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use crate::cell::UnsafeCell;
use crate::mem::{forget, MaybeUninit};
use crate::sys::cvt_nz;
use crate::sys_common::lazy_box::{LazyBox, LazyInit};
pub struct Mutex {
inner: UnsafeCell<libc::pthread_mutex_t>,
}
pub(crate) type MovableMutex = LazyBox<Mutex>;
#[inline]
pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
m.inner.get()
}
unsafe impl Send for Mutex {}
unsafe impl Sync for Mutex {}
impl LazyInit for Mutex {
fn init() -> Box<Self> {
let mut mutex = Box::new(Self::new());
unsafe { mutex.init() };
mutex
}
fn destroy(mutex: Box<Self>) {
// We're not allowed to pthread_mutex_destroy a locked mutex,
// so check first if it's unlocked.
if unsafe { mutex.try_lock() } {
unsafe { mutex.unlock() };
drop(mutex);
} else {
// The mutex is locked. This happens if a MutexGuard is leaked.
// In this case, we just leak the Mutex too.
forget(mutex);
}
}
fn cancel_init(_: Box<Self>) {
// In this case, we can just drop it without any checks,
// since it cannot have been locked yet.
}
}
impl Mutex {
pub const fn new() -> Mutex {
// Might be moved to a different address, so it is better to avoid
// initialization of potentially opaque OS data before it landed.
// Be very careful using this newly constructed `Mutex`, reentrant
// locking is undefined behavior until `init` is called!
Mutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
}
#[inline]
unsafe fn init(&mut self) {
// Issue #33770
//
// A pthread mutex initialized with PTHREAD_MUTEX_INITIALIZER will have
// a type of PTHREAD_MUTEX_DEFAULT, which has undefined behavior if you
// try to re-lock it from the same thread when you already hold a lock
// (https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_init.html).
// This is the case even if PTHREAD_MUTEX_DEFAULT == PTHREAD_MUTEX_NORMAL
// (https://github.com/rust-lang/rust/issues/33770#issuecomment-220847521) -- in that
// case, `pthread_mutexattr_settype(PTHREAD_MUTEX_DEFAULT)` will of course be the same
// as setting it to `PTHREAD_MUTEX_NORMAL`, but not setting any mode will result in
// a Mutex where re-locking is UB.
//
// In practice, glibc takes advantage of this undefined behavior to
// implement hardware lock elision, which uses hardware transactional
// memory to avoid acquiring the lock. While a transaction is in
// progress, the lock appears to be unlocked. This isn't a problem for
// other threads since the transactional memory will abort if a conflict
// is detected, however no abort is generated when re-locking from the
// same thread.
//
// Since locking the same mutex twice will result in two aliasing &mut
// references, we instead create the mutex with type
// PTHREAD_MUTEX_NORMAL which is guaranteed to deadlock if we try to
// re-lock it from the same thread, thus avoiding undefined behavior.
let mut attr = MaybeUninit::<libc::pthread_mutexattr_t>::uninit();
cvt_nz(libc::pthread_mutexattr_init(attr.as_mut_ptr())).unwrap();
let attr = PthreadMutexAttr(&mut attr);
cvt_nz(libc::pthread_mutexattr_settype(attr.0.as_mut_ptr(), libc::PTHREAD_MUTEX_NORMAL))
.unwrap();
cvt_nz(libc::pthread_mutex_init(self.inner.get(), attr.0.as_ptr())).unwrap();
}
#[inline]
pub unsafe fn lock(&self) {
let r = libc::pthread_mutex_lock(self.inner.get());
debug_assert_eq!(r, 0);
}
#[inline]
pub unsafe fn unlock(&self) {
let r = libc::pthread_mutex_unlock(self.inner.get());
debug_assert_eq!(r, 0);
}
#[inline]
pub unsafe fn try_lock(&self) -> bool {
libc::pthread_mutex_trylock(self.inner.get()) == 0
}
#[inline]
#[cfg(not(target_os = "dragonfly"))]
unsafe fn destroy(&mut self) {
let r = libc::pthread_mutex_destroy(self.inner.get());
debug_assert_eq!(r, 0);
}
#[inline]
#[cfg(target_os = "dragonfly")]
unsafe fn destroy(&mut self) {
let r = libc::pthread_mutex_destroy(self.inner.get());
// On DragonFly pthread_mutex_destroy() returns EINVAL if called on a
// mutex that was just initialized with libc::PTHREAD_MUTEX_INITIALIZER.
// Once it is used (locked/unlocked) or pthread_mutex_init() is called,
// this behaviour no longer occurs.
debug_assert!(r == 0 || r == libc::EINVAL);
}
}
impl Drop for Mutex {
#[inline]
fn drop(&mut self) {
unsafe { self.destroy() };
}
}
pub(super) struct PthreadMutexAttr<'a>(pub &'a mut MaybeUninit<libc::pthread_mutexattr_t>);
impl Drop for PthreadMutexAttr<'_> {
fn drop(&mut self) {
unsafe {
let result = libc::pthread_mutexattr_destroy(self.0.as_mut_ptr());
debug_assert_eq!(result, 0);
}
}
}
|
