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//! A priority inheriting mutex for Fuchsia.
//!
//! This is a port of the [mutex in Fuchsia's libsync]. Contrary to the original,
//! it does not abort the process when reentrant locking is detected, but deadlocks.
//!
//! Priority inheritance is achieved by storing the owning thread's handle in an
//! atomic variable. Fuchsia's futex operations support setting an owner thread
//! for a futex, which can boost that thread's priority while the futex is waited
//! upon.
//!
//! libsync is licenced under the following BSD-style licence:
//!
//! Copyright 2016 The Fuchsia Authors.
//!
//! Redistribution and use in source and binary forms, with or without
//! modification, are permitted provided that the following conditions are
//! met:
//!
//! * Redistributions of source code must retain the above copyright
//! notice, this list of conditions and the following disclaimer.
//! * Redistributions in binary form must reproduce the above
//! copyright notice, this list of conditions and the following
//! disclaimer in the documentation and/or other materials provided
//! with the distribution.
//!
//! THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
//! "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
//! LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
//! A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
//! OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
//! SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
//! LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
//! DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
//! THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//! (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
//! OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//!
//! [mutex in Fuchsia's libsync]: https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/system/ulib/sync/mutex.c
use crate::sync::atomic::{
AtomicU32,
Ordering::{Acquire, Relaxed, Release},
};
use crate::sys::futex::zircon::{
zx_futex_wait, zx_futex_wake_single_owner, zx_handle_t, zx_thread_self, ZX_ERR_BAD_HANDLE,
ZX_ERR_BAD_STATE, ZX_ERR_INVALID_ARGS, ZX_ERR_TIMED_OUT, ZX_ERR_WRONG_TYPE, ZX_OK,
ZX_TIME_INFINITE,
};
// The lowest two bits of a `zx_handle_t` are always set, so the lowest bit is used to mark the
// mutex as contested by clearing it.
const CONTESTED_BIT: u32 = 1;
// This can never be a valid `zx_handle_t`.
const UNLOCKED: u32 = 0;
pub type MovableMutex = Mutex;
pub struct Mutex {
futex: AtomicU32,
}
#[inline]
fn to_state(owner: zx_handle_t) -> u32 {
owner
}
#[inline]
fn to_owner(state: u32) -> zx_handle_t {
state | CONTESTED_BIT
}
#[inline]
fn is_contested(state: u32) -> bool {
state & CONTESTED_BIT == 0
}
#[inline]
fn mark_contested(state: u32) -> u32 {
state & !CONTESTED_BIT
}
impl Mutex {
#[inline]
pub const fn new() -> Mutex {
Mutex { futex: AtomicU32::new(UNLOCKED) }
}
#[inline]
pub unsafe fn try_lock(&self) -> bool {
let thread_self = zx_thread_self();
self.futex.compare_exchange(UNLOCKED, to_state(thread_self), Acquire, Relaxed).is_ok()
}
#[inline]
pub unsafe fn lock(&self) {
let thread_self = zx_thread_self();
if let Err(state) =
self.futex.compare_exchange(UNLOCKED, to_state(thread_self), Acquire, Relaxed)
{
self.lock_contested(state, thread_self);
}
}
#[cold]
fn lock_contested(&self, mut state: u32, thread_self: zx_handle_t) {
let owned_state = mark_contested(to_state(thread_self));
loop {
// Mark the mutex as contested if it is not already.
let contested = mark_contested(state);
if is_contested(state)
|| self.futex.compare_exchange(state, contested, Relaxed, Relaxed).is_ok()
{
// The mutex has been marked as contested, wait for the state to change.
unsafe {
match zx_futex_wait(
&self.futex,
AtomicU32::new(contested),
to_owner(state),
ZX_TIME_INFINITE,
) {
ZX_OK | ZX_ERR_BAD_STATE | ZX_ERR_TIMED_OUT => (),
// Note that if a thread handle is reused after its associated thread
// exits without unlocking the mutex, an arbitrary thread's priority
// could be boosted by the wait, but there is currently no way to
// prevent that.
ZX_ERR_INVALID_ARGS | ZX_ERR_BAD_HANDLE | ZX_ERR_WRONG_TYPE => {
panic!(
"either the current thread is trying to lock a mutex it has
already locked, or the previous owner did not unlock the mutex
before exiting"
)
}
error => panic!("unexpected error in zx_futex_wait: {error}"),
}
}
}
// The state has changed or a wakeup occurred, try to lock the mutex.
match self.futex.compare_exchange(UNLOCKED, owned_state, Acquire, Relaxed) {
Ok(_) => return,
Err(updated) => state = updated,
}
}
}
#[inline]
pub unsafe fn unlock(&self) {
if is_contested(self.futex.swap(UNLOCKED, Release)) {
// The woken thread will mark the mutex as contested again,
// and return here, waking until there are no waiters left,
// in which case this is a noop.
self.wake();
}
}
#[cold]
fn wake(&self) {
unsafe {
zx_futex_wake_single_owner(&self.futex);
}
}
}
|
