1 use crate::cell::UnsafeCell;
2 use crate::sync::atomic::{AtomicUsize, Ordering};
5 inner: UnsafeCell<libc::pthread_rwlock_t>,
6 write_locked: UnsafeCell<bool>, // guarded by the `inner` RwLock
7 num_readers: AtomicUsize,
10 unsafe impl Send for RWLock {}
11 unsafe impl Sync for RWLock {}
14 pub const fn new() -> RWLock {
16 inner: UnsafeCell::new(libc::PTHREAD_RWLOCK_INITIALIZER),
17 write_locked: UnsafeCell::new(false),
18 num_readers: AtomicUsize::new(0),
22 pub unsafe fn read(&self) {
23 let r = libc::pthread_rwlock_rdlock(self.inner.get());
25 // According to POSIX, when a thread tries to acquire this read lock
26 // while it already holds the write lock
27 // (or vice versa, or tries to acquire the write lock twice),
28 // "the call shall either deadlock or return [EDEADLK]"
29 // (https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_wrlock.html,
30 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html).
31 // So, in principle, all we have to do here is check `r == 0` to be sure we properly
34 // However, (at least) glibc before version 2.25 does not conform to this spec,
35 // and can return `r == 0` even when this thread already holds the write lock.
36 // We thus check for this situation ourselves and panic when detecting that a thread
37 // got the write lock more than once, or got a read and a write lock.
38 if r == libc::EAGAIN {
39 panic!("rwlock maximum reader count exceeded");
40 } else if r == libc::EDEADLK || (r == 0 && *self.write_locked.get()) {
41 // Above, we make sure to only access `write_locked` when `r == 0` to avoid
44 // `pthread_rwlock_rdlock` succeeded when it should not have.
47 panic!("rwlock read lock would result in deadlock");
49 // According to POSIX, for a properly initialized rwlock this can only
50 // return EAGAIN or EDEADLK or 0. We rely on that.
51 debug_assert_eq!(r, 0);
52 self.num_readers.fetch_add(1, Ordering::Relaxed);
56 pub unsafe fn try_read(&self) -> bool {
57 let r = libc::pthread_rwlock_tryrdlock(self.inner.get());
59 if *self.write_locked.get() {
60 // `pthread_rwlock_tryrdlock` succeeded when it should not have.
64 self.num_readers.fetch_add(1, Ordering::Relaxed);
72 pub unsafe fn write(&self) {
73 let r = libc::pthread_rwlock_wrlock(self.inner.get());
74 // See comments above for why we check for EDEADLK and write_locked. For the same reason,
75 // we also need to check that there are no readers (tracked in `num_readers`).
77 || (r == 0 && *self.write_locked.get())
78 || self.num_readers.load(Ordering::Relaxed) != 0
80 // Above, we make sure to only access `write_locked` when `r == 0` to avoid
83 // `pthread_rwlock_wrlock` succeeded when it should not have.
86 panic!("rwlock write lock would result in deadlock");
88 // According to POSIX, for a properly initialized rwlock this can only
89 // return EDEADLK or 0. We rely on that.
90 debug_assert_eq!(r, 0);
92 *self.write_locked.get() = true;
95 pub unsafe fn try_write(&self) -> bool {
96 let r = libc::pthread_rwlock_trywrlock(self.inner.get());
98 if *self.write_locked.get() || self.num_readers.load(Ordering::Relaxed) != 0 {
99 // `pthread_rwlock_trywrlock` succeeded when it should not have.
103 *self.write_locked.get() = true;
111 unsafe fn raw_unlock(&self) {
112 let r = libc::pthread_rwlock_unlock(self.inner.get());
113 debug_assert_eq!(r, 0);
116 pub unsafe fn read_unlock(&self) {
117 debug_assert!(!*self.write_locked.get());
118 self.num_readers.fetch_sub(1, Ordering::Relaxed);
122 pub unsafe fn write_unlock(&self) {
123 debug_assert_eq!(self.num_readers.load(Ordering::Relaxed), 0);
124 debug_assert!(*self.write_locked.get());
125 *self.write_locked.get() = false;
129 pub unsafe fn destroy(&self) {
130 let r = libc::pthread_rwlock_destroy(self.inner.get());
131 // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
132 // rwlock that was just initialized with
133 // libc::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked)
134 // or pthread_rwlock_init() is called, this behaviour no longer occurs.
135 if cfg!(target_os = "dragonfly") {
136 debug_assert!(r == 0 || r == libc::EINVAL);
138 debug_assert_eq!(r, 0);