict FutexGetOwner(struct {
        @embedded_alias("zx/Futex")
        value_ptr experimental_pointer<Futex>;
    }) -> (struct {
        koid Koid;
    }) error Status;
};