library/std/src/sys/hermit/rwlock.rs

   1 use crate::cell::UnsafeCell;
   2 use crate::sys::locks::{Condvar, Mutex};
   3
   4 pub struct RwLock {
   5     lock: Mutex,
   6     cond: Condvar,
   7     state: UnsafeCell<State>,
   8 }
   9
  10 pub type MovableRwLock = RwLock;
  11
  12 enum State {
  13     Unlocked,
  14     Reading(usize),
  15     Writing,
  16 }
  17
  18 unsafe impl Send for RwLock {}
  19 unsafe impl Sync for RwLock {}
  20
  21 // This rwlock implementation is a relatively simple implementation which has a
  22 // condition variable for readers/writers as well as a mutex protecting the
  23 // internal state of the lock. A current downside of the implementation is that
  24 // unlocking the lock will notify *all* waiters rather than just readers or just
  25 // writers. This can cause lots of "thundering stampede" problems. While
  26 // hopefully correct this implementation is very likely to want to be changed in
  27 // the future.
  28
  29 impl RwLock {
  30     pub const fn new() -> RwLock {
  31         RwLock { lock: Mutex::new(), cond: Condvar::new(), state: UnsafeCell::new(State::Unlocked) }
  32     }
  33
  34     #[inline]
  35     pub unsafe fn read(&self) {
  36         self.lock.lock();
  37         while !(*self.state.get()).inc_readers() {
  38             self.cond.wait(&self.lock);
  39         }
  40         self.lock.unlock();
  41     }
  42
  43     #[inline]
  44     pub unsafe fn try_read(&self) -> bool {
  45         self.lock.lock();
  46         let ok = (*self.state.get()).inc_readers();
  47         self.lock.unlock();
  48         return ok;
  49     }
  50
  51     #[inline]
  52     pub unsafe fn write(&self) {
  53         self.lock.lock();
  54         while !(*self.state.get()).inc_writers() {
  55             self.cond.wait(&self.lock);
  56         }
  57         self.lock.unlock();
  58     }
  59
  60     #[inline]
  61     pub unsafe fn try_write(&self) -> bool {
  62         self.lock.lock();
  63         let ok = (*self.state.get()).inc_writers();
  64         self.lock.unlock();
  65         return ok;
  66     }
  67
  68     #[inline]
  69     pub unsafe fn read_unlock(&self) {
  70         self.lock.lock();
  71         let notify = (*self.state.get()).dec_readers();
  72         self.lock.unlock();
  73         if notify {
  74             // FIXME: should only wake up one of these some of the time
  75             self.cond.notify_all();
  76         }
  77     }
  78
  79     #[inline]
  80     pub unsafe fn write_unlock(&self) {
  81         self.lock.lock();
  82         (*self.state.get()).dec_writers();
  83         self.lock.unlock();
  84         // FIXME: should only wake up one of these some of the time
  85         self.cond.notify_all();
  86     }
  87
  88     #[inline]
  89     pub unsafe fn destroy(&self) {
  90         self.lock.destroy();
  91         self.cond.destroy();
  92     }
  93 }
  94
  95 impl State {
  96     fn inc_readers(&mut self) -> bool {
  97         match *self {
  98             State::Unlocked => {
  99                 *self = State::Reading(1);
 100                 true
 101             }
 102             State::Reading(ref mut cnt) => {
 103                 *cnt += 1;
 104                 true
 105             }
 106             State::Writing => false,
 107         }
 108     }
 109
 110     fn inc_writers(&mut self) -> bool {
 111         match *self {
 112             State::Unlocked => {
 113                 *self = State::Writing;
 114                 true
 115             }
 116             State::Reading(_) | State::Writing => false,
 117         }
 118     }
 119
 120     fn dec_readers(&mut self) -> bool {
 121         let zero = match *self {
 122             State::Reading(ref mut cnt) => {
 123                 *cnt -= 1;
 124                 *cnt == 0
 125             }
 126             State::Unlocked | State::Writing => invalid(),
 127         };
 128         if zero {
 129             *self = State::Unlocked;
 130         }
 131         zero
 132     }
 133
 134     fn dec_writers(&mut self) {
 135         match *self {
 136             State::Writing => {}
 137             State::Unlocked | State::Reading(_) => invalid(),
 138         }
 139         *self = State::Unlocked;
 140     }
 141 }
 142
 143 fn invalid() -> ! {
 144     panic!("inconsistent rwlock");
 145 }