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use crate::cell::UnsafeCell;
use crate::mem::forget;
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::sys_common::lazy_box::{LazyBox, LazyInit};
pub struct RwLock {
inner: UnsafeCell<libc::pthread_rwlock_t>,
write_locked: UnsafeCell<bool>, // guarded by the `inner` RwLock
num_readers: AtomicUsize,
}
pub(crate) type MovableRwLock = LazyBox<RwLock>;
unsafe impl Send for RwLock {}
unsafe impl Sync for RwLock {}
impl LazyInit for RwLock {
fn init() -> Box<Self> {
Box::new(Self::new())
}
fn destroy(mut rwlock: Box<Self>) {
// We're not allowed to pthread_rwlock_destroy a locked rwlock,
// so check first if it's unlocked.
if *rwlock.write_locked.get_mut() || *rwlock.num_readers.get_mut() != 0 {
// The rwlock is locked. This happens if a RwLock{Read,Write}Guard is leaked.
// In this case, we just leak the RwLock too.
forget(rwlock);
}
}
fn cancel_init(_: Box<Self>) {
// In this case, we can just drop it without any checks,
// since it cannot have been locked yet.
}
}
impl RwLock {
pub const fn new() -> RwLock {
RwLock {
inner: UnsafeCell::new(libc::PTHREAD_RWLOCK_INITIALIZER),
write_locked: UnsafeCell::new(false),
num_readers: AtomicUsize::new(0),
}
}
#[inline]
pub unsafe fn read(&self) {
let r = libc::pthread_rwlock_rdlock(self.inner.get());
// According to POSIX, when a thread tries to acquire this read lock
// while it already holds the write lock
// (or vice versa, or tries to acquire the write lock twice),
// "the call shall either deadlock or return [EDEADLK]"
// (https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_wrlock.html,
// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html).
// So, in principle, all we have to do here is check `r == 0` to be sure we properly
// got the lock.
//
// However, (at least) glibc before version 2.25 does not conform to this spec,
// and can return `r == 0` even when this thread already holds the write lock.
// We thus check for this situation ourselves and panic when detecting that a thread
// got the write lock more than once, or got a read and a write lock.
if r == libc::EAGAIN {
panic!("rwlock maximum reader count exceeded");
} else if r == libc::EDEADLK || (r == 0 && *self.write_locked.get()) {
// Above, we make sure to only access `write_locked` when `r == 0` to avoid
// data races.
if r == 0 {
// `pthread_rwlock_rdlock` succeeded when it should not have.
self.raw_unlock();
}
panic!("rwlock read lock would result in deadlock");
} else {
// POSIX does not make guarantees about all the errors that may be returned.
// See issue #94705 for more details.
assert_eq!(r, 0, "unexpected error during rwlock read lock: {:?}", r);
self.num_readers.fetch_add(1, Ordering::Relaxed);
}
}
#[inline]
pub unsafe fn try_read(&self) -> bool {
let r = libc::pthread_rwlock_tryrdlock(self.inner.get());
if r == 0 {
if *self.write_locked.get() {
// `pthread_rwlock_tryrdlock` succeeded when it should not have.
self.raw_unlock();
false
} else {
self.num_readers.fetch_add(1, Ordering::Relaxed);
true
}
} else {
false
}
}
#[inline]
pub unsafe fn write(&self) {
let r = libc::pthread_rwlock_wrlock(self.inner.get());
// See comments above for why we check for EDEADLK and write_locked. For the same reason,
// we also need to check that there are no readers (tracked in `num_readers`).
if r == libc::EDEADLK
|| (r == 0 && *self.write_locked.get())
|| self.num_readers.load(Ordering::Relaxed) != 0
{
// Above, we make sure to only access `write_locked` when `r == 0` to avoid
// data races.
if r == 0 {
// `pthread_rwlock_wrlock` succeeded when it should not have.
self.raw_unlock();
}
panic!("rwlock write lock would result in deadlock");
} else {
// According to POSIX, for a properly initialized rwlock this can only
// return EDEADLK or 0. We rely on that.
debug_assert_eq!(r, 0);
}
*self.write_locked.get() = true;
}
#[inline]
pub unsafe fn try_write(&self) -> bool {
let r = libc::pthread_rwlock_trywrlock(self.inner.get());
if r == 0 {
if *self.write_locked.get() || self.num_readers.load(Ordering::Relaxed) != 0 {
// `pthread_rwlock_trywrlock` succeeded when it should not have.
self.raw_unlock();
false
} else {
*self.write_locked.get() = true;
true
}
} else {
false
}
}
#[inline]
unsafe fn raw_unlock(&self) {
let r = libc::pthread_rwlock_unlock(self.inner.get());
debug_assert_eq!(r, 0);
}
#[inline]
pub unsafe fn read_unlock(&self) {
debug_assert!(!*self.write_locked.get());
self.num_readers.fetch_sub(1, Ordering::Relaxed);
self.raw_unlock();
}
#[inline]
pub unsafe fn write_unlock(&self) {
debug_assert_eq!(self.num_readers.load(Ordering::Relaxed), 0);
debug_assert!(*self.write_locked.get());
*self.write_locked.get() = false;
self.raw_unlock();
}
#[inline]
unsafe fn destroy(&mut self) {
let r = libc::pthread_rwlock_destroy(self.inner.get());
// On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
// rwlock that was just initialized with
// libc::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked)
// or pthread_rwlock_init() is called, this behaviour no longer occurs.
if cfg!(target_os = "dragonfly") {
debug_assert!(r == 0 || r == libc::EINVAL);
} else {
debug_assert_eq!(r, 0);
}
}
}
impl Drop for RwLock {
#[inline]
fn drop(&mut self) {
unsafe { self.destroy() };
}
}
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