| 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, |
| } |
| |
| 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 the pthread_rwlock_rdlock spec, this function **may** |
| // fail with EDEADLK if a deadlock is detected. On the other hand |
| // pthread mutexes will *never* return EDEADLK if they are initialized |
| // as the "fast" kind (which ours always are). As a result, a deadlock |
| // situation may actually return from the call to pthread_rwlock_rdlock |
| // instead of blocking forever (as mutexes and Windows rwlocks do). Note |
| // that not all unix implementations, however, will return EDEADLK for |
| // their rwlocks. |
| // |
| // We roughly maintain the deadlocking behavior by panicking to ensure |
| // that this lock acquisition does not succeed. |
| // |
| // We also check whether this lock is already write locked. This |
| // is only possible if it was write locked by the current thread and |
| // the implementation allows recursive locking. The POSIX standard |
| // doesn't require recursively locking a rwlock to deadlock, but we can't |
| // allow that because it could lead to aliasing issues. |
| if r == libc::EAGAIN { |
| panic!("rwlock maximum reader count exceeded"); |
| } else if r == libc::EDEADLK || (r == 0 && *self.write_locked.get()) { |
| if r == 0 { |
| self.raw_unlock(); |
| } |
| panic!("rwlock read lock would result in deadlock"); |
| } else { |
| 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() { |
| 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. We |
| // also need to check that num_readers is 0. |
| if r == libc::EDEADLK || *self.write_locked.get() || |
| self.num_readers.load(Ordering::Relaxed) != 0 { |
| if r == 0 { |
| self.raw_unlock(); |
| } |
| panic!("rwlock write lock would result in deadlock"); |
| } else { |
| 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 { |
| 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); |
| } |
| } |
| } |