vasi_sync/scmutex.rs
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use core::{marker::PhantomData, pin::Pin};
use vasi::VirtualAddressSpaceIndependent;
use crate::sync;
#[cfg_attr(not(loom), derive(VirtualAddressSpaceIndependent))]
#[repr(transparent)]
struct AtomicFutexWord(sync::atomic::AtomicU32);
impl AtomicFutexWord {
// TODO: merge with `new` if and when loom's `AtomicU32` supports a const `new`.
#[cfg(not(loom))]
pub const fn const_new(val: FutexWord) -> Self {
Self(crate::sync::atomic::AtomicU32::new(val.to_u32()))
}
pub fn new(val: FutexWord) -> Self {
Self(crate::sync::atomic::AtomicU32::new(val.to_u32()))
}
pub fn inc_sleepers_and_fetch(&self, ord: sync::atomic::Ordering) -> FutexWord {
// The number of sleepers is stored in the low bits of the futex word,
// so we can increment the whole word.
let prev = FutexWord::from(self.0.fetch_add(1, ord));
// We'll panic here if we've overflowed she "sleepers" half of the word,
// leaving the lock in a bad state. Since UNLOCKED is 0, this will never
// cause a spurious unlock, but still-live threads using the lock
// will likely panic or deadlock.
FutexWord {
lock_state: prev.lock_state,
num_sleepers: prev.num_sleepers.checked_add(1).unwrap(),
}
}
pub fn dec_sleepers_and_fetch(&self, ord: sync::atomic::Ordering) -> FutexWord {
// The number of sleepers is stored in the low bits of the futex word,
// so we can decrement the whole word.
// Ideally we'd just use an atomic op on the "sleepers" part of the
// larger word, but that sort of aliasing breaks loom's analysis.
let prev = FutexWord::from(self.0.fetch_sub(1, ord));
// We'll panic here if we've underflowed the "sleepers" half of the word,
// leaving the lock in a bad state. This shouldn't be possible assuming
// SelfContainedMutex itself isn't buggy.
FutexWord {
lock_state: prev.lock_state,
num_sleepers: prev.num_sleepers.checked_sub(1).unwrap(),
}
}
pub fn unlock_and_fetch(&self, ord: sync::atomic::Ordering) -> FutexWord {
// We avoid having to synchronize the number of sleepers by using fetch_sub
// instead of a compare and swap.
debug_assert_eq!(UNLOCKED, 0);
let prev = FutexWord::from(self.0.fetch_sub(
u32::from(FutexWord {
lock_state: LOCKED,
num_sleepers: 0,
}),
ord,
));
assert_eq!(prev.lock_state, LOCKED);
FutexWord {
lock_state: UNLOCKED,
num_sleepers: prev.num_sleepers,
}
}
pub fn disconnect(&self, ord: sync::atomic::Ordering) {
// We avoid having to synchronize the number of sleepers by using fetch_add
// instead of a compare and swap.
//
// We'll panic here if we've somehow underflowed the word. This
// shouldn't be possible assuming SelfContainedMutex itself isn't buggy.
let to_add = LOCKED_DISCONNECTED.checked_sub(LOCKED).unwrap();
let prev = FutexWord::from(self.0.fetch_add(
u32::from(FutexWord {
lock_state: to_add,
num_sleepers: 0,
}),
ord,
));
assert_eq!(prev.lock_state, LOCKED);
}
pub fn load(&self, ord: sync::atomic::Ordering) -> FutexWord {
self.0.load(ord).into()
}
pub fn compare_exchange(
&self,
current: FutexWord,
new: FutexWord,
success: sync::atomic::Ordering,
failure: sync::atomic::Ordering,
) -> Result<FutexWord, FutexWord> {
let raw_res = self
.0
.compare_exchange(current.into(), new.into(), success, failure);
raw_res.map(FutexWord::from).map_err(FutexWord::from)
}
}
#[repr(C)]
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
struct FutexWord {
lock_state: u16,
num_sleepers: u16,
}
impl FutexWord {
const fn to_u32(self) -> u32 {
((self.lock_state as u32) << 16) | (self.num_sleepers as u32)
}
}
impl From<u32> for FutexWord {
fn from(val: u32) -> Self {
Self {
lock_state: (val >> 16).try_into().unwrap(),
num_sleepers: (val & 0xff_ff).try_into().unwrap(),
}
}
}
impl From<FutexWord> for u32 {
fn from(val: FutexWord) -> Self {
val.to_u32()
}
}
/// Simple mutex that is suitable for use in shared memory:
///
/// * It has a fixed layout (repr(C))
/// * It's self-contained; e.g. isn't boxed and doesn't refer
/// to global lock-state in this process's address space.
