1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
use crate::cell::UnsafeCell;
use crate::sync::atomic::{AtomicUsize, Ordering};
pub struct RWLock {
inner: UnsafeCell<libc::pthread_rwlock_t>,
write_locked: UnsafeCell<bool>, // guarded by the `inner` RwLock
num_readers: AtomicUsize,
}
pub type MovableRWLock = Box<RWLock>;
unsafe impl Send for RWLock {}
unsafe impl Sync for RWLock {}
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 {
// According to POSIX, for a properly initialized rwlock this can only
// return EAGAIN or EDEADLK or 0. We rely on that.
debug_assert_eq!(r, 0);
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]
pub unsafe fn destroy(&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);
}
}
}