/// * Works across processes (e.g. doesn't use FUTEX_PRIVATE_FLAG)
///
/// Performance is optimized primarily for low-contention scenarios.
#[cfg_attr(not(loom), derive(VirtualAddressSpaceIndependent))]
#[repr(C)]
pub struct SelfContainedMutex<T> {
futex: AtomicFutexWord,
val: sync::UnsafeCell<T>,
}
unsafe impl<T> Send for SelfContainedMutex<T> where T: Send {}
unsafe impl<T> Sync for SelfContainedMutex<T> where T: Send {}
const UNLOCKED: u16 = 0;
const LOCKED: u16 = 1;
const LOCKED_DISCONNECTED: u16 = 2;
impl<T> SelfContainedMutex<T> {
// TODO: merge with `new` when `AtomicFutexWord` supports a const `new`.
#[cfg(not(loom))]
pub const fn const_new(val: T) -> Self {
Self {
futex: AtomicFutexWord::const_new(FutexWord {
lock_state: UNLOCKED,
num_sleepers: 0,
}),
val: sync::UnsafeCell::new(val),
}
}
pub fn new(val: T) -> Self {
Self {
futex: AtomicFutexWord::new(FutexWord {
lock_state: UNLOCKED,
num_sleepers: 0,
}),
val: sync::UnsafeCell::new(val),
}
}
pub fn lock(&self) -> SelfContainedMutexGuard<T> {
// On first attempt, optimistically assume the lock is uncontended.
let mut current = FutexWord {
lock_state: UNLOCKED,
num_sleepers: 0,
};
loop {
if current.lock_state == UNLOCKED {
// Try to take the lock.
let current_res = self.futex.compare_exchange(
current,
FutexWord {
lock_state: LOCKED,
num_sleepers: current.num_sleepers,
},
sync::Ordering::Acquire,
sync::Ordering::Relaxed,
);
current = match current_res {
Ok(_) => {
// We successfully took the lock.
break;
}
// We weren't able to take the lock.
Err(i) => i,
};
}
// If the lock is available, try again now that we've sync'd the
// rest of the futex word (num_sleepers).
if current.lock_state == UNLOCKED {
continue;
}
// Try to sleep on the futex.
// Since incrementing is a read-modify-write operation, this does
// not break the release sequence since the last unlock.
current = self.futex.inc_sleepers_and_fetch(sync::Ordering::Relaxed);
loop {
// We may now see an UNLOCKED state from having done the increment
// above, or the load below.
if current.lock_state == UNLOCKED {
break;
}
match sync::futex_wait(&self.futex.0, current.into()) {
Ok(_) | Err(rustix::io::Errno::INTR) => break,
Err(rustix::io::Errno::AGAIN) => {
// We may have gotten this because another thread is
// also trying to sleep on the futex, and just
// incremented the sleeper count. If we naively
// decremented the sleeper count and ran the whole lock
// loop again, both threads could theoretically end up
// in a live-lock where neither ever gets to sleep on
// the futex.
//
// To avoid that, we update our current view of the
// atomic and consider trying again before removing
// ourselves from the sleeper count.
current = self.futex.load(sync::Ordering::Relaxed)
}
Err(e) => panic!("Unexpected futex error {:?}", e),
};
}
// Since decrementing is a read-modify-write operation, this does
// not break the release sequence since the last unlock.
current = self.futex.dec_sleepers_and_fetch(sync::Ordering::Relaxed);
}
SelfContainedMutexGuard {
mutex: Some(self),
ptr: Some(self.val.get_mut()),
_phantom: PhantomData,
}
}
pub fn lock_pinned<'a>(self: Pin<&'a Self>) -> Pin<SelfContainedMutexGuard<'a, T>> {
// SAFETY: `SelfContainedMutexGuard` doesn't provide DerefMut when `T`
// is `!Unpin`.
unsafe { Pin::new_unchecked(self.get_ref().lock()) }
}
fn unlock(&self) {
let current = self.futex.unlock_and_fetch(sync::Ordering::Release);
// Only perform a FUTEX_WAKE operation if other threads are actually
// sleeping on the lock.
if current.num_sleepers > 0 {
sync::futex_wake_one(&self.futex.0).unwrap();
}
}
}
pub struct SelfContainedMutexGuard<'a, T> {
mutex: Option<&'a SelfContainedMutex<T>>,
ptr: Option<sync::MutPtr<T>>,
// For purposes of deriving Send, Sync, etc.,
// this type should act as `&mut T`.
_phantom: PhantomData<&'a mut T>,
}
impl<'a, T> SelfContainedMutexGuard<'a, T> {
/// Drops the guard *without releasing the lock*.
///
/// This is useful when a lock must be held across some span of code within
/// a single thread, but it's difficult to pass the the guard between the
/// two parts of the code.
pub fn disconnect(mut self) {
self.mutex
.unwrap()
.futex
.disconnect(sync::Ordering::Relaxed);
self.mutex.take();
self.ptr.take();
}
/// Reconstitutes a guard that was previously disposed of via `disconnect`.
///
/// Panics if the lock is not disconnected (i.e. if `reconnect` was
/// already called).
///
/// Ok to reconnect from a different thread,though some external
/// synchronization may be needed to ensure the mutex is disconnected before
/// it tries to do so.
pub fn reconnect(mutex: &'a SelfContainedMutex<T>) -> Self {
let mut current = FutexWord {
lock_state: LOCKED_DISCONNECTED,
num_sleepers: 0,
};
loop {
assert_eq!(current.lock_state, LOCKED_DISCONNECTED);
let current_res = mutex.futex.compare_exchange(
current,
FutexWord {
lock_state: LOCKED,
num_sleepers: current.num_sleepers,
},
sync::Ordering::Relaxed,
sync::Ordering::Relaxed,
);
match current_res {
Ok(_) => {
// Done.
return Self {
mutex: Some(mutex),
ptr: Some(mutex.val.get_mut()),
_phantom: PhantomData,
};
}
Err(c) => {
// Try again with updated state
current = c;
}
}
}
}
/// Map the guard into a function of Pin<&mut T>.
///
/// When T implements `Unpin`, the caller can just use deref_mut instead.
///
// We can't provide an API that simply returns a Pin<&mut T>, since the Pin
// API doesn't provide a way to get to the inner guard without consuming the outer Pin.
pub fn map_pinned<F, O>(guard: Pin<Self>, f: F) -> O
where
F: FnOnce(Pin<&mut T>) -> O,
{
// SAFETY: We ensure that the &mut T made available from the unpinned guard isn't
// moved-from, by only giving `f` access to a Pin<&mut T>.
let guard: SelfContainedMutexGuard<T> = unsafe { Pin::into_inner_unchecked(guard) };
// SAFETY: The pointer is valid because it came from the mutex, which we know is live.
// The mutex ensures there can be no other live references to the internal data.
let ref_t = unsafe { guard.ptr.as_ref().unwrap().deref() };
// SAFETY: We know the original data is pinned, since the guard was Pin<Self>.
let pinned_t: Pin<&mut T> = unsafe { Pin::new_unchecked(ref_t) };
f(pinned_t)
}
}
impl<'a, T> Drop for SelfContainedMutexGuard<'a, T> {
fn drop(&mut self) {
if let Some(mutex) = self.mutex {
// We have to drop this pointer before unlocking when running
// under loom, which could otherwise detect multiple mutable
// references to the underlying cell. Under non loom, the drop
// has no effect.
#[allow(clippy::drop_non_drop)]
drop(self.ptr.take());
mutex.unlock();
}
}
}
impl<'a, T> core::ops::Deref for SelfContainedMutexGuard<'a, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// We can't call self.ptr.as_ref().unwrap().deref() here, since that
// would create a `&mut T`, and there could already exist a `&T`
// borrowed from `&self`.
// https://github.com/tokio-rs/loom/issues/293
self.ptr.as_ref().unwrap().with(|p| unsafe { &*p })
}
}
/// When T is Unpin, we can implement DerefMut. Otherwise it's unsafe
/// to do so, since SelfContainedMutex is an Archive type.
impl<'a, T> core::ops::DerefMut for SelfContainedMutexGuard<'a, T>
where
T: Unpin,
{
fn deref_mut(&mut self) -> &mut Self::Target {
unsafe { self.ptr.as_ref().unwrap().deref() }
}
}
// For unit tests see tests/scmutex-tests.rs